A modified gradient approach for the growth of low-density InAs quantum dot molecules by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Sharma, Nandlal; Reuter, Dirk

2017-11-01

Two vertically stacked quantum dots that are electronically coupled, so called quantum dot molecules, are of great interest for the realization of solid state building blocks for quantum communication networks. We present a modified gradient approach to realize InAs quantum dot molecules with a low areal density so that single quantum dot molecules can be optically addressed. The individual quantum dot layers were prepared by solid source molecular beam epitaxy depositing InAs on GaAs(100). The bottom quantum dot layer has been grown without substrate rotation resulting in an In-gradient across the surface, which translated into a density gradient with low quantum dot density in a certain region of the wafer. For the top quantum dot layer, separated from the bottom quantum dot layer by a 6 nm thick GaAs barrier, various InAs amounts were deposited without an In-gradient. In spite of the absence of an In-gradient, a pronounced density gradient is observed for the top quantum dots. Even for an In-amount slightly below the critical thickness for a single dot layer, a density gradient in the top quantum dot layer, which seems to reproduce the density gradient in the bottom layer, is observed. For more or less In, respectively, deviations from this behavior occur. We suggest that the obvious influence of the bottom quantum dot layer on the growth of the top quantum dots is due to the strain field induced by the buried dots.

Density-driven transport of gas phase chemicals in unsaturated soils

NASA Astrophysics Data System (ADS)

Fen, Chiu-Shia; Sun, Yong-tai; Cheng, Yuen; Chen, Yuanchin; Yang, Whaiwan; Pan, Changtai

2018-01-01

Variations of gas phase density are responsible for advective and diffusive transports of organic vapors in unsaturated soils. Laboratory experiments were conducted to explore dense gas transport (sulfur hexafluoride, SF6) from different source densities through a nitrogen gas-dry soil column. Gas pressures and SF6 densities at transient state were measured along the soil column for three transport configurations (horizontal, vertically upward and vertically downward transport). These measurements and others reported in the literature were compared with simulation results obtained from two models based on different diffusion approaches: the dusty gas model (DGM) equations and a Fickian-type molar fraction-based diffusion expression. The results show that the DGM and Fickian-based models predicted similar dense gas density profiles which matched the measured data well for horizontal transport of dense gas at low to high source densities, despite the pressure variations predicted in the soil column were opposite to the measurements. The pressure evolutions predicted by both models were in trend similar to the measured ones for vertical transport of dense gas. However, differences between the dense gas densities predicted by the DGM and Fickian-based models were discernible for vertically upward transport of dense gas even at low source densities, as the DGM-based predictions matched the measured data better than the Fickian results did. For vertically downward transport, the dense gas densities predicted by both models were not greatly different from our experimental measurements, but substantially greater than the observations obtained from the literature, especially at high source densities. Further research will be necessary for exploring factors affecting downward transport of dense gas in soil columns. Use of the measured data to compute flux components of SF6 showed that the magnitudes of diffusive flux component based on the Fickian-type diffusion expressions in terms of molar concentration, molar fraction and mass density fraction gradient were almost the same. However, they were greater than the result computed with the mass fraction gradient for > 24% and the DGM-based result for more than one time. As a consequence, the DGM-based total flux of SF6 was in magnitude greatly less than the Fickian result not only for horizontal transport (diffusion-dominating) but also for vertical transport (advection and diffusion) of dense gas. Particularly, the Fickian-based total flux was more than two times in magnitude as much as the DGM result for vertically upward transport of dense gas.

Interaction of lateral baroclinic forcing and turbulence in an estuary

USGS Publications Warehouse

Lacy, J.R.; Stacey, M.T.; Burau, J.R.; Monismith, Stephen G.

2003-01-01

Observations of density and velocity in a channel in northern San Francisco Bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross-channel baroclinic pressure gradient and vertical mixing due to turbulence. Profiles of velocity, salinity, temperature, and suspended sediment concentration were measured in transects across Suisun Cutoff, in northern San Francisco Bay, on two days over the 12.5-hour tidal cycle. During flood tides an axial density front developed between fresher water flowing from the shallows of Grizzly Bay into the northern side of Suisun Cutoff and saltier water flowing up the channel. North of the front, transverse currents were driven by the lateral salinity gradient, with a top-to-bottom velocity difference greater than 30 cm/s. South of the front, the secondary circulation was weak, and along-channel velocities were greater than to the north. The gradient Richardson number shows that stratification was stable north of the front, while the water column was turbulently mixed south of the front. Time-series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle. In estuaries, longitudinal dynamics predict less stratification during flood than ebb tides. These data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry.

Comparative In Situ Measurements of Plasma Instabilities in the Equatorial and Auroral Electrojets

NASA Technical Reports Server (NTRS)

Pfaff, Robert F.

2008-01-01

This presentation provides a comparison of in situ measurements of plasma instabilities gathered by rocket-borne probes in the equatorial and auroral electrojets. Specifically, using detailed measurements of the DC electric fields, current density, and plasma number density within the unstable daytime equatorial electrojet from Brazil (Guara Campaign) and in the auroral electrojet from Sweden (ERRIS Campaign), we present comparative observations and general conclusions regarding the observed physical properties of Farley-Buneman two-stream waves and large scale, gradient drift waves. The two stream observations reveal coherent-like waves propagating near the E x B direction but at reduced speeds (nearer to the presumed acoustic velocity) with wavelengths of approximately 5-10m in both the equatorial and auroral electrojet, as measured using the spaced-receiver technique. The auroral electrojet data generally shows extensions to shorter wavelengths, in concert with the fact that these waves are driven harder. With respect to gradient-drift driven waves, observations of this instability are much more pronounced in the equatorial electrojet, given the more favorable geometry for growth provided by the vertical gradient and horizontal magnetic field lines. We present new analysis of Guara rocket observations of electric field and plasma density data that reveal considerable structuring in the middle and lower portion of the electrojet (90-105 km) where the ambient plasma density gradient is unstable. Although the electric field amplitudes are largest (approximately 10-15 mV/m) in the zonal direction, considerable structure (approximately 5-10 mV/m) is also observed in the vertical electric field component as well, implying that the dominant large scale waves involve significant vertical interaction and coupling within the narrow altitude range where they are observed. Furthermore, a detailed examination of the phase of the waveforms show that on some, but not all occasions, locally enhanced eastward fields are associated with locally enhanced upwards (polarization) electric fields. The measurements are discussed in terms of theories involving the non-linear evolution and structuring of plasma waves.

Thermo-Rotational Instability in Plasma Disks Around Compact Objects*

NASA Astrophysics Data System (ADS)

Coppi, Bruno

2008-04-01

Differentially rotating plasma disks, around compact objects, that are imbedded in a ``seed'' magnetic field are shown to develop vertically localized ballooning modes that are driven by the combined radial gradient of the rotation frequency and the vertical gradients of the plasma density and temperature [1]. When the electron mean free path is shorter than the disk height and the (vertical) thermal conductivity can be neglected, the vertical particle flows produced by of these modes have the effect to drive the density and temperature profiles toward the ``adiabatic condition'' where ηT≡(dlnT/dz/(dlnn/dz)=2/3. Here T is the plasma temperature and n the particle density. The faster growth rates correspond to steeper temperature profiles (ηT>2/3) such as those produced by an internal (e.g. viscous) heating process. In the end, ballooning modes excited for various values of ηT can lead to the evolution of the disk into a different current carrying configuration such as a sequence of plasma rings[2].*Sponsored in part by the U.S. Department of Energy[1]B. Coppi, M.I.T. (LNS) Report HEP, 07/02, Cambridge, MA (2007), Invited Paper at the International Symposium on ``Momentum Transport in Jets, Disks and Laboratory Plasmas'', Alba, Piedmont, September 2007, to be published in Europhysical Letters (EPL, IOP)[2]B. Coppi andF. Rousseau, Ap. J., 641, 458, (2006)

Size and DNA distributions of electrophoretically separated cultured human kidney cells

NASA Technical Reports Server (NTRS)

Kunze, M. E.; Plank, L. D.; Todd, P. W.

1985-01-01

Electrophoretic purification of purifying cultured cells according to function presumes that the size of cycle phase of a cell is not an overriding determinant of its electrophoretic velocity in an electrophoretic separator. The size distributions and DNA distributions of fractions of cells purified by density gradient electrophoresis were determined. No systematic dependence of electrophoretic migration upward in a density gradient column upon either size or DNA content were found. It was found that human leukemia cell populations, which are more uniform function and found in all phases of the cell cycle during exponential growth, separated on a vertical sensity gradient electrophoresis column according to their size, which is shown to be strictly cell cycle dependent.

The equivalent magnetizing method applied to the design of gradient coils for MRI.

PubMed

Lopez, Hector Sanchez; Liu, Feng; Crozier, Stuart

2008-01-01

This paper presents a new method for the design of gradient coils for Magnetic Resonance Imaging systems. The method is based on the equivalence between a magnetized volume surrounded by a conducting surface and its equivalent representation in surface current/charge density. We demonstrate that the curl of the vertical magnetization induces a surface current density whose stream line defines the coil current pattern. This method can be applied for coils wounds on arbitrary surface shapes. A single layer unshielded transverse gradient coil is designed and compared, with the designs obtained using two conventional methods. Through the presented example we demonstrate that the generated unconventional current patterns obtained using the magnetizing current method produces a superior gradient coil performance than coils designed by applying conventional methods.

The GALAH survey: properties of the Galactic disc(s) in the solar neighbourhood

NASA Astrophysics Data System (ADS)

Duong, L.; Freeman, K. C.; Asplund, M.; Casagrande, L.; Buder, S.; Lind, K.; Ness, M.; Bland-Hawthorn, J.; De Silva, G. M.; D'Orazi, V.; Kos, J.; Lewis, G. F.; Lin, J.; Martell, S. L.; Schlesinger, K.; Sharma, S.; Simpson, J. D.; Zucker, D. B.; Zwitter, T.; Anguiano, B.; Da Costa, G. S.; Hyde, E.; Horner, J.; Kafle, P. R.; Nataf, D. M.; Reid, W.; Stello, D.; Ting, Y.-S.; Wyse, R. F. G.

2018-06-01

Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick discs near the solar neighbourhood. The data cover a small range of Galactocentric radius (7.9 ≲ R_GC ≲ 9.5 kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude 260° ≤ ℓ ≤ 280°). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined `thick' and `thin' discs of the Galaxy. The thin disc (low-α population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/dz = -0.18 ± 0.01 dex kpc-1, which is broadly consistent with previous studies. In contrast, its vertical α-abundance profile is almost flat, with a gradient of d[α/M]/dz = 0.008 ± 0.002 dex kpc-1. The steep vertical metallicity gradient of the low-α population is in agreement with models where radial migration has a major role in the evolution of the thin disc. The thick disc (high-α population) has a weaker vertical metallicity gradient d[M/H]/dz = -0.058 ± 0.003 dex kpc-1. The α-abundance of the thick disc is nearly constant with height, d[α/M]/dz = 0.007 ± 0.002 dex kpc-1. The negative gradient in metallicity and the small gradient in [α/M] indicate that the high-α population experienced a settling phase, but also formed prior to the onset of major Type Ia supernova enrichment. We explore the implications of the distinct α-enrichments and narrow [α/M] range of the sub-populations in the context of thick disc formation.

Heat and mass transfer analogy for condensation of humid air in a vertical channel

NASA Astrophysics Data System (ADS)

Desrayaud, G.; Lauriat, G.

This study examines energy transport associated with liquid film condensation in natural convection flows driven by differences in density due to temperature and concentration gradients. The condensation problem is based on the thin-film assumptions. The most common compositional gradient, which is encountered in humid air at ambient temperature is considered. A steady laminar Boussinesq flow of an ideal gas-vapor mixture is studied for the case of a vertical parallel plate channel. New correlations for the latent and sensible Nusselt numbers are established, and the heat and mass transfer analogy between the sensible Nusselt number and Sherwood number is demonstrated.

Comprehensive gravitational modeling of the vertical cylindrical prism by Gauss-Legendre quadrature integration

NASA Astrophysics Data System (ADS)

Asgharzadeh, M. F.; Hashemi, H.; von Frese, R. RB

2018-01-01

Forward modeling is the basis of gravitational anomaly inversion that is widely applied to map subsurface mass variations. This study uses numerical least-squares Gauss-Legendre quadrature (GLQ) integration to evaluate the gravitational potential, anomaly and gradient components of the vertical cylindrical prism element. These results, in turn, may be integrated to accurately model the complete gravitational effects of fluid bearing rock formations and other vertical cylinder-like geological bodies with arbitrary variations in shape and density. Comparing the GLQ gravitational effects of uniform density, vertical circular cylinders against the effects calculated by a number of other methods illustrates the veracity of the GLQ modeling method and the accuracy limitations of the other methods. Geological examples include modeling the gravitational effects of a formation washout to help map azimuthal variations of the formation's bulk densities around the borehole wall. As another application, the gravitational effects of a seismically and gravimetrically imaged salt dome within the Laurentian Basin are evaluated for the velocity, density and geometric properties of the Basin's sedimentary formations.

Study on cyclic injection gas override in condensate gas reservoir

NASA Astrophysics Data System (ADS)

Sun, Yan; Zhu, Weiyao; Xia, Jing; Li, Baozhu

2018-02-01

Cyclic injection gas override in condensate gas reservoirs for the large density difference between injection gas and condensate gas has been studied, but no relevant mathematical models have been built. In this paper, a mathematical model of cyclic injection gas override in condensate gas reservoir is established, considering density difference between the injected gas and the remaining condensate gas in the formation. The vertical flow ratio and override degree are used to reflect the override law of injected dry gas. Combined with the actual data of Tarim gas condensate reservoir, the parameters of injected dry gas override are calculated and analysed. The results show that the radial pressure rises or falls rapidly and the pressure gradient varies greatly in the near wells. The radial pressure varies slowly and the pressure gradient changes little in the reservoir which is within a certain distance from the wells. In the near injection well, the injected dry gas mainly migrates along the radial direction, and the vertical migration is relatively not obvious. With the distance from the injection well, the vertical flow ratio and override degree of injected dry gas increases, and the vertical flow ratio reaches the maximum in the middle of the injection well and the production well.

Formation of temperature front in stably stratified turbulence

NASA Astrophysics Data System (ADS)

Kimura, Yoshifumi; Sullivan, Peter; Herring, Jackson

2016-11-01

An important feature of stably stratified turbulence is the significant influence of internal gravity waves which makes stably stratified turbulence unique compared to homogeneous isotropic turbulence. In this paper, we investigate the genesis of temperature fronts-a crucial subject both practically and fundamentally-in stably stratified turbulence using Direct Numerical Simulations (DNS) of the Navier-Stokes equation under the Boussinesq approximation with 10243 grid points. Vertical profiles of temperature fluctuations show almost vertically periodic sawtooth wavy structures with negative and positive layers stacked together with clear boundaries implying a sharp temperature fronts. The sawtooth waves consist of gradual decreasing temperature fluctuations with rapid recovery to a positive value as the frontal boundary is crossed vertically. This asymmetry of gradients comes from the structure that warm temperature region lies on top of cool temperature region, and can be verified in the skewed probability density function (PDF) of vertical temperature gradient. We try to extract the flow structures and mechanism for the formation and maintenance of the strong temperature front numerically.

Atmospheric gravity waves with small vertical-to-horizotal wavelength ratios

NASA Astrophysics Data System (ADS)

Song, I. S.; Jee, G.; Kim, Y. H.; Chun, H. Y.

2017-12-01

Gravity wave modes with small vertical-to-horizontal wavelength ratios of an order of 10-3 are investigated through the systematic scale analysis of governing equations for gravity wave perturbations embedded in the quasi-geostrophic large-scale flow. These waves can be categorized as acoustic gravity wave modes because their total energy is given by the sum of kinetic, potential, and elastic parts. It is found that these waves can be forced by density fluctuations multiplied by the horizontal gradients of the large-scale pressure (geopotential) fields. These theoretical findings are evaluated using the results of a high-resolution global model (Specified Chemistry WACCM with horizontal resolution of 25 km and vertical resolution of 600 m) by computing the density-related gravity-wave forcing terms from the modeling results.

Vertical heterogeneity in predation pressure in a temperate forest canopy

PubMed Central

Aikens, Kathleen R.; Buddle, Christopher M.

2013-01-01

The forest canopy offers a vertical gradient across which variation in predation pressure implies variation in refuge quality for arthropods. Direct and indirect experimental approaches were combined to assess whether canopy strata differ in ability to offer refuge to various arthropod groups. Vertical heterogeneity in impact of avian predators was quantified using exclosure cages in the understory, lower, mid, and upper canopy of a north-temperate deciduous forest near Montreal, Quebec. Bait trials were completed in the same strata to investigate the effects of invertebrate predators. Exclusion of birds yielded higher arthropod densities across all strata, although treatment effects were small for some taxa. Observed gradients in predation pressure were similar for both birds and invertebrate predators; the highest predation pressure was observed in the understory and decreased with height. Our findings support a view of the forest canopy that is heterogeneous with respect to arthropod refuge from natural enemies. PMID:24010017

Vertical and Horizontal Vegetation Structure across Natural and Modified Habitat Types at Mount Kilimanjaro.

PubMed

Rutten, Gemma; Ensslin, Andreas; Hemp, Andreas; Fischer, Markus

2015-01-01

In most habitats, vegetation provides the main structure of the environment. This complexity can facilitate biodiversity and ecosystem services. Therefore, measures of vegetation structure can serve as indicators in ecosystem management. However, many structural measures are laborious and require expert knowledge. Here, we used consistent and convenient measures to assess vegetation structure over an exceptionally broad elevation gradient of 866-4550 m above sea level at Mount Kilimanjaro, Tanzania. Additionally, we compared (human)-modified habitats, including maize fields, traditionally managed home gardens, grasslands, commercial coffee farms and logged and burned forests with natural habitats along this elevation gradient. We distinguished vertical and horizontal vegetation structure to account for habitat complexity and heterogeneity. Vertical vegetation structure (assessed as number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) displayed a unimodal elevation pattern, peaking at intermediate elevations in montane forests, whereas horizontal structure (assessed as coefficient of variation of number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) was lowest at intermediate altitudes. Overall, vertical structure was consistently lower in modified than in natural habitat types, whereas horizontal structure was inconsistently different in modified than in natural habitat types, depending on the specific structural measure and habitat type. Our study shows how vertical and horizontal vegetation structure can be assessed efficiently in various habitat types in tropical mountain regions, and we suggest to apply this as a tool for informing future biodiversity and ecosystem service studies.

Effects of buoyancy-driven convection on nucleation and growth of protein crystals.

PubMed

Nanev, Christo N; Penkova, Anita; Chayen, Naomi

2004-11-01

Protein crystallization has been studied in presence or absence of buoyancy-driven convection. Gravity-driven flow was created, or suppressed, in protein solutions by means of vertically directed density gradients that were caused by generating suitable temperature gradients. The presence of enhanced mixing was demonstrated directly by experiments with crustacyanin, a blue-colored protein, and other materials. Combined with the vertical tube position the enhanced convection has two main effects. First, it reduces the number of nucleated hen-egg-white lysozyme (HEWL) crystals, as compared with those in a horizontal capillary. By enabling better nutrition from the protein in the solution, convection results in growth of fewer larger HEWL crystals. Second, we observe that due to convection, trypsin crystals grow faster. Suppression of convection, achieved by decreasing solution density upward in the capillary, can to some extent mimic conditions of growth in microgravity. Thus, impurity supply, which may have a detrimental effect on crystal quality, was avoided.

The surface morphology of crystals melting under solutions of different densities

NASA Technical Reports Server (NTRS)

Fang, Dacheng; Hellawell, A.

1988-01-01

Examples of solids melting beneath liquids are described for cases where the bulk liquid volume is stabilized against convection by a positive vertical temperature gradient, either with, or without local density inversion at the melting interface. The examples include ice melting beneath brine or methanol solutions and tin or lead melting under molten Sn-20 wt pct Pb or Pb-20 wt pct Sn, respectively. Without density inversion the melting is slow, purely diffusion controlled and the interfaces are smooth; with convection assisted melting the rate increases by some two orders of magnitude and the interfaces develop a rough profile - in the case of ice both irregular and quasi-steady state features are observed. The observations are discussed in terms of prevailing temperature and concentration gradients.

Response of bird species densities to habitat structure and fire history along a Midwestern open-forest gradient

USGS Publications Warehouse

Grundel, R.; Pavlovic, N.B.

2007-01-01

Oak savannas were historically common but are currently rare in the Midwestern United States. We assessed possible associations of bird species with savannas and other threatened habitats in the region by relating fire frequency and vegetation characteristics to seasonal densities of 72 bird species distributed across an open-forest gradient in northwestern Indiana. About one-third of the species did not exhibit statistically significant relationships with any combination of seven vegetation characteristics that included vegetation cover in five vertical strata, dead tree density, and tree height. For 40% of the remaining species, models best predicting species density incorporated tree density. Therefore, management based solely on manipulating tree density may not be an adequate strategy for managing bird populations along this open-forest gradient. Few species exhibited sharp peaks in predicted density under habitat conditions expected in restored savannas, suggesting that few savanna specialists occur among Midwestern bird species. When fire frequency, measured over fifteen years, was added to vegetation characteristics as a predictor of species density, it was incorporated into models for about one-quarter of species, suggesting that fire may modify habitat characteristics in ways that are important for birds but not captured by the structural habitat variables measured. Among those species, similar numbers had peaks in predicted density at low, intermediate, or high fire frequency. For species suggested by previous studies to have a preference for oak savannas along the open-forest gradient, estimated density was maximized at an average fire return interval of about one fire every three years. ?? The Cooper Ornithological Society 2007.

Horizontal cryogenic bushing for the termination of a superconducting power-transmission line

DOEpatents

Minati, K.F.; Morgan, G.H.; McNerney, A.J.; Schauer, F.

1982-07-29

A termination for a superconducting power transmission line is disclosed which is comprised of a standard air entrance insulated vertical bushing with an elbow, a horizontal cryogenic bushing linking the pressurized cryogenic cable environment to the ambient temperature bushing and a stress cone which terminated the cable outer shield and transforms the large radial voltage gradient in the cable dielectric into a much lower radial voltage gradient in the high density helium coolant at the cold end of the cryogenic bushing.

Termination for a superconducting power transmission line including a horizontal cryogenic bushing

DOEpatents

Minati, Kurt F.; Morgan, Gerry H.; McNerney, Andrew J.; Schauer, Felix

1984-01-01

A termination for a superconducting power transmission line is disclosed which is comprised of a standard air entrance insulated vertical bushing with an elbow, a horizontal cryogenic bushing linking the pressurized cryogenic cable environment to the ambient temperature bushing and a stress cone which terminates the cable outer shield and transforms the large radial voltage gradient in the cable dielectric into a much lower radial voltage gradient in the high density helium coolant at the cold end of the cryogenic bushing.

Altitudinal patterns in breeding bird species richness and density in relation to climate, habitat heterogeneity, and migration influence in a temperate montane forest (South Korea).

PubMed

Kim, Jin-Yong; Lee, Sanghun; Shin, Man-Seok; Lee, Chang-Hoon; Seo, Changwan; Eo, Soo Hyung

2018-01-01

Altitudinal patterns in the population ecology of mountain bird species are useful for predicting species occurrence and behavior. Numerous hypotheses about the complex interactions among environmental factors have been proposed; however, these still remain controversial. This study aimed to identify the altitudinal patterns in breeding bird species richness or density and to test the hypotheses that climate, habitat heterogeneity (horizontal and vertical), and heterospecific attraction in a temperate forest, South Korea. We conducted a field survey of 142 plots at altitudes between 200 and 1,400 m a.s.l in the breeding season. A total of 2,771 individuals from 53 breeding bird species were recorded. Altitudinal patterns of species richness and density showed a hump-shaped pattern, indicating that the highest richness and density could be observed at moderate altitudes. Models constructed with 13 combinations of six variables demonstrated that species richness was positively correlated with vertical and horizontal habitat heterogeneity. Density was positively correlated with vertical, but not horizontal habitat heterogeneity, and negatively correlated with migratory bird ratio. No significant relationships were found between spring temperature and species richness or density. Therefore, the observed patterns in species richness support the hypothesis that habitat heterogeneity, rather than climate, is the main driver of species richness. Also, neither habitat heterogeneity nor climate hypotheses fully explains the observed patterns in density. However, vertical habitat heterogeneity does likely help explain observed patterns in density. The heterospecific attraction hypothesis did not apply to the distribution of birds along the altitudinal gradient. Appropriate management of vertical habitat heterogeneity, such as vegetation cover, should be maintained for the conservation of bird diversity in this area.

Vertical and Horizontal Vegetation Structure across Natural and Modified Habitat Types at Mount Kilimanjaro

PubMed Central

Rutten, Gemma; Ensslin, Andreas; Hemp, Andreas; Fischer, Markus

2015-01-01

In most habitats, vegetation provides the main structure of the environment. This complexity can facilitate biodiversity and ecosystem services. Therefore, measures of vegetation structure can serve as indicators in ecosystem management. However, many structural measures are laborious and require expert knowledge. Here, we used consistent and convenient measures to assess vegetation structure over an exceptionally broad elevation gradient of 866–4550m above sea level at Mount Kilimanjaro, Tanzania. Additionally, we compared (human)-modified habitats, including maize fields, traditionally managed home gardens, grasslands, commercial coffee farms and logged and burned forests with natural habitats along this elevation gradient. We distinguished vertical and horizontal vegetation structure to account for habitat complexity and heterogeneity. Vertical vegetation structure (assessed as number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) displayed a unimodal elevation pattern, peaking at intermediate elevations in montane forests, whereas horizontal structure (assessed as coefficient of variation of number, width and density of vegetation layers, maximum canopy height, leaf area index and vegetation cover) was lowest at intermediate altitudes. Overall, vertical structure was consistently lower in modified than in natural habitat types, whereas horizontal structure was inconsistently different in modified than in natural habitat types, depending on the specific structural measure and habitat type. Our study shows how vertical and horizontal vegetation structure can be assessed efficiently in various habitat types in tropical mountain regions, and we suggest to apply this as a tool for informing future biodiversity and ecosystem service studies. PMID:26406985

Observations of Inner Shelf Flows Influenced by a Small-Scale River Plume in the Northern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Roth, M.; MacMahan, J.; Reniers, A.; Ozgokmen, T. M.

2016-02-01

Recent work has demonstrated that wind and waves are important forcing mechanisms for the inner shelf vertical current structure. Here, the inner shelf flows are evaluated away from an adjacent inlet where a small-scale buoyant plume emerges. The plume's nearshore extent, speed, vertical thickness, and density are controlled by the passage of low-pressure extratropical cyclones that are common in the northern Gulf of Mexico. The colder, brackish plume water provides vertical stratification and a cross-shore density gradient with the warmer, saline oceanic water. An Acoustic Doppler Current Profiler (ADCP) was deployed in 10m water depth as part of an intensive 2-week experiment (SCOPE), which also obtained wind and cross-shelf temperature, salinity, and velocity. The 10m ADCP remained collecting an additional year of velocity observations. The plume was not always present, but episodically influenced the experiment site. When the plume reached the site, the alongshore surface and subsurface typically flowed in opposite directions, likely caused by plume-induced pressure gradients. Plumes that extended into the subsurface appear to have caused depth-averaged onshore flow above that expected from wind and wave-driven forcing. Observations from SCOPE and the 1-year ADCP are used to describe seasonal full-depth flow patterns influenced by wind, waves, and plume presence.

Atomic oxygen between 80 and 120 km - Evidence for a latitudinal variation in vertical transport near the mesopause

NASA Technical Reports Server (NTRS)

Wasser, B.; Donahue, T. M.

1979-01-01

Analysis of the OGO 6 OI green line nightglow photometer experiment has been carried out for eight cases when the alignment of the spacecraft was such that local emission rates could be determined below the altitude of maximum emission and down to about 80 km. The results obtained show that the vertical gradient of the emission rate between 95 and 80 km alternates between regions of very rapid variation and very slow variation spaced on a scale of 5-10 deg of latitude. Maps showing isoemissivity contours and isodensity contours for atomic oxygen concentration in vertical meridional planes are presented. The densities are computed under three assumptions concerning excitation mechanisms. Comparisons of the vertical variations of oxygen density with the results of a time dependent theory suggest the regions of strong downward transport alternate in latitude with regions of weak transport near 90 km. In the first case, conversion of O to O3 at night appears to be overwhelmed by downward transport of O.

Gravity and gravity gradient changes caused by a point dislocation

NASA Astrophysics Data System (ADS)

Huang, Jian-Liang; Li, Hui; Li, Rui-Hao

1995-02-01

In this paper we studied gravitational potential, gravity and its gradient changes, which are caused by a point dislocation, and gave the concise mathematical deduction with definite physical implication in dealing with the singular integral at a seismic source. We also analysed the features of the fields of gravity and gravity gradient, gravity-vertical-displacement gradient. The conclusions are: (1) Gravity and gravity gradient changes are very small with the change of vertical position; (2) Gravity change is much greater than the gravity gradient change which is not so distinct; (3) The gravity change due to redistribution of mass accounts for 10 50 percent of the total gravity change caused by dislocation. The signs (positive or negative) of total gravity change and vertical displacement are opposite each other at the same point for strike slip and dip slip; (4) Gravity-vertical-displacement-gradient is not constant; it manifests a variety of patterns for different dislocation models; (5) Gravity-vertical-displacement-gradient is approximately equal to apparent gravity-vertical-displacement-gradient.

The development of concentration gradients in a suspension of chemotactic bacteria

NASA Technical Reports Server (NTRS)

Hillesdon, A. J.; Pedley, T. J.; Kessler, J. O.

1995-01-01

When a suspension of bacterial cells of the species Bacillus subtilis is placed in a chamber with its upper surface open to the atmosphere complex bioconvection patterns are observed. These arise because the cells: (1) are denser than water; and (2) usually swim upwards, so that the density of an initially uniform suspension becomes greater at the top than the bottom. When the vertical density gradient becomes large enough, an overturning instability occurs which ultimately evolves into the observed patterns. The reason that the cells swim upwards is that they are aerotactic, i.e., they swim up gradients of oxygen, and they consume oxygen. These properties are incorporated in conservation equations for the cell (N) and oxygen (C) concentrations, and these are solved in the pre-instability phase of development when N and C depend only on the vertical coordinate and time. Numerical results are obtained for both shallow- and deep-layer chambers, which are intrinsically different and require different mathematical and numerical treatments. It is found that, for both shallow and deep chambers, a thin boundary layer, densely packed with cells, forms near the surface. Beneath this layer the suspension becomes severely depleted of cells. Furthermore, in the deep chamber cases, a discontinuity in the cell concentration arises between this cell-depleted region and a cell-rich region further below, where no significant oxygen concentration gradients develop before the oxygen is fully consumed. The results obtained from the model are in good qualitative agreement with the experimental observations.

Vertical Mixing In Western Lake Constance Due To Long Internal Waves

NASA Astrophysics Data System (ADS)

Boehrer, B.

Current profiles in the pelagic waters of western Lake Constance have been broken up into modes of the internal wave equation [1,2]. All current profiles can be well represented by a combination of the first and second mode wave. The temporal vari- ation of the modal composition with the interaction of the first and second mode im- plies current shear at varying depths. From current and density profiles, the gradient Richardson number can be evaluated in its spatial and temporal pattern with occa- tional occurence of supercritical values at all depths, also in the deep hypolimnion. An empiric connection between gradient Richardson number and diapycnical mixing [3] is applied to yield a profile of vertical transport coefficients, which can be com- pared with transport coefficients from gradient flux calculations of temperature and electrical conductivity profiles [4]. [1] B. Boehrer, J. Ilmberger and K.O. Münnich (2000): Vertical Structure of Current in Western Lake Constance, JGR-Oceans, 105 (12), 28823-28835 [2] B. Boehrer (2000): Modal Response of a Deep Stratified Lake: Western Lake Con- stance, JGR-Oceans, 105 (12), 28837-28845 [3] H. Peeters, M.C. Gregg and J.M. Toole (1988): On the parameterization of equa- torial turbulence, JGR, 93, 1199-1218 [4] G. Heinz, J. Ilmberger and M. Schimmele (1990): Vertical Mixing in Überlinger See, western part of Lake Constance, Aquat. Sci., 52(3), 256-268

Growth of Ca 4YO(BO 3) 3 crystals by vertical Bridgman method

NASA Astrophysics Data System (ADS)

Luo, Jun; Fan, Shiji; Wang, Jinchang; Zhong, Zhenwu; Qian, Guoxing; Sun, Renying

2001-07-01

Growth of single crystals of Ca 4YO(BO 3) 3 (YCOB) by the vertical Bridgman method is reported in this paper. By using near-sealed Pt crucibles to prevent volatilization of B 2O 3, the high-optical-quality YCOB crystals of 25 mm diameter and more than 40 mm in length have been grown at the furnace temperature of 50-80°C above the melting point of YCOB and the crucible lowering rates of 0.2-0.6 mm/h. Owing to the low vertical and radial temperature gradient, crack-free YCOB crystals have been obtained in the <0 1 0> and <0 0 1> directions. At the top of a YCOB boule, the dislocation density was found to decrease from the center to the outer area, and the average dislocation density is about 600/cm 2.

Bottomside Ionospheric Electron Density Specification using Passive High Frequency Signals

NASA Astrophysics Data System (ADS)

Kaeppler, S. R.; Cosgrove, R. B.; Mackay, C.; Varney, R. H.; Kendall, E. A.; Nicolls, M. J.

2016-12-01

The vertical bottomside electron density profile is influenced by a variety of natural sources, most especially traveling ionospheric disturbances (TIDs). These disturbances cause plasma to be moved up or down along the local geomagnetic field and can strongly impact the propagation of high frequency radio waves. While the basic physics of these perturbations has been well studied, practical bottomside models are not well developed. We present initial results from an assimilative bottomside ionosphere model. This model uses empirical orthogonal functions based on the International Reference Ionosphere (IRI) to develop a vertical electron density profile, and features a builtin HF ray tracing function. This parameterized model is then perturbed to model electron density perturbations associated with TIDs or ionospheric gradients. Using the ray tracing feature, the model assimilates angle of arrival measurements from passive HF transmitters. We demonstrate the effectiveness of the model using angle of arrival data. Modeling results of bottomside electron density specification are compared against suitable ancillary observations to quantify accuracy of our model.

Shadowgraph Study of Gradient Driven Fluctuations

NASA Technical Reports Server (NTRS)

Cannell, David; Nikolaenko, Gennady; Giglio, Marzio; Vailati, Alberto; Croccolo, Fabrizio; Meyer, William

2002-01-01

A fluid or fluid mixture, subjected to a vertical temperature and/or concentration gradient in a gravitational field, exhibits greatly enhanced light scattering at small angles. This effect is caused by coupling between the vertical velocity fluctuations due to thermal energy and the vertically varying refractive index. Physically, small upward or downward moving regions will be displaced into fluid having a refractive index different from that of the moving region, thus giving rise to the enhanced scattering. The scattered intensity is predicted to vary with scattering wave vector q, as q(sup -4), for sufficiently large q, but the divergence is quenched by gravity at small q. In the absence of gravity, the long wavelength fluctuations responsible for the enhanced scattering are predicted to grow until limited by the sample dimensions. It is thus of interest to measure the mean-squared amplitude of such fluctuations in the microgravity environment for comparison with existing theory and ground based measurements. The relevant wave vectors are extremely small, making traditional low-angle light scattering difficult or impossible because of stray elastically scattered light generated by optical surfaces. An alternative technique is offered by the shadowgraph method, which is normally used to visualize fluid flows, but which can also serve as a quantitative tool to measure fluctuations. A somewhat novel shadowgraph apparatus and the necessary data analysis methods will be described. The apparatus uses a spatially coherent, but temporally incoherent, light source consisting of a super-luminescent diode coupled to a single-mode optical fiber in order to achieve extremely high spatial resolution, while avoiding effects caused by interference of light reflected from the various optical surfaces that are present when using laser sources. Results obtained for a critical mixture of aniline and cyclohexane subjected to a vertical temperature gradient will be presented. The sample was confined between two horizontal parallel sapphire plates with a vertical spacing of 1 mm. The temperatures of the sapphire plates were controlled by independent circulating water loops that used Peltier devices to add or remove heat from the room air as required. For a mixture with a temperature gradient, two effects are involved in generating the vertical refractive index gradient, namely thermal expansion and the Soret effect, which generates a concentration gradient in response to the applied temperature gradient. For the aniline/cyclohexane system, the denser component (aniline) migrates toward the colder surface. Consequently, when heating from above, both effects result in the sample density decreasing with altitude and are stabilizing in the sense that no convective motion occurs regardless of the magnitude of the applied temperature gradient. The Soret effect is strong near a binary liquid critical point, and thus the dominant effect is due to the induced concentration gradient. The results clearly show the divergence at low q and the predicted gravitational quenching. Results obtained for different applied temperature gradients at varying temperature differences from the critical temperature, clearly demonstrate the predicted divergence of the thermal diffusion ratio. Thus, the more closely the critical point is approached, the smaller becomes the temperature gradient required to generate the same signal. Two different methods have been used to generate pure concentration gradients. In the first, a sample cell was filled with a single fluid, ethylene glycol, and a denser miscible fluid, water, was added from below thus establishing a sharp interface to begin the experiment. As time went on the two fluids diffused into each other, and large amplitude fluctuations were clearly observed at low q. The effects of gravitational quenching were also evident. In the second method, the aniline/cyclohexane sample was used, and after applying a vertical temperature gradient for several hours, the top and bottom temperatures were set equal and the thermal gradient died on a time scale of seconds, leaving the Soret induced concentration gradient in place. Again, large-scale fluctuations were observed and died away slowly in amplitude as diffusion destroyed the initial concentration gradient.

Direct mapping of the temperature and velocity gradients in discs. Imaging the vertical CO snow line around IM Lupi

NASA Astrophysics Data System (ADS)

Pinte, C.; Ménard, F.; Duchêne, G.; Hill, T.; Dent, W. R. F.; Woitke, P.; Maret, S.; van der Plas, G.; Hales, A.; Kamp, I.; Thi, W. F.; de Gregorio-Monsalvo, I.; Rab, C.; Quanz, S. P.; Avenhaus, H.; Carmona, A.; Casassus, S.

2018-01-01

Accurate measurements of the physical structure of protoplanetary discs are critical inputs for planet formation models. These constraints are traditionally established via complex modelling of continuum and line observations. Instead, we present an empirical framework to locate the CO isotopologue emitting surfaces from high spectral and spatial resolution ALMA observations. We apply this framework to the disc surrounding IM Lupi, where we report the first direct, i.e. model independent, measurements of the radial and vertical gradients of temperature and velocity in a protoplanetary disc. The measured disc structure is consistent with an irradiated self-similar disc structure, where the temperature increases and the velocity decreases towards the disc surface. We also directly map the vertical CO snow line, which is located at about one gas scale height at radii between 150 and 300 au, with a CO freeze-out temperature of 21 ± 2 K. In the outer disc (>300 au), where the gas surface density transitions from a power law to an exponential taper, the velocity rotation field becomes significantly sub-Keplerian, in agreement with the expected steeper pressure gradient. The sub-Keplerian velocities should result in a very efficient inward migration of large dust grains, explaining the lack of millimetre continuum emission outside of 300 au. The sub-Keplerian motions may also be the signature of the base of an externally irradiated photo-evaporative wind. In the same outer region, the measured CO temperature above the snow line decreases to ≈15 K because of the reduced gas density, which can result in a lower CO freeze-out temperature, photo-desorption, or deviations from local thermodynamic equilibrium.

A Theory of Density Layering in Stratified Turbulence using Statistical State Dynamics

NASA Astrophysics Data System (ADS)

Fitzgerald, J.; Farrell, B.

2016-12-01

Stably stratified turbulent fluids commonly develop density structures that are layered in the vertical direction (e.g., Manucharyan et al., 2015). Within layers, density is approximately constant and stratification is weak. Between layers, density varies rapidly and stratification is strong. A common explanation for the existence of layers invokes the negative diffusion mechanism of Phillips (1972) & Posmentier (1977). The physical principle underlying this mechanism is that the flux-gradient relationship connecting the turbulent fluxes of buoyancy to the background stratification must have the special property of weakening fluxes with strengthening gradient. Under these conditions, the evolution of the stratification is governed by a negative diffusion problem which gives rise to spontaneous layer formation. In previous work on stratified layering, this flux-gradient property is often assumed (e.g, Posmentier, 1977) or drawn from phenomenological models of turbulence (e.g., Balmforth et al., 1998).In this work we develop the theoretical underpinnings of layer formation by applying stochastic turbulence modeling and statistical state dynamics (SSD) to predict the flux-gradient relation and analyze layer formation directly from the equations of motion. We show that for stochastically-forced homogeneous 2D Boussinesq turbulence, the flux-gradient relation can be obtained analytically and indicates that the fluxes always strengthen with stratification. The Phillips mechanism thus does not operate in this maximally simplified scenario. However, when the problem is augmented to include a large scale background shear, we show that the flux-gradient relationship is modified so that the fluxes weaken with stratification. Sheared and stratified 2D Boussinesq turbulence thus spontaneously forms density layers through the Phillips mechanism. Using SSD (Farrell & Ioannou 2003), we obtain a closed, deterministic dynamics for the stratification and the statistical turbulent state. We show that density layers form as a linear instability of the sheared turbulence, associated with a supercritical bifurcation. We further show that SSD predicts the nonlinear equilibration and maintenance of the layers, and captures the phenomena of layer growth and mergers (Radko, 2007).

Intra-pixel variability in satellite tropospheric NO2 column densities derived from simultaneous space-borne and airborne observations over the South African Highveld

NASA Astrophysics Data System (ADS)

Broccardo, Stephen; Heue, Klaus-Peter; Walter, David; Meyer, Christian; Kokhanovsky, Alexander; van der A, Ronald; Piketh, Stuart; Langerman, Kristy; Platt, Ulrich

2018-05-01

Aircraft measurements of NO2 using an imaging differential optical absorption spectrometer (iDOAS) instrument over the South African Highveld region in August 2007 are presented and compared to satellite measurements from OMI and SCIAMACHY. In situ aerosol and trace-gas vertical profile measurements, along with aerosol optical thickness and single-scattering albedo measurements from the Aerosol Robotic Network (AERONET), are used to devise scenarios for a radiative transfer modelling sensitivity study. Uncertainty in the air-mass factor due to variations in the aerosol and NO2 profile shape is constrained and used to calculate vertical column densities (VCDs), which are compared to co-located satellite measurements. The lower spatial resolution of the satellites cannot resolve the detailed plume structures revealed in the aircraft measurements. The airborne DOAS in general measured steeper horizontal gradients and higher peak NO2 vertical column density. Aircraft measurements close to major sources, spatially averaged to the satellite resolution, indicate NO2 column densities more than twice those measured by the satellite. The agreement between the high-resolution aircraft instrument and the satellite instrument improves with distance from the source, this is attributed to horizontal and vertical dispersion of NO2 in the boundary layer. Despite the low spatial resolution, satellite images reveal point sources and plumes that retain their structure for several hundred kilometres downwind.

Along-shelf current variability on the Catalan inner-shelf (NW Mediterranean)

USGS Publications Warehouse

Grifoll, Manel; Aretxabaleta, Alfredo L.; Espino, Manuel; Warner, John C.

2012-01-01

We examine the circulation over the inner shelf of the Catalan Sea using observations of currents obtained from three ADCPs within the inner-shelf (24 and 50 m depth) during March-April 2011. The along-shelf current fluctuations during that period are mainly controlled by the local wind stress on short time scales and by remote pressure gradients on synoptic time scales. Different forcing mechanisms are involved in the along-shelf momentum balance. During storm conditions, wind stress, sea level gradients and the non-linear terms dominate the balance. During weak wind conditions, the momentum balance is controlled by the pressure gradient, while during periods of moderate wind in the presence of considerable stratification, the balance is established between the Coriolis and wind stress terms. Vertical variations of velocity are affected by the strong observed density gradient. The increased vertical shear is accompanied by the development of stratified conditions due to local heating when the wind is not able to counteract (and destroy) stratification. The occasional influence of the Besòs river plume is observed in time scales of hours to days in a limited area in front of Barcelona. The area affected by the plume depends on the vertical extend of the fresher layer, the fast river discharge peak, and the relaxation of cross-shore velocities after northeast storm events. This contribution provides a first interpretation of the inner-shelf dynamics in the Catalan Sea.

Spiral density waves and vertical circulation in protoplanetary discs

NASA Astrophysics Data System (ADS)

Riols, A.; Latter, H.

2018-06-01

Spiral density waves dominate several facets of accretion disc dynamics - planet-disc interactions and gravitational instability (GI) most prominently. Though they have been examined thoroughly in two-dimensional simulations, their vertical structures in the non-linear regime are somewhat unexplored. This neglect is unwarranted given that any strong vertical motions associated with these waves could profoundly impact dust dynamics, dust sedimentation, planet formation, and the emissivity of the disc surface. In this paper, we combine linear calculations and shearing box simulations in order to investigate the vertical structure of spiral waves for various polytropic stratifications and wave amplitudes. For sub-adiabatic profiles, we find that spiral waves develop a pair of counter-rotating poloidal rolls. Particularly strong in the non-linear regime, these vortical structures issue from the baroclinicity supported by the background vertical entropy gradient. They are also intimately connected to the disc's g modes which appear to interact non-linearly with the density waves. Furthermore, we demonstrate that the poloidal rolls are ubiquitous in gravitoturbulence, emerging in the vicinity of GI spiral wakes, and potentially transporting grains off the disc mid-plane. Other than hindering sedimentation and planet formation, this phenomena may bear on observations of the disc's scattered infrared luminosity. The vortical features could also impact on the turbulent dynamo operating in young protoplanetary discs subject to GI, or possibly even galactic discs.

The role of the density gradient on intermittent cross-field transport events in a simple magnetized toroidal plasma

NASA Astrophysics Data System (ADS)

Theiler, C.; Diallo, A.; Fasoli, A.; Furno, I.; Labit, B.; Podestà, M.; Poli, F. M.; Ricci, P.

2008-04-01

Intermittent cross-field particle transport events (ITEs) are studied in the basic toroidal device TORPEX [TORoidal Plasma EXperiment, A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], with focus on the role of the density gradient. ITEs are due to the intermittent radial elongation of an interchange mode. The elongating positive wave crests can break apart and form blobs. This is not necessary, however, for plasma particles to be convected a considerable distance across the magnetic field lines. Conditionally sampled data reveal two different scenarios leading to ITEs. In the first case, the interchange mode grows radially from a slab-like density profile and leads to the ITE. A novel analysis technique reveals a monotonic dependence between the vertically averaged inverse radial density scale length and the probability for a subsequent ITE. In the second case, the mode is already observed before the start of the ITE. It does not elongate radially in a first stage, but at a later time. It is shown that this elongation is preceded by a steepening of the density profile as well.

From convection rolls to finger convection in double-diffusive turbulence

PubMed Central

Verzicco, Roberto; Lohse, Detlef

2016-01-01

Double-diffusive convection (DDC), which is the buoyancy-driven flow with fluid density depending on two scalar components, is ubiquitous in many natural and engineering environments. Of great interests are scalars' transfer rate and flow structures. Here we systematically investigate DDC flow between two horizontal plates, driven by an unstable salinity gradient and stabilized by a temperature gradient. Counterintuitively, when increasing the stabilizing temperature gradient, the salinity flux first increases, even though the velocity monotonically decreases, before it finally breaks down to the purely diffusive value. The enhanced salinity transport is traced back to a transition in the overall flow pattern, namely from large-scale convection rolls to well-organized vertically oriented salt fingers. We also show and explain that the unifying theory of thermal convection originally developed by Grossmann and Lohse for Rayleigh–Bénard convection can be directly applied to DDC flow for a wide range of control parameters (Lewis number and density ratio), including those which cover the common values relevant for ocean flows. PMID:26699474

Stomatal Conductance, Plant Hydraulics, and Multilayer Canopies: A New Paradigm for Earth System Models or Unnecessary Uncertainty

NASA Astrophysics Data System (ADS)

Bonan, G. B.

2016-12-01

Soil moisture stress is a key regulator of canopy transpiration, the surface energy budget, and land-atmosphere coupling. Many land surface models used in Earth system models have an ad-hoc parameterization of soil moisture stress that decreases stomatal conductance with soil drying. Parameterization of soil moisture stress from more fundamental principles of plant hydrodynamics is a key research frontier for land surface models. While the biophysical and physiological foundations of such parameterizations are well-known, their best implementation in land surface models is less clear. Land surface models utilize a big-leaf canopy parameterization (or two big-leaves to represent the sunlit and shaded canopy) without vertical gradients in the canopy. However, there are strong biometeorological and physiological gradients in plant canopies. Are these gradients necessary to resolve? Here, I describe a vertically-resolved, multilayer canopy model that calculates leaf temperature and energy fluxes, photosynthesis, stomatal conductance, and leaf water potential at each level in the canopy. In this model, midday leaf water stress manifests in the upper canopy layers, which receive high amounts of solar radiation, have high leaf nitrogen and photosynthetic capacity, and have high stomatal conductance and transpiration rates (in the absence of leaf water stress). Lower levels in the canopy become water stressed in response to longer-term soil moisture drying. I examine the role of vertical gradients in the canopy microclimate (solar radiation, air temperature, vapor pressure, wind speed), structure (leaf area density), and physiology (leaf nitrogen, photosynthetic capacity, stomatal conductance) in determining above canopy fluxes and gradients of transpiration and leaf water potential within the canopy.

Transport of particles, drops, and small organisms in density stratified fluids

NASA Astrophysics Data System (ADS)

Ardekani, Arezoo M.; Doostmohammadi, Amin; Desai, Nikhil

2017-10-01

Sedimenting particles and motile organisms are ubiquitously found in oceans and lakes, where density stratification naturally occurs due to temperature or salinity gradients. We explore the effects of stratification on the fundamental hydrodynamics of settling particles, rising drops, and small organisms. The results of our direct numerical simulations of the sedimentation of particles show that the presence of vertical density gradients in the water column can substantially affect the settling dynamics of a particle, interaction between a pair of particles, and settling rates and microstructure of suspension of particles. We show that elongation of particles affects both the settling orientation and the settling rate of particles in stratified fluids, which will have direct consequences on the vertical flux of particulate matter and carbon flux in the ocean. We further demonstrate an unexpected effect of buoyancy, potentially affecting a broad range of processes at pycnoclines in oceans and lakes. In particular, stratification has a major effect on the flow field, energy expenditure, and nutrient uptake of small organisms. In addition, the role of stratification in pattern formation of bioconvection plumes of algal cells and in biogenic mixing is investigated. In particular, the numerical approach allows for considering the effects of background turbulence and hydrodynamic perturbations produced by swimming organisms, shedding light on the contribution of organisms in the mixing process in aqueous environments.

A study of the conditions necessary for the onset of mid-latitude spread F

NASA Technical Reports Server (NTRS)

Zinchenko, G. N.

1976-01-01

Ionospheric conditions associated with the initiation of spread F in the mid-latitude ionosphere were observed. The morphology of spread F at Puerto Rico was investigated. Data from 7 nights was examined for Arecibo, five with spread F and two without. The relative height of the F layer maximum and the vertically integreted Pedersen conductivity, the relation between E and F region conductivities, the coupling lengths between the E and F regions, and vertical and horizontal gradients of electron density were examined. At Millstone Hill 13 nights were examined for all of which spread F was observed. The EW and NS velocities and the vertical velocities and the electric ion temperature ratio were examined.

Physical vapor transport of mercurous chloride under a nonlinear thermal profile

NASA Technical Reports Server (NTRS)

Mennetrier, Christophe; Duval, Walter M. B.; Singh, Narsingh B.

1992-01-01

Our study investigates numerically the flow field characteristics during the growth of mercurous chloride (Hg2Cl2) crystals in a rectangular ampoule under terrestrial and microgravity conditions for a nonlinear thermal gradient. With a residual gas lighter than the nutrient, the solutal Grashof number is dominant. We observe that in tilted configurations, when solutal convection is dominant, the maximum transport rate occurs at approximately 40 percent. For the vertical configurations, we were able to obtain solutions only for the cases either below the critical Rayleigh numbers or the stabilized configurations. The total mass flux decreases exponentially with an increase of pressure of residual gas, but it increases following a power law with the temperature difference driving the transport. The nonlinear thermal gradient appears to destabilize the flow field when thermal convection is dominant for both vertical top-heated and bottom-heated configurations. However, when the solutal Grashof number is dominant, the density gradient resulting from the solutal gradient appears to stabilize the flow for the bottom-heated configuration. The flow field for the top-heated configuration is destabilized for high Grashof numbers. The microgravity environment provides a means for lowering convection. For gravity levels of 10(exp -3) g(0) or less, the Stefan wind drives the flow, and no recirculating cell is predicted.

Observations and analysis of a stratification-destratification event in a tropical estuary

NASA Astrophysics Data System (ADS)

Uncles, R. J.; Ong, J. E.; Gong, W. K.

1990-11-01

A data set comprising 31 continuous tidal cycles was collected in the Sungai Merbok Estuary, Malaysia, in June 1987 as part of an ecological study of nutrient fluxes from a tropical mangrove estuary. Currents, salinity and salinity stratification at a deep-channel (15 m) station near the mouth of the Merbok Estuary showed a pronounced spring-neap variability. The slow currents and weak vertical mixing at neap tides favoured the formation of a stratified water column and generated a neap-spring cycle of water column stabilization and destabilization. A strong stratification event occurred during the period of observations. This was partly driven by a modest freshwater spate which coincided with neap tides. An eddy viscosity-diffusivity model of the stratification, which assumed a constant, longitudinal salinity gradient, demonstrated a pronounced stratification-destratification cycle due to neap-spring variations in vertical mixing. Larger and more realistic stratification was modelled when the estimated, time-varying longitudinal salinity gradient was incorporated. This gradient maximized in response to the peak in freshwater runoff. The measured and modelled density-driven circulations showed qualitative similarities and were of the order of 10 cm s -1 at neap tides. The circulation was weaker during spring tides. The tidally-filtered salt transport due to vertical shear was directed up-estuary and was an order of magnitude smaller during spring tides. The results are discussed in terms of their relevance to mangrove system oceanography.

Frontal dynamics at the edge of the Columbia River plume

NASA Astrophysics Data System (ADS)

Akan, Çiğdem; McWilliams, James C.; Moghimi, Saeed; Özkan-Haller, H. Tuba

2018-02-01

In the tidal ebb-cycle at the Mouth of the Columbia River, strong density and velocity fronts sometimes form perpendicular to the coast at the edges of the freshwater plume. They are distinct from previously analyzed fronts at the offshore western edge of the plume that evolve as a gravity-wave bore. We present simulation results to demonstrate their occurrence and investigate the mechanisms behind their frontogenesis and evolution. Tidal velocities on average ranged between 1.5 m s-1 in flood and 2.5 m s-1 in ebb during the brief hindcast period. The tidal fronts exhibit strong horizontal velocity and buoyancy gradients on a scale ∼ 100 m in width with normalized relative vorticity (ζz/f) values reaching up to 50. We specifically focus on the front on the northern edge of the plume and examine the evolution in plume characteristics such as its water mass gradients, horizontal and vertical velocity structure, vertical velocity, turbulent vertical mixing, horizontal propagation, cross-front momentum balance, and Lagrangian frontogenetic tendencies in both buoyancy and velocity gradients. Advective frontogenesis leads to a very sharp front where lateral mixing near the grid-resolution limit arrests its further contraction. The negative vorticity within the front is initiated by the positive bottom drag curl on the north side of the Columbia estuary and against the north jetty. Because of the large negative vorticity and horizontal vorticity gradient, centrifugal and lateral shear instability begins to develop along the front, but frontal fragmentation and decay set in only after the turn of the tide because of the briefness of the ebb interval.

A fully consistent and conservative vertically adaptive coordinate system for SLIM 3D v0.4 with an application to the thermocline oscillations of Lake Tanganyika

NASA Astrophysics Data System (ADS)

Delandmeter, Philippe; Lambrechts, Jonathan; Legat, Vincent; Vallaeys, Valentin; Naithani, Jaya; Thiery, Wim; Remacle, Jean-François; Deleersnijder, Eric

2018-03-01

The discontinuous Galerkin (DG) finite element method is well suited for the modelling, with a relatively small number of elements, of three-dimensional flows exhibiting strong velocity or density gradients. Its performance can be highly enhanced by having recourse to r-adaptivity. Here, a vertical adaptive mesh method is developed for DG finite elements. This method, originally designed for finite difference schemes, is based on the vertical diffusion of the mesh nodes, with the diffusivity controlled by the density jumps at the mesh element interfaces. The mesh vertical movement is determined by means of a conservative arbitrary Lagrangian-Eulerian (ALE) formulation. Though conservativity is naturally achieved, tracer consistency is obtained by a suitable construction of the mesh vertical velocity field, which is defined in such a way that it is fully compatible with the tracer and continuity equations at a discrete level. The vertically adaptive mesh approach is implemented in the three-dimensional version of the geophysical and environmental flow Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM 3D; www.climate.be/slim). Idealised benchmarks, aimed at simulating the oscillations of a sharp thermocline, are dealt with. Then, the relevance of the vertical adaptivity technique is assessed by simulating thermocline oscillations of Lake Tanganyika. The results are compared to measured vertical profiles of temperature, showing similar stratification and outcropping events.

The boundary condition for vertical velocity and its interdependence with surface gas exchange

NASA Astrophysics Data System (ADS)

Kowalski, Andrew S.

2017-07-01

The law of conservation of linear momentum is applied to surface gas exchanges, employing scale analysis to diagnose the vertical velocity (w) in the boundary layer. Net upward momentum in the surface layer is forced by evaporation (E) and defines non-zero vertical motion, with a magnitude defined by the ratio of E to the air density, as w = E/ρ. This is true even right down at the surface where the boundary condition is w|0 = E/ρ|0 (where w|0 and ρ|0 represent the vertical velocity and density of air at the surface). This Stefan flow velocity implies upward transport of a non-diffusive nature that is a general feature of the troposphere but is of particular importance at the surface, where it assists molecular diffusion with upward gas migration (of H2O, for example) but opposes that of downward-diffusing species like CO2 during daytime. The definition of flux-gradient relationships (eddy diffusivities) requires rectification to exclude non-diffusive transport, which does not depend on scalar gradients. At the microscopic scale, the role of non-diffusive transport in the process of evaporation from inside a narrow tube - with vapour transport into an overlying, horizontal airstream - was described long ago in classical mechanics and is routinely accounted for by chemical engineers, but has been neglected by scientists studying stomatal conductance. Correctly accounting for non-diffusive transport through stomata, which can appreciably reduce net CO2 transport and marginally boost that of water vapour, should improve characterisations of ecosystem and plant functioning.

A model for thin layer formation by delayed particle settling at sharp density gradients

NASA Astrophysics Data System (ADS)

Prairie, Jennifer C.; White, Brian L.

2017-02-01

Thin layers - regions where plankton or particles accumulate vertically on scales of a few meters or less - are common in coastal waters, and have important implications for both trophic dynamics and carbon cycling. These features can form by a variety of biological and physical mechanisms, including localized growth, shear-thinning, and directed swimming. An additional mechanism may result in the formation of thin layers of marine aggregates, which have been shown to decrease their settling velocity when passing through sharp density gradients, a behavior termed delayed settling. Here, we apply a simple vertical advection-diffusion model to predict the properties of aggregate thin layers formed by this process. We assume a constant vertical flux of particles from the surface, which is parameterized by observations from laboratory experiments with marine aggregates. The formation, maintenance, and shape of the layers are described in relation to non-dimensional numbers that depend on environmental conditions and particle settling properties. In particular, model results demonstrate layer intensity and sharpness both increase with higher Péclet number (Pe), that is, under conditions with weaker mixing relative to layer formation. Similarly, more intense and sharper layers are found when the delayed settling behavior of aggregates is characterized by a lower velocity minimum. The model also predicts layers that are vertically asymmetric and highly "peaky" when compared with a Gaussian distribution, features often seen in thin layers in natural environments. Lastly, by comparing model predictions with observations of thin layers in the field, we are able to gain some insight into the applicability of delayed settling as a thin layer formation mechanism in different environmental conditions.

Density interface topography recovered by inversion of satellite gravity gradiometry observations

NASA Astrophysics Data System (ADS)

Ramillien, G. L.

2017-08-01

A radial integration of spherical mass elements (i.e. tesseroids) is presented for evaluating the six components of the second-order gravity gradient (i.e. second derivatives of the Newtonian mass integral for the gravitational potential) created by an uneven spherical topography consisting of juxtaposed vertical prisms. The method uses Legendre polynomial series and takes elastic compensation of the topography by the Earth's surface into account. The speed of computation of the polynomial series increases logically with the observing altitude from the source of anomaly. Such a forward modelling can be easily applied for reduction of observed gravity gradient anomalies by the effects of any spherical interface of density. An iterative least-squares inversion of measured gravity gradient coefficients is also proposed to estimate a regional set of juxtaposed topographic heights. Several tests of recovery have been made by considering simulated gradients created by idealistic conical and irregular Great Meteor seamount topographies, and for varying satellite altitudes and testing different levels of uncertainty. In the case of gravity gradients measured at a GOCE-type altitude of ˜ 300 km, the search converges down to a stable but smooth topography after 10-15 iterations, while the final root-mean-square error is ˜ 100 m that represents only 2 % of the seamount amplitude. This recovery error decreases with the altitude of the gravity gradient observations by revealing more topographic details in the region of survey.

Compressional velocities from multichannel refraction arrivals on Georges Bank: northwest Atlantic Ocean

USGS Publications Warehouse

McGinnis, L. D.; Otis, R. M.

1979-01-01

Velocities were obtained from unreversed, refracted arrivals on analog records from a 48‐channel, 3.6-km hydrophone cable (3.89 km from the airgun array to the last hydrophone array). Approximately 200 records were analyzed along 1500 km of ship track on Georges Bank, northwest Atlantic Ocean, to obtain regional sediment velocity distribution to a depth of 1.4 km below sea level. This technique provides nearly continuous coverage of refraction velocities and vertical velocity gradients. Because of the length of the hydrophone cable and the vertical velocity gradients, the technique is applicable only to the Continental Shelf and the shallower parts of the Continental Slope in water depths less than 300 m. Sediment diagenesis, the influence of overburden pressure on compaction, lithology, density, and porosity are inferred from these data. Velocities of the sediment near the water‐sediment interface range from less than 1500 m/sec on the north edge of Georges Bank to 1830 m/sec for glacial deposits in the northcentral part of the bank. Velocity gradients in the upper 400 m range from 1.0km/sec/km(sec−1) on the south edge of the bank to 1.7sec−1 on the north. Minimum gradients of 0.8sec−1 were observed south of Nantucket Island. Velocities and velocity gradients are explained in relation to physical properties of the Cretaceous, Tertiary, and Pleistocene sediments. Isovelocity contours at 100-m/sec intervals are nearly horizontal in the upper 400 m. Isovelocity contours at greater depths show a greater difference from a mean depth because of the greater structural and lithological variation. Bottom densities inferred from the velocities range from 1.7 to 1.9g/cm3 and porosities range from 48 to 62 percent. The most significant factor controlling velocity distribution on Georges Bank is overburden pressure and resulting compaction. From the velocity data we conclude that Georges Bank has been partially overridden by a continental ice sheet.

On vertical advection truncation errors in terrain-following numerical models: Comparison to a laboratory model for upwelling over submarine canyons

NASA Astrophysics Data System (ADS)

Allen, S. E.; Dinniman, M. S.; Klinck, J. M.; Gorby, D. D.; Hewett, A. J.; Hickey, B. M.

2003-01-01

Submarine canyons which indent the continental shelf are frequently regions of steep (up to 45°), three-dimensional topography. Recent observations have delineated the flow over several submarine canyons during 2-4 day long upwelling episodes. Thus upwelling episodes over submarine canyons provide an excellent flow regime for evaluating numerical and physical models. Here we compare a physical and numerical model simulation of an upwelling event over a simplified submarine canyon. The numerical model being evaluated is a version of the S-Coordinate Rutgers University Model (SCRUM). Careful matching between the models is necessary for a stringent comparison. Results show a poor comparison for the homogeneous case due to nonhydrostatic effects in the laboratory model. Results for the stratified case are better but show a systematic difference between the numerical results and laboratory results. This difference is shown not to be due to nonhydrostatic effects. Rather, the difference is due to truncation errors in the calculation of the vertical advection of density in the numerical model. The calculation is inaccurate due to the terrain-following coordinates combined with a strong vertical gradient in density, vertical shear in the horizontal velocity and topography with strong curvature.

The vertical metallicity gradients of mono-age stellar populations in the Milky Way with the RAVE and Gaia data

NASA Astrophysics Data System (ADS)

Ciucǎ, Ioana; Kawata, Daisuke; Lin, Jane; Casagrande, Luca; Seabroke, George; Cropper, Mark

2018-03-01

We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution and Radial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. The metallicity at disc plane remains almost constant between 2 and 8 Gyr, and it becomes significantly lower for the 8 < τ ≤ 11 Gyr population. The current analysis also reveals that the intrinsic dispersion in metallicity increases steadily with age. We discuss that our results are consistent with a scenario that (thin) disc stars formed from a flaring (thin) star-forming disc.

System noise analysis of the dumbbell tethered satellite for gravity-gradient measurements

NASA Technical Reports Server (NTRS)

Colombo, G.

1979-01-01

An analysis of the dumbbell gravity gradiometer concept for measuring short wavelength variations in the earth's gravity gradient is presented. Variations in the gradient are recorded by measuring tension variations in a vertically stabilized satellite consisting of heavy masses connected by a long wire or rod. Tension noise arises from the excitation of various mechanical oscillations of the system. The principal noise sources that were identified are fluctuations in atmospheric drag heating and drag force resulting from density variations and winds. Approximate analytical expressions are presented for the tension noise as a function of the system design parameters for various possible configurations. Computer simulations using numerical integration were performed to study the tension noise for several sample cases. Three designs consistent with Shuttle launch capabilities are discussed.

Stable Computation of the Vertical Gradient of Potential Field Data Based on Incorporating the Smoothing Filters

NASA Astrophysics Data System (ADS)

Baniamerian, Jamaledin; Liu, Shuang; Abbas, Mahmoud Ahmed

2018-04-01

The vertical gradient is an essential tool in interpretation algorithms. It is also the primary enhancement technique to improve the resolution of measured gravity and magnetic field data, since it has higher sensitivity to changes in physical properties (density or susceptibility) of the subsurface structures than the measured field. If the field derivatives are not directly measured with the gradiometers, they can be calculated from the collected gravity or magnetic data using numerical methods such as those based on fast Fourier transform technique. The gradients behave similar to high-pass filters and enhance the short-wavelength anomalies which may be associated with either small-shallow sources or high-frequency noise content in data, and their numerical computation is susceptible to suffer from amplification of noise. This behaviour can adversely affect the stability of the derivatives in the presence of even a small level of the noise and consequently limit their application to interpretation methods. Adding a smoothing term to the conventional formulation of calculating the vertical gradient in Fourier domain can improve the stability of numerical differentiation of the field. In this paper, we propose a strategy in which the overall efficiency of the classical algorithm in Fourier domain is improved by incorporating two different smoothing filters. For smoothing term, a simple qualitative procedure based on the upward continuation of the field to a higher altitude is introduced to estimate the related parameters which are called regularization parameter and cut-off wavenumber in the corresponding filters. The efficiency of these new approaches is validated by computing the first- and second-order derivatives of noise-corrupted synthetic data sets and then comparing the results with the true ones. The filtered and unfiltered vertical gradients are incorporated into the extended Euler deconvolution to estimate the depth and structural index of a magnetic sphere, hence, quantitatively evaluating the methods. In the real case, the described algorithms are used to enhance a portion of aeromagnetic data acquired in Mackenzie Corridor, Northern Mainland, Canada.

Lateral baroclinic forcing enhances sediment transport from shallows to channel in an estuary

USGS Publications Warehouse

Lacy, Jessica R.; Gladding, Steve; Brand, Andreas; Collignon, Audric; Stacey, Mark

2014-01-01

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologic and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperature- and SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.

Polyhedral shape model for terrain correction of gravity and gravity gradient data based on an adaptive mesh

NASA Astrophysics Data System (ADS)

Guo, Zhikui; Chen, Chao; Tao, Chunhui

2016-04-01

Since 2007, there are four China Da yang cruises (CDCs), which have been carried out to investigate polymetallic sulfides in the southwest Indian ridge (SWIR) and have acquired both gravity data and bathymetry data on the corresponding survey lines(Tao et al., 2014). Sandwell et al. (2014) published a new global marine gravity model including the free air gravity data and its first order vertical gradient (Vzz). Gravity data and its gradient can be used to extract unknown density structure information(e.g. crust thickness) under surface of the earth, but they contain all the mass effect under the observation point. Therefore, how to get accurate gravity and its gradient effect of the existing density structure (e.g. terrain) has been a key issue. Using the bathymetry data or ETOPO1 (http://www.ngdc.noaa.gov/mgg/global/global.html) model at a full resolution to calculate the terrain effect could spend too much computation time. We expect to develop an effective method that takes less time but can still yield the desired accuracy. In this study, a constant-density polyhedral model is used to calculate the gravity field and its vertical gradient, which is based on the work of Tsoulis (2012). According to gravity field attenuation with distance and variance of bathymetry, we present an adaptive mesh refinement and coarsening strategies to merge both global topography data and multi-beam bathymetry data. The local coarsening or size of mesh depends on user-defined accuracy and terrain variation (Davis et al., 2011). To depict terrain better, triangular surface element and rectangular surface element are used in fine and coarse mesh respectively. This strategy can also be applied to spherical coordinate in large region and global scale. Finally, we applied this method to calculate Bouguer gravity anomaly (BGA), mantle Bouguer anomaly(MBA) and their vertical gradient in SWIR. Further, we compared the result with previous results in the literature. Both synthetic model tests and field applications indicate that the adaptive terrain correction method can be adopted as a rapid and accurate tool of marine gravity data processing. References Davis, K. &Kass, M.A. & Li, Y., 2011. Rapid gravity and gravity gradiometry terrain corrections via an adaptive quadtree mesh discretization, EXPLOR GEOPHYS, 42, 88-97. Sandwell, D.T., Müller, R.D., Smith, W.H., Garcia, E. & Francis, R., 2014. New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure, SCIENCE, 346, 65-67. Tao, C., Li, H., Jin, X., Zhou, J., Wu, T., He, Y., Deng, X., Gu, C., Zhang, G. & Liu, W., 2014. Seafloor hydrothermal activity and polymetallic sulfide exploration on the southwest Indian ridge, CHINESE SCI BULL, 59, 2266-2276. Tsoulis, D., 2012. Analytical computation of the full gravity tensor of a homogeneous arbitrarily shaped polyhedral source using line integrals, GEOPHYSICS, 77, F1-F11.

Optimum gradient of mountain paths.

PubMed

Minetti, A E

1995-11-01

By combining the experiment results of R. Margaria (Atti Accad. Naz. Lincei Memorie 7: 299-368, 1938), regarding the metabolic cost of gradient locomotion, together with recent insights on gait biomechanics, a prediction about the most economical gradient of mountain paths (approximately 25%) is obtained and interpreted. The pendulum-like mechanism of walking produces a waste of mechanical work against gravity within the gradient range of up to 15% (the overall efficiency is dominated by the low transmission efficiency), whereas for steeper values only the muscular efficiency is responsible for the (slight) metabolic change (per meter of vertical displacement) with respect to gradient. The speeds at the optimum gradient turned out to be approximately 0.65 m/s (+0.16 m/s vertical) and 1.50 m/s (-0.36 m/s vertical), for uphill and downhill walking, respectively, and the ascensional energy expenditure was 0.4 and 2.0 ml O2.kg body mass-1.vertical m-1 climbed or descended. When the metabolic power becomes a burden, as in high-altitude mountaineering, the optimum gradient should be reduced. A sample of real mountain path gradients, experimentally measured, mimics the obtained predictions.

ADSORPTION AND DISSOCIATION OF O2 ON Ti3Al (0001) STUDIED BY FIRST-PRINCIPLES

NASA Astrophysics Data System (ADS)

Wei, Li-Jing; Guo, Jian-Xin; Dai, Xiu-Hong; Wang, Ying-Long; Liu, Bao-Ting

2015-05-01

The adsorption and dissociation of oxygen molecule on Ti3Al (0001) surface have been investigated by density functional theory (DFT) with the generalized gradient approximation (GGA). All possible adsorption sites including nine vertical and fifteen parallel sites of O2 are considered on Ti3Al (0001) surface. It is found that all oxygen molecules dissociate except for three vertical adsorption sites after structure optimization. This indicates that oxygen molecules prefer to dissociate on the junction site between Ti and Al atoms. Oxygen atoms coming from dissociation of oxygen molecule tend to occupy the most stable adsorption sites of the Ti3Al (0001) surface. The distance of O-O is related to the surface dissociation distance of Ti3Al (0001) surface. The valence electron localization function (ELF) and projected density of states (DOS) show that the bonds of O-O are breakaway at parallel adsorption end structures.

Diurnal evolution of the F region electron density local time gradient at low and middle latitudes resolved by the Swarm constellation

NASA Astrophysics Data System (ADS)

Xiong, Chao; Zhou, Yun-Liang; Lühr, Hermann; Ma, Shu-Ying

2016-09-01

In this study we have provided new insights into the local time gradient of F region electron density (ΔNe) derived from the lower pair of Swarm satellites flying side by side. Our result shows that the electron density (Ne) increase starts just at sunrise, around 06:00 LT, simultaneously at low and middle latitudes due to the increased photoionization. At equatorial latitudes the increase in electron density gets even steeper after 07:00 LT, and the steepest increase of electron density (about 3 · 1010 m-3 within 6 min) occurs around 09:00 LT. We suggest that the upward vertical plasma drift in connection with the buildup of the equatorial fountain effect plays a major role. We also found that the local time variations of the equatorial ionization anomaly (EIA) crest electron density during daytime are similar to the respective evolutions at the equator, but about 1-2 h delayed. We relate this delay to the response time between the equatorial electric field and the buildup of the plasma fountain. At equinox months a fast decrease of the F region electron density is seen at the EIA trough region during the prereversal enhancement, while an increase is found meanwhile at crest regions. Afterward, a fast decrease of the EIA crest electron density occurs between 19:00 and 23:00 LT, with seasonal dependence. The local time gradient between Swarm A and C shows also prominent longitudinal wave-4 pattern around August months, and the phase of DE3 in ΔNe is found to be delayed by 6 h compared to that in Ne.

Planetesimal formation by an axisymmetric radial bump of the column density of the gas in a protoplanetary disk

NASA Astrophysics Data System (ADS)

Onishi, Isamu K.; Sekiya, Minoru

2017-04-01

We investigate the effect of a radial pressure bump in a protoplanetary disk on planetesimal formation. We performed the two-dimensional numerical simulation of the dynamical interaction of solid particles and gas with an initially defined pressure bump under the assumption of axisymmetry. The aim of this work is to elucidate the effects of the stellar vertical gravity that were omitted in a previous study. Our results are very different from the previous study, which omitted the vertical gravity. Because dust particles settle toward the midplane because of the vertical gravity to form a thin dust layer, the regions outside of the dust layer are scarcely affected by the back-reaction of the dust. Hence, the gas column density keeps its initial profile with a bump, and dust particles migrate toward the bump. In addition, the turbulence due to the Kelvin-Helmholtz instability caused by the difference of the azimuthal velocities between the inside and outside of the dust layer is suppressed where the radial pressure gradient is reduced by the pressure bump. The dust settling proceeds further where the turbulence is weak, and a number of dust clumps are formed. The dust density in some clumps exceeds the Roche density. Planetesimals are considered to be formed from these clumps owing to the self-gravity.[Figure not available: see fulltext.

Advective and diapycnal diffusive oceanic flux in Tenerife - La Gomera Channel

NASA Astrophysics Data System (ADS)

Marrero-Díaz, A.; Rodriguez-Santana, A.; Hernández-Arencibia, M.; Machín, F.; García-Weil, L.

2012-04-01

During the year 2008, using the commercial passenger ship Volcán de Tauce of the Naviera Armas company several months, it was possible to obtain vertical profiles of temperature from expandable bathythermograph probes in eight stations across the Tenerife - La Gomera channel. With these data of temperature we have been estimated vertical sections of potential density and geostrophic transport with high spatial and temporal resolution (5 nm between stations, and one- two months between cruises). The seasonal variability obtained for the geostrophic transport in this channel shows important differences with others Canary Islands channels. From potential density and geostrophic velocity data we estimated the vertical diffusion coefficients and diapycnal diffusive fluxes, using a parameterization that depends of Richardson gradient number. In the center of the channel and close to La Gomera Island, we found higher values for these diffusive fluxes. Convergence and divergence of these fluxes requires further study so that we can draw conclusions about its impact on the distribution of nutrients in the study area and its impact in marine ecosystems. This work is being used in research projects TRAMIC and PROMECA.

Vertical and temporal variation in phytoplankton assemblages correlated with environmental conditions in the Mundaú reservoir, semi-arid northeastern Brazil.

PubMed

Lira, G A S T; Moura, A N; Vilar, M C P; Cordeiro-Araújo, M K; Bittencourt-Oliveira, M C

2014-08-01

The goal of this study was to analyse the vertical structure of the phytoplankton community at the Mundaú reservoir, located in the semi-arid region of northeastern Brazil, and to correlate it to environmental conditions over two distinct seasons, dry and rainy. Samples were collected bimonthly at eight depths in the dry and rainy season for analyses of the physical and chemical variables of the water, as well as density, abundance, dominance, species diversity index and equitability of the community. Analysis of variance (ANOVA-two way) was used to analyse the vertical and seasonal differences, and Canonical Correspondence Analysis (CCA) was used to assess associations between phytoplankton and environmental variables Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju was the only dominant species and Geitlerinema amphibium (C. Agardh) Anagnostidis, Merismopedia punctata Meyen and Synedra rumpens Kützing. Others six taxa were abundant in at least one of the samples. Distinct vertical distribution patterns were observed for the abundant taxa between depths and seasons. The cyanobacteria, with the exception of C. raciborskii, showed similar seasonal patterns, with higher densities in the dry season. The CCA showed a strong correlation between the density of the phytoplanktonic species and abiotic variables. The vertical changes in abundant taxa revealed distinct patterns regulated by the variation in the environmental factors that were directly linked to seasonality, with the success of one or more species being dependent on their life strategies and ecological needs. The present study restates the importance of environmental and seasonal factors for phytoplankton composition and distribution in a freshwater tropical reservoir through a vertical gradient.

Magnetized stratified rotating shear waves.

PubMed

Salhi, A; Lehner, T; Godeferd, F; Cambon, C

2012-02-01

We present a spectral linear analysis in terms of advected Fourier modes to describe the behavior of a fluid submitted to four constraints: shear (with rate S), rotation (with angular velocity Ω), stratification, and magnetic field within the linear spectral theory or the shearing box model in astrophysics. As a consequence of the fact that the base flow must be a solution of the Euler-Boussinesq equations, only radial and/or vertical density gradients can be taken into account. Ertel's theorem no longer is valid to show the conservation of potential vorticity, in the presence of the Lorentz force, but a similar theorem can be applied to a potential magnetic induction: The scalar product of the density gradient by the magnetic field is a Lagrangian invariant for an inviscid and nondiffusive fluid. The linear system with a minimal number of solenoidal components, two for both velocity and magnetic disturbance fields, is eventually expressed as a four-component inhomogeneous linear differential system in which the buoyancy scalar is a combination of solenoidal components (variables) and the (constant) potential magnetic induction. We study the stability of such a system for both an infinite streamwise wavelength (k(1) = 0, axisymmetric disturbances) and a finite one (k(1) ≠ 0, nonaxisymmetric disturbances). In the former case (k(1) = 0), we recover and extend previous results characterizing the magnetorotational instability (MRI) for combined effects of radial and vertical magnetic fields and combined effects of radial and vertical density gradients. We derive an expression for the MRI growth rate in terms of the stratification strength, which indicates that purely radial stratification can inhibit the MRI instability, while purely vertical stratification cannot completely suppress the MRI instability. In the case of nonaxisymmetric disturbances (k(1) ≠ 0), we only consider the effect of vertical stratification, and we use Levinson's theorem to demonstrate the stability of the solution at infinite vertical wavelength (k(3) = 0): There is an oscillatory behavior for τ > 1+|K(2)/k(1)|, where τ = St is a dimensionless time and K(2) is the radial component of the wave vector at τ = 0. The model is suitable to describe instabilities leading to turbulence by the bypass mechanism that can be relevant for the analysis of magnetized stratified Keplerian disks with a purely azimuthal field. For initial isotropic conditions, the time evolution of the spectral density of total energy (kinetic + magnetic + potential) is considered. At k(3) = 0, the vertical motion is purely oscillatory, and the sum of the vertical (kinetic + magnetic) energy plus the potential energy does not evolve with time and remains equal to its initial value. The horizontal motion can induce a rapid transient growth provided K(2)/k(1)>1. This rapid growth is due to the aperiodic velocity vortex mode that behaves like K(h)/k(h) where k(h)(τ)=[k(1)(2) + (K(2) - k(1)τ)(2)](1/2) and K(h) =k(h)(0). After the leading phase (τ > K(2)/k(1)>1), the horizontal magnetic energy and the horizontal kinetic energy exhibit a similar (oscillatory) behavior yielding a high level of total energy. The contribution to energies coming from the modes k(1) = 0 and k(3) = 0 is addressed by investigating the one-dimensional spectra for an initial Gaussian dense spectrum. For a magnetized Keplerian disk with a purely vertical field, it is found that an important contribution to magnetic and kinetic energies comes from the region near k(1) = 0. The limit at k(1) = 0 of the streamwise one-dimensional spectra of energies, or equivalently, the streamwise two-dimensional (2D) energy, is then computed. The comparison of the ratios of these 2D quantities with their three-dimensional counterparts provided by previous direct numerical simulations shows a quantitative agreement.

Renewed circulation scheme of the Baltic Sea - based on the 40-year simulation with GETM.

NASA Astrophysics Data System (ADS)

Maljutenko, Ilja; Raudsepp, Urmas

2015-04-01

The general circulation of the Baltic Sea has been characterized as cyclonic in all sub-basins based on numerous measurements and model simulations. From the long-term hydrodynamical simulation our model results have verified the general cyclonic circulation in the Baltic Proper and in the Gulf of Bothnia, but the Gulf of Finland and the Gulf of Riga have shown tendency to anticyclonic circulation. We have applied the General Estuarine Transport Model ( GETM ) for the period of 1966 - 2006 with a 1 nautical mile horizontal resolution and density adaptive bottom following vertical coordinates to make it possible to simulate horizontal and vertical density gradients with better precision. The atmospheric forcing from dynamically downscaled ERA40-HIRLAM and parametrized lateral boundary conditions are applied. Model simulation show close agreement with measurements conducted in the main monitoring stations in the BS during the simulation period. The geostrophic adjustment of density driven currents along with the upward salinity flux due to entrainment could explain the anticyclonic circulation and strong coastal current. Mean vertical velocities show that upward and downward movements are forming closed vertical circulation loops along the bottom slope of the Baltic Proper and the Gulf of Bothnia. The model has also reproduced patchy vertical movement across the BS with some distinctive areas of upward advective fluxes in the GoF along the thalweg. The distinctive areas of deepwater upwelling are also evident in the Gdansk Basin, western Gotland Basin, northern Gotland Basin and in the northen part of the Bothnia Sea.

A mechanism for tectonic deformation on Venus

NASA Technical Reports Server (NTRS)

Phillips, Roger J.

1986-01-01

In the absence of identifiable physiographic features directly associated with plate tectonics, alternate mechanisms are sought for the intense tectonic deformation observed in radar images of Venus. One possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior. Spectral Green's function solutions have been obtained for stresses in an elastic lithosphere overlying a Newtonian interior with an exponential depth dependence of viscosity, and a specified surface-density distribution driving the flow. At long wavelengths and for a rigid elastic/fluid boundary condition, horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid. The depth and strength of density anomalies in the Venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 MPa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses. The great apparent depth of compensation of topographic features argues against this, however, thus supporting the importance of the coupling mechanism. If there is no elastic lid, stresses will also be very high near the surface, providing also that the viscosity gradient is negative.

Microgravimetry and the Measurement and Application of Gravity Gradients,

DTIC Science & Technology

1980-06-01

Neumann, R., 1972, High precision gravimetry--recent develop- ments: Report to Paris Commission of E.A.E.G., Compagnie Generale de Geophysique , Massy...experimentation on vertical gradient: Compagnie Generale de Geophysique , Massy, France. 12. Fajklewicz, Z. J., 1976, Gravity vertical gradient

Generalised photon skyshine calculations.

PubMed

Hayes, Robert

2004-01-01

The energy-dependent dose contributions from monoenergetic photon source points located 1.5 m above the ground have been tabulated. These values are intended to be used for regulatory compliance with site boundary dose limitations and as such are all presented in effective dose units. Standard air and soil are modelled where the air has vertical density gradient approximation. Energies from 0.05 up to 10 MeV are evaluated for dose transport up to 40 mean free paths.

Gradient Drift Turbulence from Electron Bite-Outs: Dependence on Atmospheric Parameters.

NASA Astrophysics Data System (ADS)

Young, M.; Oppenheim, M. M.; Dimant, Y. S.

2017-12-01

Electron bite-outs are regions of decreased electron density without a corresponding decrease in ion density, often caused by electron attachment to dust grains. They typically occur in the upper D-/lower E-region ionosphere and the accompanying electron gradient provides free energy to drive the gradient drift instability (GDI). The major difference between classical GDI and electron bite-out driven GDI is that the instability occurs on the top side of the bite-out region in the latter, as opposed to the bottom side in the former, in the presence of a vertical background electric field. Moreover, the mobile plasma population contains a gradient in only one species while the entire system remains quasineutral. This modified geometry presents new pathways for instabilities as the ions build up near the bite-out layer, leaving behind depletions that ascend away from the layer. Previous simulation runs showed that the presence of an electron gradient drives GDI-like turbulence even when ions and electrons start in momentum balance. Furthermore, a simulation run that replaced the electron bite-out with a layer of enhanced ion density, as though ions and electrons had filled in the bite-out region, did not lead to instability. This work examines the role of atmospheric parameters at altitudes between 80-100 km in instability formation and turbulence development, including the role of collisions in impeding instability growth as altitude decreases. Key parameters include the ambient electric field, which plays a critical role in triggering the gradient-drift instability; collision frequencies and temperature, which vary with altitude and effect the turbulent growth rate; and relative charge density of the bite-out, which increases the electron gradient strength. This work provides insight into how electron bite-out layers can produce turbulence that ground-based high frequency (HF) radars may be able to observe. The upper D-/lower E-region ionosphere is generally difficult to study in situ, making simulations of ground-based observables much more important. Assuming that electron bite-out layers result from dust charging in particular will allow the community to use the predictions of this work to study the ionospheric dust population.

Preparative liquid column electrophoresis of T and B lymphocytes at gravity = 1

NASA Technical Reports Server (NTRS)

Van Oss, C. J.; Bigazzi, P. E.; Gillman, C. F.; Allen, R. E.

1974-01-01

Vertical liquid columns containing low-molecular-weight dextran density gradients can be used for preparative lymphocyte electrophoresis on earth, in simulation of zero gravity conditions. Another method that has been tested at 1 g, is the electrophoresis of lymphocytes in an upward direction in vertical columns. By both methods up to 100 million lymphocytes can be separated at one time in a 30-cm glass column of 8-mm inside diameter, at 12 V/cm, in two hours. Due to convection and sedimentation problems, the separation at 1 g is less than ideal, but it is expected that at zero gravity electrophoresis will probe to be a uniquely powerful cell separation tool.

Accurate pressure gradient calculations in hydrostatic atmospheric models

NASA Technical Reports Server (NTRS)

Carroll, John J.; Mendez-Nunez, Luis R.; Tanrikulu, Saffet

1987-01-01

A method for the accurate calculation of the horizontal pressure gradient acceleration in hydrostatic atmospheric models is presented which is especially useful in situations where the isothermal surfaces are not parallel to the vertical coordinate surfaces. The present method is shown to be exact if the potential temperature lapse rate is constant between the vertical pressure integration limits. The technique is applied to both the integration of the hydrostatic equation and the computation of the slope correction term in the horizontal pressure gradient. A fixed vertical grid and a dynamic grid defined by the significant levels in the vertical temperature distribution are employed.

System for loading slab-gel holders for electrophoresis separation

DOEpatents

Anderson, Norman G.; Anderson, Norman L.

1979-01-01

A slab-gel loading system includes a prismatic chamber for filling a plurality of slab-gel holders simultaneously. Each slab-gel holder comprises a pair of spaced apart plates defining an intermediate volume for gel containment. The holders are vertically positioned in the chamber with their major surfaces parallel to the chamber end walls. A liquid inlet is provided at the corner between the bottom and a side wall of the chamber for distributing a polymerizable monomer solution or a coagulable colloidal solution into each of the holders. The chamber is rotatably supported so that filling can begin with the corner having the liquid inlet directed downwardly such that the solution is gently funneled upwardly, without mixing, along the diverging side and bottom surfaces. As filling proceeds, the chamber is gradually rotated to position the bottom wall in a horizontal mode. The liquid filling means includes a plastic envelope with a septum dividing it into two compartments for intermixing two solutions of different density and thereby providing a liquid flow having a density gradient. The resulting gels have a density gradient between opposite edges for subsequent use in electrophoresis separations.

A wet/wet differential pressure sensor for measuring vertical hydraulic gradient.

PubMed

Fritz, Brad G; Mackley, Rob D

2010-01-01

Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater-surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.

Particle dispersion in a stably stratified channel flow

NASA Astrophysics Data System (ADS)

Pasquero, C.; Armenio, V.

2003-04-01

The motion of particles in a stably stratified channel flow is relevant in geophysic and environmental applications. In the present research this problem has been studied numerically using a mixed Lagrangian-Eulerian technique (Lagrangian motion of an ensemble of particles in an Eulerian field) by means of large eddy simulation. A stratified channel flows can be decomposed into a buoyancy affected region, with a strong turbulent activity, close to the walls, and into a buoyancy dominated region, where turbulence is strongly inhibited, in the center of the channel. For strong stratifications, counter gradient heat fluxes steepen the density gradient moving hot fluid up and cold fluid down. The stratification in the central region of the channel becomes extremely stable. However, the vertical turbulent energy, defined as the difference between the total vertical kinetic energy and its temporal average, is very strong. Particle statistics have shown that this can be related to the presence of high frequency internal waves, that do not contribute to dispersion because of their highly coherent behavior. Vertical stratification is shown to reduce or increase the decorrelation time for vertical motion, depending on the Richardson number. When stratification is increased there are two competing effects: Structures have a smaller vertical scale (acting to reduce the decorrelation time) and vertical velocities are smaller (acting to increase the decorrelation time, since particles stay for a longer time into a given structure in the flow). It has been shown that for low stratification the first mechanism dominates, while for large stratification the second effect is more important. The research is in progress and results for both fluid and inertial particles will be presented at the conference.

Vertical Gradients in Regional Alveolar Oxygen Tension in Supine Human Lung Imaged by Hyperpolarized 3He MRI

PubMed Central

Hamedani, Hooman; Shaghaghi, Hoora; Kadlecek, Stephen J.; Xin, Yi; Han, Biao; Siddiqui, Sarmad; Rajaei, Jennia; Ishii, Masaru; Rossman, Milton; Rizi, Rahim R.

2015-01-01

Purpose To evaluate whether regional alveolar oxygen tension (PAO2) vertical gradients imaged with hyperpolarized 3He can identify smoking-induced pulmonary alterations. To compare these gradients with common clinical measurements including pulmonary function tests, the six minute walk test, and the St. George’s Respiratory Questionnaire. Materials and Methods 8 healthy nonsmokers, 12 asymptomatic smokers, and 7 symptomatic subjects with chronic obstructive pulmonary disease (COPD) underwent two sets of back-to-back PAO2 imaging acquisitions in supine position with two opposite directions (top to bottom and bottom to top), followed by clinically standard pulmonary tests. The whole-lung mean, standard deviation (DPAO2) and vertical gradients of PAO2 along the slices were extracted, and the results were compared with clinically derived metrics. Statistical tests were performed to analyze the differences between cohorts. Results The anterior-posterior vertical gradients and DPAO2 effectively differentiated all three cohorts (p<0.05). The average vertical gradient PAO2 in healthy subjects was −1.03 ± 0.51 Torr/cm toward lower values in the posterior/dependent regions. The directional gradient was absent in smokers (0.36 ± 1.22 Torr/cm) and was in the opposite direction in COPD subjects (2.18 ± 1.54 Torr/cm). The vertical gradients correlated with Smoking History (p=0.004); BMI (p=0.037), PFT metrics (FEV1, p=0.025; and %RV/TLC, p=0.033) and with distance walked in six minutes (p=0.009). Discussion Regional PAO2 data indicate that cigarette smoke induces physiological alterations that are not being detected by the most widely used physiologic tests. PMID:25395184

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wong, May Wai San; Ovchinnikov, Mikhail; Wang, Minghuai

Potential ways of parameterizing vertical turbulent fluxes of hydrometeors are examined using a high-resolution cloud-resolving model. The cloud-resolving model uses the Morrison microphysics scheme, which contains prognostic variables for rain, graupel, ice, and snow. A benchmark simulation with a horizontal grid spacing of 250 m of a deep convection case carried out to evaluate three different ways of parameterizing the turbulent vertical fluxes of hydrometeors: an eddy-diffusion approximation, a quadrant-based decomposition, and a scaling method that accounts for within-quadrant (subplume) correlations. Results show that the down-gradient nature of the eddy-diffusion approximation tends to transport mass away from concentrated regions, whereasmore » the benchmark simulation indicates that the vertical transport tends to transport mass from below the level of maximum to aloft. Unlike the eddy-diffusion approach, the quadri-modal decomposition is able to capture the signs of the flux gradient but underestimates the magnitudes. The scaling approach is shown to perform the best by accounting for within-quadrant correlations, and improves the results for all hydrometeors except for snow. A sensitivity study is performed to examine how vertical transport may affect the microphysics of the hydrometeors. The vertical transport of each hydrometeor type is artificially suppressed in each test. Results from the sensitivity tests show that cloud-droplet-related processes are most sensitive to suppressed rain or graupel transport. In particular, suppressing rain or graupel transport has a strong impact on the production of snow and ice aloft. Lastly, a viable subgrid-scale hydrometeor transport scheme in an assumed probability density function parameterization is discussed.« less

Hybrid gravity survey to search for submarine ore deposit

NASA Astrophysics Data System (ADS)

Araya, A.; Kanazawa, T.; Fujimoto, H.; Shinohara, M.; Yamada, T.; Mochizuki, K.; Iizasa, K.; Ishihara, T.; Omika, S.

2011-12-01

Along with seismic surveys, gravity survey is a useful method to profile the underground density structure. We propose a hybrid gravity survey using gravimeters and gravity gradiometers to detect submarine ore deposits as density anomalies by towing the instruments using an AUV (Autonomous Underwater Vehicle) or an ROV (Remotely Operated Vehicle). Gravimeters measure the regional density structure below the seafloor, whereas gravity gradiometers are sensitive to localized mass distribution. A gravity gradiometer comprises two accelerometers arranged with a vertical separation, and a gravity gradient can be obtained from the acceleration difference. Compared to gravimeters, gravity gradiometers are insensitive to common disturbances such as parallel acceleration, thermal drift, and apparent gravity effect (Eötvös effect). We made two accelerometers using astatic pendulums, and obtained common acceleration reduction more than two orders of magnitude. With these pendulums of 500-mm separation, resolution of 7E (=7x10^{-9}(1/s^2)), enough to detect a typical ore deposit buried 50m below the seafloor, was evaluated. During measurements using a submersible mobile object, instrument orientation is required to be controlled to keep verticality and to reduce centrifugal force associated with rotation of the instrument. Using a gyro and a tiltmeter, angular rotation was shown to be controlled within 0.001deg/s which corresponds to 0.3E in effective gravity gradient due to the centrifugal force. In this paper, target of this research, details of the instruments and their performance, and development for the submarine gravity survey using an AUV will be presented.

Gravity-driven soap film dynamics in subcritical regimes

NASA Astrophysics Data System (ADS)

Auliel, M. I.; Castro, F.; Sosa, R.; Artana, G.

2015-10-01

We undertake the analysis of soap-film dynamics with the classical approach of asymptotic expansions. We focus our analysis in vertical soap film tunnels operating in subcritical regimes with elastic Mach numbers Me=O(10-1) . Considering the associated set of nondimensional numbers that characterize this flow, we show that the flow behaves as a two-dimensional (2D) divergence free flow with variable mass density. When the soap film dynamics agrees with that of a 2D and almost constant mass density flow, the regions where the second invariant of the velocity gradient is non-null correspond to regions where the rate of change of film thickness is non-negligible.

Method and apparatus for determining vertical heat flux of geothermal field

DOEpatents

Poppendiek, Heinz F.

1982-01-01

A method and apparatus for determining vertical heat flux of a geothermal field, and mapping the entire field, is based upon an elongated heat-flux transducer (10) comprised of a length of tubing (12) of relatively low thermal conductivity with a thermopile (20) inside for measuring the thermal gradient between the ends of the transducer after it has been positioned in a borehole for a period sufficient for the tube to reach thermal equilibrium. The transducer is thermally coupled to the surrounding earth by a fluid annulus, preferably water or mud. A second transducer comprised of a length of tubing of relatively high thermal conductivity is used for a second thermal gradient measurement. The ratio of the first measurement to the second is then used to determine the earth's thermal conductivity, k.sub..infin., from a precalculated graph, and using the value of thermal conductivity thus determined, then determining the vertical earth temperature gradient, b, from predetermined steady state heat balance equations which relate the undisturbed vertical earth temperature distributions at some distance from the borehole and earth thermal conductivity to the temperature gradients in the transducers and their thermal conductivity. The product of the earth's thermal conductivity, k.sub..infin., and the earth's undisturbed vertical temperature gradient, b, then determines the earth's vertical heat flux. The process can be repeated many times for boreholes of a geothermal field to map vertical heat flux.

Modeling Vertical Structure and Heat Transport within the Oceans of Ice-covered Worlds (Invited)

NASA Astrophysics Data System (ADS)

Goodman, J. C.

2010-12-01

Indirect observational evidence provides a strong case for liquid oceans beneath the icy crust of Europa and several other frozen moons in the outer solar system. However, little is known about the fluid circulation within these exotic oceans. As a first step toward understanding circulations driven by buoyancy (rather than mechanical forcing from tides), one must understand the typical vertical structure of temperature, salinity, and thus density within the ocean. Following a common approach from terrestrial oceanography, I have built a "single column convection model" for icy world oceans, which describes the density structure of the ocean as a function of depth only: horizontal variations are ignored. On Earth, this approach is of limited utility, because of the strong influence of horizontal wind-driven currents and sea-surface temperature gradients set in concert with the overlying atmosphere. Neither of these confounding issues is present in an icy world's ocean. In the model, mixing of fluid properties via overturning convection is modeled as a strong diffusive process which only acts when the ocean is vertically unstable. "Double diffusive" processes (salt fingering and diffusive layering) are included: these are mixing processes resulting from the unequal molecular diffusivities of heat and salt. Other important processes, such as heating on adiabatic compression, and freshwater fluxes from melting overlying ice, are also included. As a simple test case, I considered an ocean of Europa-like depth (~100 km) and gravity, heated from the seafloor. To simplify matters, I specified an equation of state appropriate to terrestrial seawater, and a simple isothermal ocean as an initial condition. As expected, convection gradually penetrates upward, warming the ocean to an adiabatic, unstratified equilibrium density profile on a timescale of 50 kyr if 4.5 TW of heat are emitted by the silicate interior; the same result is achieved in proportionally more/less time for weaker/stronger internal heating. Unlike Earth's oceans, I predict that since icy worlds' oceans are heated from below, they will generally be unstratified, with constant potential density from top to bottom. There will be no pycnocline as on Earth, so global ocean currents supported by large-scale density gradients seem unlikely. However, icy world oceans may be "weird" in ways which are unheard-of in terrestrial oceanography The density of sulfate brine has a very different equation of state than chloride brines: does this affect the vertical structure? If the ocean water is very pure, cold water can be less dense than warm. Can this lead to periodic catastrophic overturning, as proposed by other authors? These and other questions are currently being investigated using the single-column convection model as a primary tool.

Modeling lateral circulation and its influence on the along-channel flow in a branched estuary

NASA Astrophysics Data System (ADS)

Zhu, Lei; He, Qing; Shen, Jian

2018-02-01

A numerical modeling study of the influence of the lateral flow on the estuarine exchange flow was conducted in the north passage of the Changjiang estuary. The lateral flows show substantial variabilities within a flood-ebb tidal cycle. The strong lateral flow occurring during flood tide is caused primarily by the unique cross-shoal flow that induces a strong northward (looking upstream) barotropic force near the surface and advects saltier water toward the northern part of the channel, resulting in a southward baroclinic force caused by the lateral density gradient. Thus, a two-layer structure of lateral flows is produced during the flood tide. The lateral flows are vigorous near the flood slack and the magnitude can exceed that of the along-channel tidal flow during that period. The strong vertical shear of the lateral flows and the salinity gradient in lateral direction generate lateral tidal straining, which are out of phase with the along-channel tidal straining. Consequently, stratification is enhanced at the early stage of the ebb tide. In contrast, strong along-channel straining is apparent during the late ebb tide. The vertical mixing disrupts the vertical density gradient, thus suppressing stratification. The impact of lateral straining on stratification during spring tide is more pronounced than that of along-channel straining during late flood and early ebb tides. The momentum balance along the estuary suggests that lateral flow can augment the residual exchange flow. The advection of lateral flows brings low-energy water from the shoal to the deep channel during the flood tide, whereas the energetic water is moved to the shoal via lateral advection during the ebb tide. The impact of lateral flow on estuarine circulation of this multiple-channel estuary is different from single-channel estuary. A model simulation by blocking the cross-shoal flow shows that the magnitudes of lateral flows and tidal straining are reduced. Moreover, the reduced lateral tidal straining results in a decrease in vertical stratification from the late flood to early ebb tides during the spring tide. By contrast, the along-channel tidal straining becomes dominant. The model results illustrate the important dynamic linkage between lateral flows and estuarine dynamics in the Changjiang estuary.

Neutral winds and electric fields from model studies using reduced ionograms

NASA Technical Reports Server (NTRS)

Baran, D. E.

1974-01-01

A relationship between the vertical component of the ion velocity and electron density profiles derived from reduced ionograms is developed. Methods for determining the horizontal components of the neutral winds and electric fields by using this relationship and making use of the variations of the inclinations and declinations of the earth's magnetic field are presented. The effects that electric fields have on the neutral wind calculations are estimated to be small but not second order. Seasonal and latitudinal variations of the calculated neutral winds are presented. From the calculated neutral winds a new set of neutral pressure gradients is determined. The new pressure gradients are compared with those generated from several static neutral atmospheric models. Sensitivity factors relating the pressure gradients and neutral winds are calculated and these indicate that mode coupling and harmonic generation are important to studies which assume linearized theories.

Automated apparatus for producing gradient gels

DOEpatents

Anderson, N.L.

1983-11-10

Apparatus for producing a gradient gel which serves as a standard medium for a two-dimensional analysis of proteins, the gel having a density gradient along its height formed by a variation in gel composition, with the apparatus including first and second pumping means each including a plurality of pumps on a common shaft and driven by a stepping motor capable of providing small incremental changes in pump outputs for the gel ingredients, the motors being controlled, by digital signals from a digital computer, a hollow form or cassette for receiving the gel composition, means for transferring the gel composition including a filler tube extending near the bottom of the cassette, adjustable horizontal and vertical arms for automatically removing and relocating the filler tube in the next cassette, and a digital computer programmed to automatically control the stepping motors, arm movements, and associated sensing operations involving the filling operation.

Automated apparatus for producing gradient gels

DOEpatents

Anderson, Norman L.

1986-01-01

Apparatus for producing a gradient gel which serves as a standard medium for a two-dimensional analysis of proteins, the gel having a density gradient along its height formed by a variation in gel composition, with the apparatus including first and second pumping means each including a plurality of pumps on a common shaft and driven by a stepping motor capable of providing small incremental changes in pump outputs for the gel ingredients, the motors being controlled, by digital signals from a digital computer, a hollow form or cassette for receiving the gel composition, means for transferring the gel composition including a filler tube extending near the bottom of the cassette, adjustable horizontal and vertical arms for automatically removing and relocating the filler tube in the next cassette, and a digital computer programmed to automatically control the stepping motors, arm movements, and associated sensing operations involving the filling operation.

Light acclimation optimizes leaf functional traits despite height-related constraints in a canopy shading experiment.

PubMed

Coble, Adam P; Cavaleri, Molly A

2015-04-01

Within-canopy gradients of leaf functional traits have been linked to both light availability and vertical gradients in leaf water potential. While observational studies can reveal patterns in leaf traits, within-canopy experimental manipulations can provide mechanistic insight to tease apart multiple interacting drivers. Our objectives were to disentangle effects of height and light environment on leaf functional traits by experimentally shading branches along vertical gradients within a sugar maple (Acer saccharum) forest. Shading reduced leaf mass per area (LMA), leaf density, area-based leaf nitrogen (N(area)), and carbon:nitrogen (C:N) ratio, and increased mass-based leaf nitrogen (N(mass)), highlighting the importance of light availability on leaf morphology and chemistry. Early in the growing season, midday leaf water potential (Ψ(mid)), LMA, and N(area) were driven primarily by height; later in the growing season, light became the most important driver for LMA and Narea. Carbon isotope composition (δ(13)C) displayed strong, linear correlations with height throughout the growing season, but did not change with shading, implying that height is more influential than light on water use efficiency and stomatal behavior. LMA, leaf density, N(mass), C:N ratio, and δ(13)C all changed seasonally, suggesting that leaf ageing effects on leaf functional traits are equally as important as microclimatic conditions. Overall, our results indicate that: (1) stomatal sensitivity to vapor pressure deficit or Ψ(mid) constrains the supply of CO2 to leaves at higher heights, independent of light environment, and (2) LMA and N(area) distributions become functionally optimized through morphological acclimation to light with increasing leaf age despite height-related constraints.

Plasma Drift Rates During and Preceding Equatorial Spread F Inferred by the HF Doppler Technique

NASA Astrophysics Data System (ADS)

Miller, E. S.; Hilton, A. J.; Chartier, A.

2017-12-01

The quiet time afternoon and evening equatorial and low-latitude ionosphere is characterized by increasing vertical drift and sharpening plasma density gradient in the lower F region. This combination of effects leads to the plasma instability cascade known imprecisely as "equatorial spread F." In this work, we utilize a simple transequatorial HF Doppler observation to infer the vertical and horizontal plasma drifts preceding and during spread-F conditions. The data exhibit three behavior regimes indicative of three different processes: The first is a slow vertical drift that may be due to either increasing vertical plasma drifts or recombination of the bottomside. The second is an explosive spread Doppler signature (indicating relative velocities of 600 m/s or more) that is associated with the initiation of the spread-F depletions. Finally, the third is a structure that represents a changing HF propagation channel as radio rays propagate through the regions of depleted-but still unstable-plasma. Observations of the March 2016 Pacific total solar eclipse will also be included as a test case for the effects of vertical drifts versus recombination.

The Upper 1000-m Slope Currents North of the South Shetland Islands and Elephant Island Based on Ship Cruise Observations

NASA Astrophysics Data System (ADS)

Du, Guangqian; Zhang, Zhaoru; Zhou, Meng; Zhu, Yiwu; Zhong, Yisen

2018-04-01

While the Antarctic Slope Current (ASC) has been intensively studied for the East Antarctica slope area and the Weddell Sea, its fate in the western Antarctic Peninsula (WAP) region remains much less known. Data from two cruises conducted near the South Shetland Islands (SSIs) and the Elephant Island (EI), one in austral summer of 2004 and one in austral winter of 2006, were analyzed to provide a broad picture of the circulation pattern over the continental slope of the surveyed area, and an insight into the dynamical balance of the circulation. The results indicate that southwestward currents are present over the upper slope in the study area, indicating the ASC in the WAP region. Near the Shackleton Gap (SG) north of the EI, the southwestward slope currents near the shelf break are characterized by a water mass colder and fresher than the ambient water, which produces cross-slope density gradients and then vertical shear of the along-slope (or along-isobath) velocity. The vertical shear is associated with a reversal of the along-slope current from northeastward at surface to southwestward in deeper layers, or a depth-intensification of the southwestward slope currents. The water mass with temperature and salinity characteristics similar to the observed cold and fresh water is also revealed on the southern slope of the Scotia Sea, suggesting that this cold and fresh water is originated from the Scotia Sea slope and flows southwestward through the SG. Over the shelf north of the SSIs, the cold and fresh water mass is also observed and originates mainly from the Bransfield Strait. In this area, vertical structure of the southwestward slope currents is associated with the onshore intrusion of the upper Circumpolar Deep Water that creates cross-slope density gradients.

Vertical structure of tropospheric winds on gas giants

NASA Astrophysics Data System (ADS)

Scott, R. K.; Dunkerton, T. J.

2017-04-01

Zonal mean zonal velocity profiles from cloud-tracking observations on Jupiter and Saturn are used to infer latitudinal variations of potential temperature consistent with a shear stable potential vorticity distribution. Immediately below the cloud tops, density stratification is weaker on the poleward and stronger on the equatorward flanks of midlatitude jets, while at greater depth the opposite relation holds. Thermal wind balance then yields the associated vertical shears of midlatitude jets in an altitude range bounded above by the cloud tops and bounded below by the level where the latitudinal gradient of static stability changes sign. The inferred vertical shear below the cloud tops is consistent with existing thermal profiling of the upper troposphere. The sense of the associated mean meridional circulation in the upper troposphere is discussed, and expected magnitudes are given based on existing estimates of the radiative timescale on each planet.

Mechanical models for dikes: A third school of thought

NASA Astrophysics Data System (ADS)

Townsend, Meredith R.; Pollard, David D.; Smith, Richard P.

2017-04-01

Geological and geophysical data from continental volcanic centers and giant radial swarms, and from oceanic shield volcanoes and rift zones, indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height. What traps dikes within the shallow lithosphere and promotes these highly eccentric shapes? Gravity-induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms; pressure gradients due to magma flow are secondary to insignificant, because the flow direction is dominantly horizontal. This configuration of vertical, blade-shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes. In School I, a dike is disconnected from its source and ascends under the influence of buoyancy. In School II, a dike is connected to a magma reservoir and is driven upward by magma flux from the source. We review the geological and geophysical data supporting the vertical dike - horizontal flow/propagation configuration and suggest the abundance and veracity of these data in many different geological settings, and the modeling results that address this physical process, warrant adding this as a third school of thought. A new analytical solution for the boundary-value problem of a homogeneous, isotropic, and linear elastic solid with a vertical, fluid-filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution, dike-tip stress intensity, and stable height. Model results indicate that in a homogeneous crust, dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness KIC 100 MPa · m1/2. However, density stratification of the crust is an effective mechanism for trapping kilometer-scale dikes even if the host rock is very weak (KIC = 0). This analysis may explain why vertical dikes propagate laterally for great distances, but reside within a narrow range of depths in the crust.

Increase in the Random Dopant Induced Threshold Fluctuations and Lowering in Sub 100 nm MOSFETs Due to Quantum Effects: A 3-D Density-Gradient Simulation Study

NASA Technical Reports Server (NTRS)

Asenov, Asen; Slavcheva, G.; Brown, A. R.; Davies, J. H.; Saini, S.

2000-01-01

In this paper we present a detailed simulation study of the influence of quantum mechanical effects in the inversion layer on random dopant induced threshold voltage fluctuations and lowering in sub 100 nm MOSFETs. The simulations have been performed using a 3-D implementation of the density gradient (DG) formalism incorporated in our established 3-D atomistic simulation approach. This results in a self-consistent 3-D quantum mechanical picture, which implies not only the vertical inversion layer quantisation but also the lateral confinement effects related to current filamentation in the 'valleys' of the random potential fluctuations. We have shown that the net result of including quantum mechanical effects, while considering statistical dopant fluctuations, is an increase in both threshold voltage fluctuations and lowering. At the same time, the random dopant induced threshold voltage lowering partially compensates for the quantum mechanical threshold voltage shift in aggressively scaled MOSFETs with ultrathin gate oxides.

Seasonal changes in the tropospheric carbon monoxide profile over the remote Southern Hemisphere evaluated using multi-model simulations and aircraft observations

NASA Astrophysics Data System (ADS)

Fisher, J. A.; Wilson, S. R.; Zeng, G.; Williams, J. E.; Emmons, L. K.; Langenfelds, R. L.; Krummel, P. B.; Steele, L. P.

2014-11-01

We use aircraft observations from the 1991-2000 Cape Grim Overflight Program and the 2009-2011 HIAPER Pole-to-Pole Observations (HIPPO), together with output from four chemical transport and chemistry-climate models, to better understand the vertical distribution of carbon monoxide (CO) in the remote Southern Hemisphere. Observed CO vertical gradients at Cape Grim vary from 1.6 ppbv km-1 in austral autumn to 2.2 ppbv km-1 in austral spring. CO vertical profiles from Cape Grim are remarkably consistent with those observed over the southern mid-latitudes Pacific during HIPPO, despite major differences in time periods, flight locations, and sampling strategies between the two datasets. Using multi-model simulations from the Southern Hemisphere Model Intercomparison Project (SHMIP), we find that observed CO vertical gradients in austral winter-spring are well-represented in models and can be attributed to primary CO emissions from biomass burning. In austral summer-autumn, inter-model variability in simulated gradients is much larger, and two of the four SHMIP models significantly underestimate the Cape Grim observations. Sensitivity simulations show that CO vertical gradients at this time of year are driven by long-range transport of secondary CO of biogenic origin, implying a large sensitivity of the remote Southern Hemisphere troposphere to biogenic emissions and chemistry. Inter-model variability in summer-autumn gradients can be explained by differences in both the chemical mechanisms that drive secondary production of CO from biogenic sources and the vertical transport that redistributes this CO throughout the Southern Hemisphere. This suggests that the CO vertical gradient in the remote Southern Hemisphere provides a sensitive test of the chemistry and transport processes that define the chemical state of the background atmosphere.

Neutral surfaces and potential vorticity in the world's oceans

NASA Astrophysics Data System (ADS)

You, Yuzhu; McDougall, Trevor J.

1990-08-01

Several neutral surfaces are mapped in this paper and their properties are contrasted with those of potential density surfaces. It is shown that the Pacific is relatively forgiving to the use of potential density, while more care must be taken in the Atlantic and Indian oceans because of the larger compensating lateral gradients of potential temperature and salinity along neutral surfaces in these oceans. The dynamically important tracer, neutral-surface potential vorticity (NSPV), defined to be proportional to f/h (where f is the Coriolis frequency and h is the height between two neutral surfaces), is mapped on several neutral surfaces in each of the world's oceans. At a depth of 1000m in the Atlantic and Indian oceans, the epineutral gradient of NSPV is different to the isopycnal variations of fN2 by as much as a factor of two (here N is the buoyancy frequency). Maps of isopycnal potential vorticity (IPV) resemble those of fN2, but the values of IPV are less by the simple factor μ, defined by μ = c[Rρ-1]/[Rρ-c], where Rρ is the stability ratio of the water column and c is the ratio of the values of α/β at the in situ pressure to that at the reference pressure (α and β being the thermal expansion and saline contraction coefficients, respectively). Layered models of the ocean circulation often take the vertical shear between layers (the thermal wind) to be given by the product of the interface slope and the contrast of potential density across the interface. The true thermal wind equation involves the interfaeial difference of in situ density, which is larger than the corresponding difference of potential density by the factor μ that is mapped in this paper, taking values up to 1.25 at a depth of 1000 m. This implies that the thermal wind is currently underestimated by up to 25% in layered ocean models. The differences between the slopes of neutral surfaces and potential density surfaces can be quantified Using the factory μ. The magnitudes of these slopes are illustrated here with contour maps and with vertical profiles, One would think that by choosing the reference pressure of potential density to be at the central pressure of a data set, the conservation equation of potential vorticity could be expressed with respect to these potential density surfaces with sufficient accuracy. Here it is shown that even the best potential density variable is significantly in error at thermoclinic frontal regions. This is linked to the fact that diapycnal velocities are not simply due to vertical mixing processes, but are also partly caused by epineutral mixing.

Numerical simulation of life cycles of advection warm fog

NASA Technical Reports Server (NTRS)

Hung, R. J.; Vaughan, O. H.

1977-01-01

The formation, development and dissipation of advection warm fog is investigated. The equations employed in the model include the equation of continuity, momentum and energy for the descriptions of density, wind component and potential temperature, respectively, together with two diffusion equations for the modification of water-vapor mixing ratio and liquid-water mixing ratios. A description of the vertical turbulent transfer of heat, moisture and momentum has been taken into consideration. The turbulent exchange coefficients adopted in the model are based on empirical flux-gradient relations.

Temperature Versus Salinity Gradients Below the Ocean Mixed Layer

DTIC Science & Technology

2012-05-03

where salinity controls the depth of the mixed layer are understood to have “barrier” layers [Lukas and Lindstrom , 1991], where the depth of vertically...the horizontal. For example, Rudnick and Martin [2002] have shown that the ocean mixed layer at sub-mesoscales is horizontally well density compensated...Res., 102, 23,063–23,078, doi:10.1029/97JC01443. Barron, C. N., A. B. Kara, P. J. Martin , R. C. Rhodes, and L. F. Smedstad (2006), Formulation

The influence of vertical disparity gradient and cue conflict on EEG omega complexity in Panum's limiting case.

PubMed

Li, Huayun; Jia, Huibin; Yu, Dongchuan

2018-03-01

Using behavioral measures and ERP technique, researchers discovered at least two factors could influence the final perception of depth in Panum's limiting case, which are the vertical disparity gradient and the degree of cue conflict between two- and three-dimensional shapes. Although certain event-related potential components have been proved to be sensitive to the different levels of these two factors, some methodological limitations existed in this technique. In this study, we proposed that the omega complexity of EEG signal may serve as an important supplement of the traditional event-related potential technique. We found that the trials with lower vertical gradient disparity have lower omega complexity (i.e., higher global functional connectivity) of the occipital region, especially that of the right-occipital hemisphere. Moreover, for occipital omega complexity, the trials with low-cue conflict have significantly larger omega complexity than those with medium- and high-cue conflict. It is also found that the electrodes located in the middle line of the occipital region (i.e., POz and Oz) are more crucial to the impact of different levels of cue conflict on omega complexity than the other electrodes located in the left- and right-occipital hemispheres. These evidences demonstrated that the EEG omega complexity could reflect distinct neural activities evoked by Panum's limiting case configurations, with different levels of vertical disparity gradient and cue conflict. Besides, the influence of vertical disparity gradient and cue conflict on omega complexity may be regional dependent. NEW & NOTEWORTHY The EEG omega complexity could reflect distinct neural activities evoked by Panum's limiting case configurations with different levels of vertical disparity gradient and cue conflict. The influence of vertical disparity gradient and cue conflict on omega complexity is regional dependent. The omega complexity of EEG signal can serve as an important supplement of the traditional ERP technique.

Florida Harvester Ant Nest Architecture, Nest Relocation and Soil Carbon Dioxide Gradients

PubMed Central

Tschinkel, Walter R.

2013-01-01

Colonies of the Florida harvester ant, Pogonomyrmex badius, excavate species-typical subterranean nests up the 3 m deep with characteristic vertical distribution of chamber area/shape, spacing between levels and vertical arrangement of the ants by age and brood stage. Colonies excavate and occupy a new nest about once a year, and doing so requires that they have information about the depth below ground. Careful excavation and mapping of vacated and new nests revealed that there was no significant difference between the old and new nests in any measure of nest size, shape or arrangement. Colonies essentially built a replicate of the just-vacated nest (although details differed), and they did so in less than a week. The reason for nest relocation is not apparent. Tschinkel noted that the vertical distribution of chamber area, worker age and brood type was strongly correlated to the soil carbon dioxide gradient, and proposed that this gradient serves as a template for nest excavation and vertical distribution. To test this hypothesis, the carbon dioxide gradient of colonies that were just beginning to excavate a new nest was eliminated by boring 6 vent holes around the forming nest, allowing the soil CO2 to diffuse into the atmosphere and eliminating the gradient. Sadly, neither the nest architecture nor the vertical ant distribution of vented nests differed from either unvented control or from their own vacated nest. In a stronger test, workers excavated a new nest under a reversed carbon dioxide gradient (high concentration near the surface, low below). Even under these conditions, the new and old nests did not differ significantly, showing that the soil carbon dioxide gradient does not serve as a template for nest construction or vertical worker distribution. The possible importance of soil CO2 gradients for soil-dwelling animals is discussed. PMID:23555829

Importance of closely spaced vertical sampling in delineating chemical and microbiological gradients in groundwater studies

USGS Publications Warehouse

Smith, R.L.; Harvey, R.W.; LeBlanc, D.R.

1991-01-01

Vertical gradients of selected chemical constituents, bacterial populations, bacterial activity and electron acceptors were investigated for an unconfined aquifer contaminated with nitrate and organic compounds on Cape Cod, Massachusetts, U.S.A. Fifteen-port multilevel sampling devices (MLS's) were installed within the contaminant plume at the source of the contamination, and at 250 and 2100 m downgradient from the source. Depth profiles of specific conductance and dissolved oxygen at the downgradient sites exhibited vertical gradients that were both steep and inversely related. Narrow zones (2-4 m thick) of high N2O and NH4+ concentrations were also detected within the contaminant plume. A 27-fold change in bacterial abundance; a 35-fold change in frequency of dividing cells (FDC), an indicator of bacterial growth; a 23-fold change in 3H-glucose uptake, a measure of heterotrophic activity; and substantial changes in overall cell morphology were evident within a 9-m vertical interval at 250 m downgradient. The existence of these gradients argues for the need for closely spaced vertical sampling in groundwater studies because small differences in the vertical placement of a well screen can lead to incorrect conclusions about the chemical and microbiological processes within an aquifer.Vertical gradients of selected chemical constituents, bacterial populations, bacterial activity and electron acceptors were investigated for an unconfined aquifer contaminated with nitrate and organic compounds on Cape Cod, Massachusetts, USA. Fifteen-port multilevel sampling devices (MLS's) were installed within the contaminant plume at the source of the contamination, and at 250 and 2100 m downgradient from the source. Depth profiles of specific conductance and dissolved oxygen at the downgradient sites exhibited vertical gradients that were both steep and inversely related. Narrow zones (2-4 m thick) of high N2O and NH4+ concentrations were also detected within the contaminant plume. A 27-fold change in bacterial abundance; a 35-fold change in frequency of dividing cells (FDC), an indicator of bacterial growth; a 23-fold change in 3H-glucose uptake, a measure of heterotrophic activity; and substantial changes in overall cell morphology were evident within a 9-m vertical interval at 250 m downgradient. The existence of these gradients argues for the need for closely spaced vertical sampling in ground-water studies because small differences in the vertical placement of a well screen can lead to incorrect conclusions about the chemical and microbiological processes within an aquifer.

Solvent effects in time-dependent self-consistent field methods. II. Variational formulations and analytical gradients

DOE PAGES

Bjorgaard, J. A.; Velizhanin, K. A.; Tretiak, S.

2015-08-06

This study describes variational energy expressions and analytical excited state energy gradients for time-dependent self-consistent field methods with polarizable solvent effects. Linear response, vertical excitation, and state-specific solventmodels are examined. Enforcing a variational ground stateenergy expression in the state-specific model is found to reduce it to the vertical excitation model. Variational excited state energy expressions are then provided for the linear response and vertical excitation models and analytical gradients are formulated. Using semiempiricalmodel chemistry, the variational expressions are verified by numerical and analytical differentiation with respect to a static external electric field. Lastly, analytical gradients are further tested by performingmore » microcanonical excited state molecular dynamics with p-nitroaniline.« less

Assimilation of temperature and hydraulic gradients for quantifying the spatial variability of streambed hydraulics

NASA Astrophysics Data System (ADS)

Huang, Xiang; Andrews, Charles B.; Liu, Jie; Yao, Yingying; Liu, Chuankun; Tyler, Scott W.; Selker, John S.; Zheng, Chunmiao

2016-08-01

Understanding the spatial and temporal characteristics of water flux into or out of shallow aquifers is imperative for water resources management and eco-environmental conservation. In this study, the spatial variability in the vertical specific fluxes and hydraulic conductivities in a streambed were evaluated by integrating distributed temperature sensing (DTS) data and vertical hydraulic gradients into an ensemble Kalman filter (EnKF) and smoother (EnKS) and an empirical thermal-mixing model. The formulation of the EnKF/EnKS assimilation scheme is based on a discretized 1D advection-conduction equation of heat transfer in the streambed. We first systematically tested a synthetic case and performed quantitative and statistical analyses to evaluate the performance of the assimilation schemes. Then a real-world case was evaluated to calculate assimilated specific flux. An initial estimate of the spatial distributions of the vertical hydraulic gradients was obtained from an empirical thermal-mixing model under steady-state conditions using a constant vertical hydraulic conductivity. Then, this initial estimate was updated by repeatedly dividing the assimilated specific flux by estimates of the vertical hydraulic gradients to obtain a refined spatial distribution of vertical hydraulic gradients and vertical hydraulic conductivities. Our results indicate that optimal parameters can be derived with fewer iterations but greater simulation effort using the EnKS compared with the EnKF. For the field application in a stream segment of the Heihe River Basin in northwest China, the average vertical hydraulic conductivities in the streambed varied over three orders of magnitude (5 × 10-1 to 5 × 102 m/d). The specific fluxes ranged from near zero (qz < ±0.05 m/d) to ±1.0 m/d, while the vertical hydraulic gradients were within the range of -0.2 to 0.15 m/m. The highest and most variable fluxes occurred adjacent to a debris-dam and bridge pier. This phenomenon is very likely the result of heterogeneous streambed hydraulic characteristics in these areas. Our results have significant implications for hyporheic micro-habitats, fish spawning and other wildlife incubation, regional flow and hyporheic solute transport models in the Heihe River Basin, as well as in other similar hydrologic settings.

Atmospheric stability effects on wind fields and scalar mixing within and just above a subalpine forest in sloping terrain

USGS Publications Warehouse

Burns, Sean P.; Sun, Jielun; Lenschow, D.H.; Oncley, S.P.; Stephens, B.B.; Yi, C.; Anderson, D.E.; Hu, Jiawen; Monson, Russell K.

2011-01-01

Air temperature Ta, specific humidity q, CO2 mole fraction ??c, and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Rib. For stable conditions, we found vertical scalar differences developed over a "transition" region between 0.05 < Rib < 0.5. For strongly stable conditions (Rib > 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships q and ??c have with Rib are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns. At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO2 near the ground being 5-7 ??mol mol-1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Rib-binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients, detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results for CO2 advection over heterogenous, complex terrain are discussed. ?? 2010 Springer Science+Business Media B.V.

Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients

NASA Astrophysics Data System (ADS)

Guy, Alexandra; Holzrichter, Nils; Ebbing, Jörg

2017-09-01

The main purpose of this study is to construct a new 3-D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3-D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3-D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Corotating solar wind structures and recurrent trains of enhanced diurnal variation in galactic cosmic rays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yeeram, T.; Ruffolo, D.; Sáiz, A.

Data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, with a vertical cutoff rigidity of 16.8 GV, were utilized to determine the diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) near Earth during solar minimum conditions between 2007 November and 2010 November. We identified trains of enhanced DA over several days, which often recur after a solar rotation period (∼27 days). By investigating solar coronal holes as identified from synoptic maps and solar wind parameters, we found that the intensity and anisotropy of cosmic rays are associated with the high-speed streams (HSSs) in the solar wind, which aremore » in turn related to the structure and evolution of coronal holes. An enhanced DA was observed after the onset of some, but not all, HSSs. During time periods of recurrent trains, the DA was often enhanced or suppressed according to the sign of the interplanetary magnetic field B, which suggests a contribution from a mechanism involving a southward gradient in the GCR density, n, and a gradient anisotropy along B × ∇n. In one non-recurrent and one recurrent sequence, an HSS from an equatorial coronal hole was merged with that from a trailing mid-latitude extension of a polar coronal hole, and the slanted HSS structure in space with suppressed GCR density can account for the southward GCR gradient. We conclude that the gradient anisotropy is a source of temporary changes in the GCR DA under solar minimum conditions, and that the latitudinal GCR gradient can sometimes be explained by the coronal hole morphology.« less

Preparative electrophoresis of living lymphocytes

NASA Technical Reports Server (NTRS)

Vanoss, C. J.; Bigazzi, P. E.; Gillman, C. F.; Allen, R. E.

1974-01-01

Vertical liquid columns containing low molecular weight dextran density gradients can be used for preparative lymphocyte electrophoresis on earth, in simulation of 0 gravity conditions. Another method that has been tested at 1 G, is the electrophoresis of lymphocytes in a upward direction in vertical columns. By both methods up to 10 to the 7th power lymphocytes can be separated at one time in a 30 cm glass column of 8 mm inside diameter, at 12 v/cm, in 2 hours. Due to convection and sedimentation problems, the separation at 1 G is less than ideal, but it is expected that at 0 gravity electrophoresis will prove to be a uniquely powerful cell separation tool. The technical feasibility of electrophoresing inert particles at 0 G has been proven earlier, during the flight of Apollo 16.

New methods for interpretation of magnetic vector and gradient tensor data I: eigenvector analysis and the normalised source strength

NASA Astrophysics Data System (ADS)

Clark, David A.

2012-09-01

Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.

Using absolute gravimeter data to determine vertical gravity gradients

USGS Publications Warehouse

Robertson, D.S.

2001-01-01

The position versus time data from a free-fall absolute gravimeter can be used to estimate the vertical gravity gradient in addition to the gravity value itself. Hipkin has reported success in estimating the vertical gradient value using a data set of unusually good quality. This paper explores techniques that may be applicable to a broader class of data that may be contaminated with "system response" errors of larger magnitude than were evident in the data used by Hipkin. This system response function is usually modelled as a sum of exponentially decaying sinusoidal components. The technique employed here involves combining the x0, v0 and g parameters from all the drops made during a site occupation into a single least-squares solution, and including the value of the vertical gradient and the coefficients of system response function in the same solution. The resulting non-linear equations must be solved iteratively and convergence presents some difficulties. Sparse matrix techniques are used to make the least-squares problem computationally tractable.

Convective flows in enclosures with vertical temperature or concentration gradients

NASA Technical Reports Server (NTRS)

Wang, L. W.; Chai, A. T.; Sun, D. J.

1988-01-01

The transport process in the fluid phase during the growth of a crystal has a profound influence on the structure and quality of the solid phase. In vertical growth techniques the fluid phase is often subjected to vertical temperature and concentration gradients. The main objective is to obtain more experimental data on convective flows in enclosures with vertical temperature or concentration gradients. Among actual crystal systems the parameters vary widely. The parametric ranges studied for mass transfer are mainly dictated by the electrochemical system employed to impose concentration gradients. Temperature or concentration difference are maintained between two horizontal end walls. The other walls are kept insulated. Experimental measurements and observations were made of the heat transfer or mass transfer, flow patterns, and the mean and fluctuating temperature distribution. The method used to visualize the flow pattern in the thermal cases is an electrochemical pH-indicator method. Laser shadowgraphs are employed to visualize flow patterns in the solutal cases.

Convective flows in enclosures with vertical temperature or concentration gradients

NASA Technical Reports Server (NTRS)

Wang, L. W.; Chai, A. T.; Sun, D. J.

1989-01-01

The transport process in the fluid phase during the growth of a crystal has a profound influence on the structure and quality of the solid phase. In vertical growth techniques the fluid phase is often subjected to vertical temperature and concentration gradients. The main objective is to obtain more experimental data on convective flows in enclosures with vertical temperature or concentration gradients. Among actual crystal systems the parameters vary widely. The parametric ranges studied for mass transfer are mainly dictated by the electrochemical system employed to impose concentration gradients. Temperature or concentration difference are maintained between two horizontal end walls. The other walls are kept insulated. Experimental measurements and observations were made of the heat transfer or mass transfer, flow patterns, and the mean and fluctuating temperature distribution. The method used to visualize the flow pattern in the thermal cases is an electrochemical pH-indicator method. Laser shadowgraphs are employed to visualize flow patterns in the solutal cases.

Short-term thermal stratification and partial overturning events in a warm polymictic reservoir: effects on distribution of phytoplankton community.

PubMed

Santos, R M; Saggio, A A; Silva, T L R; Negreiros, N F; Rocha, O

2015-01-01

In lentic freshwater ecosystems, patterns of thermal stratification play a considerable part in determining the population dynamics of phytoplankton. In this study we investigated how these thermal patterns and the associated hydrodynamic processes affect the vertical distribution of phytoplankton during two consecutive diel cycles in a warm polymictic urban reservoir in the metropolitan region of São Paulo, Brazil. Water samples were taken and physical, chemical and biological data collected at half-meter intervals of depth along a water column at a fixed site, every 3 hours throughout the 48-hour period. Two events of stratification, followed by deepening of the thermocline occurred during the study period and led to changes in the vertical distribution of phytoplankton populations. Aphanocapsa delicatissima Nägeli was the single dominant species throughout the 48-hour period. In the second diel cycle, the density gradient induced by temperature differences avoided the sedimentation of Mougeotia sp. C. Agardh to the deepest layers. On the other hand, Pseudanabaena galeata Böcher remained in the 4.0-5.5 m deep layer. The thermal structure of the water was directly affected by two meteorological factors: air temperature and wind speed. Changes in the cell density and vertical distribution of the phytoplankton were controlled by the thermal and hydrodynamic events.

Nutrient interleaving below the mixed layer of the Kuroshio Extension Front

NASA Astrophysics Data System (ADS)

Nagai, Takeyoshi; Clayton, Sophie

2017-08-01

Nitrate interleaving structures were observed below the mixed layer during a cruise to the Kuroshio Extension in October 2009. In this paper, we investigate the formation mechanisms for these vertical nitrate anomalies, which may be an important source of nitrate to the oligotrphoc surface waters south of the Kuroshio Extension Front. We found that nitrate concentrations below the main stream of the Kuroshio Extension were elevated compared to the ambient water of the same density ( σ 𝜃 = 23.5-25). This appears to be analogous to the "nutrient stream" below the mixed layer, associated with the Gulf Stream. Strong turbulence was observed above the vertical nitrate anomaly, and we found that this can drive a large vertical turbulent nitrate flux >O (1 mmol N m-2 day-1). A realistic, high-resolution (2 km) numerical simulation reproduces the observed Kuroshio nutrient stream and nitrate interleaving structures, with similar lateral and vertical scales. The model results suggest that the nitrate interleaving structures are first generated at the western side of the meander crest on the south side of the Kuroshio Extension, where the southern tip of the mixed layer front is under frontogenesis. Lagrangian analyses reveal that the vertical shear of geostrophic and subinertial ageostrophic flow below the mixed layer tilts the existing along-isopycnal nitrate gradient of the Kuroshio nutrient stream to form nitrate interleaving structures. This study suggests that the multi-scale combination of (i) the lateral stirring of the Kuroshio nutrient stream by developed mixed layer fronts during fall to winter, (ii) the associated tilting of along-isopycnal nitrate gradient of the nutrient stream by subinertial shear, which forms vertical interleaving structures, and (iii) the strong turbulent diffusion above them, may provide a route to supply nutrients to oligotrophic surface waters on the south side of the Kuroshio Extension.

Geohydrology and potential for upward movement of saline water in the Cocoa well field, East Orange County, Florida

USGS Publications Warehouse

Phelps, G.G.; Schiffer, D.M.

1996-01-01

The Floridan aquifer system, an approximately 2,000-foot thick sequence of Eocene-age limestone and dolomite, is the main source of water supply in central Florida. Hydraulic conductivity is different in strata of different lithology and is the basis for separating the aquifer system into the Upper Floridan aquifer, a middle semi- confining unit, and the Lower Floridan aquifer. The coastal city of Cocoa withdraws about 26 million gallons of water per day from the Upper Floridan aquifer from a well field in east Orange County, about 25 miles inland. About 60 million gallons per day are withdrawn from the Upper Floridan aquifer and 56 million gallons per day from the Lower Floridan aquifer in the Orlando area, about 15 miles west of the Cocoa well field. Wells drilled in the Cocoa well field from 1955-61 yielded water with chloride concentrations ranging from 25-55 milligrams per liter. Soon after the wells were put in service, chloride concentrations increased; therefore, new wells were drilled further inland. Chloride concen- trations in water from many of the new wells also have increased. Possible sources of saline water are lateral movement of relict seawater in the Upper Floridan aquifer from the east, regional upconing of saline water from the Lower Floridan aquifer or underlying older rocks, or localized upward movement of saline water through fractures. Several test wells were drilled to provide information about chloride concentration changes with depth and to monitor changes with time, including a multi-zone well drilled in 1965 (well C) and two wells drilled in the 1990's (wells R and S). Chloride concentrations have increased in the zone pumped by the supply wells (the upper 500 feet of the aquifer) and in the 1,351-1,357-foot deep zone of well C, but not in the two intervening zones. This indicates that the source of saline water is located laterally, rather than vertically, from the pumped zone in the area of well C. The potential for upward movement of saline water depends on the direction of the vertical hydraulic gradient and on the vertical hydraulic conductivity of the aquifer. A series of aquifer tests was run in 1993-94 and existing water-level and water-quality data were analyzed to evaluate the potential for upward movement of saline water in the well field. The transmissivity of the upper 500 feet of the aquifer is about 100,000 feet squared per day (the horizontal hydraulic conductivity is about 200 feet per day) and the storage coefficient is about 2x10 -4. Horizontal hydraulic conductivities determined from slug tests of the three deepest zones of well C ranged from 20-50 feet per day. Vertical hydraulic conductivities probably do not exceed 0.05 feet per day. The vertical hydraulic gradient is determined by comparing water levels in the various zones, but because of density differences, unadjusted water levels in the deepest zone investigated cannot be directly compared to water levels in the overlying freshwater zones. The difference between environmental-water heads (adjusted for density differences) in the saline-water zone of well C and the overlying freshwater zone were calculated from measured water levels for the period 1966 to 1994. During most of this time period, the gradient was downward, indicating that saline water did not move upward. Upconing of saline water probably is not taking place in the center and western part of the well field, based on the low vertical hydraulic conductivity values estimated for the middle semi-confining unit, the generally downward vertical hydraulic gradient, and the constant chloride concentrations in the intermediate zones of well C. However, there is no information about the extent of the zone of low vertical hydraulic conductivity gradient in the eastern part of the well field. Thus, increased chloride concentrations in supply wells in the eastern part of the well field could be caused either by lateral movement of saline water from the east, or by upwar

Crosswind Shear Gradient Affect on Wake Vortices

NASA Technical Reports Server (NTRS)

Proctor, Fred H.; Ahmad, Nashat N.

2011-01-01

Parametric simulations with a Large Eddy Simulation (LES) model are used to explore the influence of crosswind shear on aircraft wake vortices. Previous studies based on field measurements, laboratory experiments, as well as LES, have shown that the vertical gradient of crosswind shear, i.e. the second vertical derivative of the environmental crosswind, can influence wake vortex transport. The presence of nonlinear vertical shear of the crosswind velocity can reduce the descent rate, causing a wake vortex pair to tilt and change in its lateral separation. The LES parametric studies confirm that the vertical gradient of crosswind shear does influence vortex trajectories. The parametric results also show that vortex decay from the effects of shear are complex since the crosswind shear, along with the vertical gradient of crosswind shear, can affect whether the lateral separation between wake vortices is increased or decreased. If the separation is decreased, the vortex linking time is decreased, and a more rapid decay of wake vortex circulation occurs. If the separation is increased, the time to link is increased, and at least one of the vortices of the vortex pair may have a longer life time than in the case without shear. In some cases, the wake vortices may never link.

Vertical Structure and Dynamics of the Beaufort Gyre Subsurface Layer from ADCP Obervations

NASA Astrophysics Data System (ADS)

Torres, D. J.; Krishfield, R. A.; Proshutinsky, A. Y.; Timmermans, M. L. E.

2014-12-01

As part of the Beaufort Gyre Observing System (BGOS), several Acoustic Doppler Current Profilers (ADCPs) have been maintained at moorings in different locations in the Canada Basin since 2005 to measure upper ocean velocities and sea ice motion. The ADCP data have been analyzed to better understand relationships among different components of forcing driving the sea ice and upper ocean layer including: winds, tides, and horizontal and vertical density gradients in the ocean. Specific attention is paid to data processing and analysis to separate inertial and tidal motions in these regions in the vicinity of the critical latitudes. In addition, we describe the dynamic characteristics of halocline eddies and estimate their kinetic energy and their role in the total energy balance in this region. Ice-Tethered Profiler (ITP) data are used in conjunction with the ADCP measurements to identify relationships between T-S and vertical velocity structures in the mixed layer and deeper. Seasonal and interannual variability in all parameters are also discussed and causes of observed changes are suggested.

Instabilities in a staircase stratified shear flow

NASA Astrophysics Data System (ADS)

Ponetti, G.; Balmforth, N. J.; Eaves, T. S.

2018-01-01

We study stratified shear flow instability where the density profile takes the form of a staircase of interfaces separating uniform layers. Internal gravity waves riding on density interfaces can resonantly interact due to a background shear flow, resulting in the Taylor-Caulfield instability. The many steps of the density profile permit a multitude of interactions between different interfaces, and a rich variety of Taylor-Caulfield instabilities. We analyse the linear instability of a staircase with piecewise-constant density profile embedded in a background linear shear flow, locating all the unstable modes and identifying the strongest. The interaction between nearest-neighbour interfaces leads to the most unstable modes. The nonlinear dynamics of the instabilities are explored in the long-wavelength, weakly stratified limit (the defect approximation). Unstable modes on adjacent interfaces saturate by rolling up the intervening layer into a distinctive billow. These nonlinear structures coexist when stacked vertically and are bordered by the sharp density gradients that are the remnants of the steps of the original staircase. Horizontal averages remain layer-like.

The innate origin of radial and vertical gradients in a simulated galaxy disc

NASA Astrophysics Data System (ADS)

Navarro, Julio F.; Yozin, Cameron; Loewen, Nic; Benítez-Llambay, Alejandro; Fattahi, Azadeh; Frenk, Carlos S.; Oman, Kyle A.; Schaye, Joop; Theuns, Tom

2018-05-01

We examine the origin of radial and vertical gradients in the age/metallicity of the stellar component of a galaxy disc formed in the APOSTLE cosmological hydrodynamical simulations. Some of these gradients resemble those in the Milky Way, where they have sometimes been interpreted as due to internal evolution, such as scattering off giant molecular clouds, radial migration driven by spiral patterns, or orbital resonances with a bar. Secular processes play a minor role in the simulated galaxy, which lacks strong spiral or bar patterns, and where such gradients arise as a result of the gradual enrichment of a gaseous disc that is born thick but thins as it turns into stars and settles into centrifugal equilibrium. The settling is controlled by the feedback of young stars; which links the star formation, enrichment, and equilibration time-scales, inducing radial and vertical gradients in the gaseous disc and its descendent stars. The kinematics of coeval stars evolve little after birth and provide a faithful snapshot of the gaseous disc structure at the time of their formation. In this interpretation, the age-velocity dispersion relation would reflect the gradual thinning of the disc rather than the importance of secular orbit scattering; the outward flaring of stars would result from the gas disc flare rather than from radial migration; and vertical gradients would arise because the gas disc gradually thinned as it enriched. Such radial and vertical trends might just reflect the evolving properties of the parent gaseous disc, and are not necessarily the result of secular evolutionary processes.

Vertical leaf mass per area gradient of mature sugar maple reflects both height-driven increases in vascular tissue and light-driven increases in palisade layer thickness.

PubMed

Coble, Adam P; Cavaleri, Molly A

2017-10-01

A key trait used in canopy and ecosystem function modeling, leaf mass per area (LMA), is influenced by changes in both leaf thickness and leaf density (LMA = Thickness × Density). In tall trees, LMA is understood to increase with height through two primary mechanisms: (i) increasing palisade layer thickness (and thus leaf thickness) in response to light and/or (ii) reduced cell expansion and intercellular air space in response to hydrostatic constraints, leading to increased leaf density. Our objective was to investigate within-canopy gradients in leaf anatomical traits in order to understand environmental factors that influence leaf morphology in a sugar maple (Acer saccharum Marshall) forest canopy. We teased apart the effects of light and height on anatomical traits by sampling at exposed and closed canopies that had different light conditions at similar heights. As expected, palisade layer thickness responded strongly to cumulative light exposure. Mesophyll porosity, however, was weakly and negatively correlated with light and height (i.e., hydrostatic gradients). Reduced mesophyll porosity was not likely caused by limitations on cell expansion; in fact, epidermal cell width increased with height. Palisade layer thickness was better related to LMA, leaf density and leaf thickness than was mesophyll porosity. Vein diameter and fraction of vascular tissue also increased with height and LMA, density and thickness, revealing that greater investment in vascular and support tissue may be a third mechanism for increased LMA with height. Overall, decreasing mesophyll porosity with height was likely due to palisade cells expanding into the available air space and also greater investments in vascular and support tissue, rather than a reduction of cell expansion due to hydrostatic constraints. Our results provide evidence that light influences both palisade layer thickness and mesophyll porosity and indicate that hydrostatic gradients influence leaf vascular and support tissues in mature Acer saccharum trees. © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Operational Data Reduction Procedure for Determining Density and Vertical Structure of the Martian Upper Atmosphere from Mars Global Surveyor Accelerometer Measurements

NASA Technical Reports Server (NTRS)

Cancro, George J.; Tolson, Robert H.; Keating, Gerald M.

1998-01-01

The success of aerobraking by the Mars Global Surveyor (MGS) spacecraft was partly due to the analysis of MGS accelerometer data. Accelerometer data was used to determine the effect of the atmosphere on each orbit, to characterize the nature of the atmosphere, and to predict the atmosphere for future orbits. To interpret the accelerometer data, a data reduction procedure was developed to produce density estimations utilizing inputs from the spacecraft, the Navigation Team, and pre-mission aerothermodynamic studies. This data reduction procedure was based on the calculation of aerodynamic forces from the accelerometer data by considering acceleration due to gravity gradient, solar pressure, angular motion of the MGS, instrument bias, thruster activity, and a vibration component due to the motion of the damaged solar array. Methods were developed to calculate all of the acceleration components including a 4 degree of freedom dynamics model used to gain a greater understanding of the damaged solar array. The total error inherent to the data reduction procedure was calculated as a function of altitude and density considering contributions from ephemeris errors, errors in force coefficient, and instrument errors due to bias and digitization. Comparing the results from this procedure to the data of other MGS Teams has demonstrated that this procedure can quickly and accurately describe the density and vertical structure of the Martian upper atmosphere.

Chemical vapor deposition of low reflective cobalt (II) oxide films

NASA Astrophysics Data System (ADS)

Amin-Chalhoub, Eliane; Duguet, Thomas; Samélor, Diane; Debieu, Olivier; Ungureanu, Elisabeta; Vahlas, Constantin

2016-01-01

Low reflective CoO coatings are processed by chemical vapor deposition from Co2(CO)8 at temperatures between 120 °C and 190 °C without additional oxygen source. The optical reflectivity in the visible and near infrared regions stems from 2 to 35% depending on deposition temperature. The combination of specific microstructural features of the coatings, namely a fractal ⿿cauliflower⿿ morphology and a grain size distribution more or less covering the near UV and IR wavelength ranges enhance light scattering and gives rise to a low reflectivity. In addition, the columnar morphology results in a density gradient in the vertical direction that we interpret as a refractive index gradient lowering reflectivity further down. The coating formed at 180 °C shows the lowest average reflectivity (2.9%), and presents an interesting deep black diffuse aspect.

Spiral MRI on a 9.4T Vertical-bore Superconducting Magnet Using Unshielded and Self-shielded Gradient Coils

PubMed Central

Kodama, Nao; Setoi, Ayana; Kose, Katsumi

2018-01-01

Spiral MRI sequences were developed for a 9.4T vertical standard bore (54 mm) superconducting magnet using unshielded and self-shielded gradient coils. Clear spiral images with 64-shot scan were obtained with the self-shielded gradient coil, but severe shading artifacts were observed for the spiral-scan images acquired with the unshielded gradient coil. This shading artifact was successfully corrected with a phase-correction technique using reference scans that we developed based on eddy current field measurements. We therefore concluded that spiral imaging sequences can be installed even for unshielded gradient coils if phase corrections are performed using the reference scans. PMID:28367906

Spiral MRI on a 9.4T Vertical-bore Superconducting Magnet Using Unshielded and Self-shielded Gradient Coils.

PubMed

Kodama, Nao; Setoi, Ayana; Kose, Katsumi

2018-04-10

Spiral MRI sequences were developed for a 9.4T vertical standard bore (54 mm) superconducting magnet using unshielded and self-shielded gradient coils. Clear spiral images with 64-shot scan were obtained with the self-shielded gradient coil, but severe shading artifacts were observed for the spiral-scan images acquired with the unshielded gradient coil. This shading artifact was successfully corrected with a phase-correction technique using reference scans that we developed based on eddy current field measurements. We therefore concluded that spiral imaging sequences can be installed even for unshielded gradient coils if phase corrections are performed using the reference scans.

Testing hydrodynamics schemes in galaxy disc simulations

NASA Astrophysics Data System (ADS)

Few, C. G.; Dobbs, C.; Pettitt, A.; Konstandin, L.

2016-08-01

We examine how three fundamentally different numerical hydrodynamics codes follow the evolution of an isothermal galactic disc with an external spiral potential. We compare an adaptive mesh refinement code (RAMSES), a smoothed particle hydrodynamics code (SPHNG), and a volume-discretized mesh-less code (GIZMO). Using standard refinement criteria, we find that RAMSES produces a disc that is less vertically concentrated and does not reach such high densities as the SPHNG or GIZMO runs. The gas surface density in the spiral arms increases at a lower rate for the RAMSES simulations compared to the other codes. There is also a greater degree of substructure in the SPHNG and GIZMO runs and secondary spiral arms are more pronounced. By resolving the Jeans length with a greater number of grid cells, we achieve more similar results to the Lagrangian codes used in this study. Other alterations to the refinement scheme (adding extra levels of refinement and refining based on local density gradients) are less successful in reducing the disparity between RAMSES and SPHNG/GIZMO. Although more similar, SPHNG displays different density distributions and vertical mass profiles to all modes of GIZMO (including the smoothed particle hydrodynamics version). This suggests differences also arise which are not intrinsic to the particular method but rather due to its implementation. The discrepancies between codes (in particular, the densities reached in the spiral arms) could potentially result in differences in the locations and time-scales for gravitational collapse, and therefore impact star formation activity in more complex galaxy disc simulations.

Laboratory-Scale Internal Wave Apparatus for Studying Copepod Behavior

NASA Astrophysics Data System (ADS)

Jung, S.; Webster, D. R.; Haas, K. A.; Yen, J.

2016-02-01

Internal waves are ubiquitous features in coastal marine environments and have been observed to mediate vertical distributions of zooplankton in situ. Internal waves create fine-scale hydrodynamic cues that copepods and other zooplankton are known to sense, such as fluid density gradients and velocity gradients (quantified as shear deformation rate). The role of copepod behavior in response to cues associated with internal waves is largely unknown. The objective is to provide insight to the bio-physical interaction and the role of biological versus physical forcing in mediating organism distributions. We constructed a laboratory-scale internal wave apparatus to facilitate fine-scale observations of copepod behavior in flows that replicate in situ conditions of internal waves in two-layer stratification. Two cases were chosen with density jump of 1 and 1.5 sigma-t units. Analytical analysis of the two-layer system provided guidance to the target forcing frequency needed to generate a standing internal wave with a single dominate frequency of oscillation. Flow visualization and signal processing of the interface location were used to quantify the wave characteristics. The results show a close match to the target wave parameters. Marine copepod (mixed population of Acartia tonsa, Temora longicornis, and Eurytemora affinis) behavior assays were conducted for three different physical arrangements: (1) no density stratification, (2) stagnant two-layer density stratification, and (3) two-layer density stratification with internal wave motion. Digitized trajectories of copepod swimming behavior indicate that in the control (case 1) the animals showed no preferential motion in terms of direction. In the stagnant density jump treatment (case 2) copepods preferentially moved horizontally, parallel to the density interface. In the internal wave treatment (case 3) copepods demonstrated orbital trajectories near the density interface.

Determination of the rate of crystal growth from the gas phase under conditions of turbulent free convection

NASA Astrophysics Data System (ADS)

Alad'Ev, S. I.

1987-04-01

Crystal growth in vertical and horizontal cylindrical vials, with the substrate and the source serving as the vial ends, is investigated analytically, assuming that the medium consists of a binary mixture of an active and an inert gas. The active gas is made up of the gaseous products of reactions taking place at the substrate and at the source. It is shown that turbulent free convection leads to an increase in crystal growth rate. All other conditions being equal, crystal growth in vertical vials is greater than that in horizontal ones; in both cases crystal growth rate increases with the vial radius, temperature gradient in the gas phase, and gas phase density. The results are compared with experimental data on the growth of Ge crystals in the Ge-GeI4 system.

Laboratory Layered Latte

NASA Astrophysics Data System (ADS)

Xue, Nan; Khodaparast, Sepideh; Zhu, Lailai; Nunes, Janine; Kim, Hyoungsoo; Stone, Howard

2017-11-01

Layered composite fluids are sometimes observed in confined systems of rather chaotic initial states, for example, layered lattes formed by pouring espresso into a glass of warm milk. In such configurations, pouring forces a lower density liquid (espresso) into a higher density ambient, which is similar to the fountain effects that characterize a wide range of flows driven by injecting a fluid into a second miscible phase. Although the initial state of the mixture is complex and chaotic, there are conditions where the mixture cools at room temperature and exhibits an organized layered pattern. Here we report controlled experiments injecting a fluid into a miscible phase and show that, above a critical injection velocity, layering naturally emerges over the time scale of minutes. We perform experimental and numerical analyses of the time-dependent flows to observe and understand the convective circulation in the layers. We identify critical conditions to produce the layering and relate the results quantitatively to the critical Rayleigh number in double-diffusive convection, which indicates the competition between the horizontal thermal gradient and the vertical density gradient generated by the fluid injection. Based on this understanding, we show how to employ this single-step process to produce layered structures in soft materials, where the local elastic properties as well as the local material concentration vary step-wise along the length of the material.

Altitude Variation of the Plasmapause Signature in the Main Ionospheric Trough

NASA Technical Reports Server (NTRS)

Grebowsky, Joseph M.; Benson, Robert F.; Webb, Phillip A.; Truhlik, Vladimir; Bilitza, Dieter

2009-01-01

The projection of the plasmapause magnetic-field lines to low altitudes, where the light-ion chemistry is dominated by O(+), tends to occur near the minimum electron density in the main (midlatitude) electron density trough at night. With increasing attitude in the trough, where H(+) emerges as the dominant iota on the low-latitude boundary, we have found cases where the plasmapause field lines are located on the sharp low-Latitude side of the trough as expected if this topside ionosphere H(+) distribution varies in step with the plasmapause gradient in the distant plasmasphere. These conclusions are based on near-equatorial crossings of the plasmapause (corresponding to the steep gradient in the dominant species H(+) by the Explorer-45 satellite as determined from electric-field measurements by Maynard and Cauffman in the early 1970s and ISIS-2 ionospheric topside-sounder measurements. The former data have now been converted to digital form and made available at http://nssdcftp.gsfc.nasa.gov. The latter provide samples of nearly coincident observations of ionospheric main trough crossings near the same magnetic-field lines of the Explorer 45-determined equatorial plasmapause. The ISIS-2 vertical electron density profiles are used to infer where the F-region transitions from an O(+) to a H(+) dominated plasma through the main trough boundaries.

Linking boundary-layer circulations and surface processes during FIFE89. Part 1: Observational analysis

NASA Technical Reports Server (NTRS)

Smith, Eric A.; Wai, Mickey M.-K.; Cooper, Harry J.; Rubes, Michael T.; Hsu, Ann

1994-01-01

Surface, aircraft, and satellite observations are analyzed for the 21-day 1989 intensive field campaign of the First ISLSCP Field Experiment (FIFE) to determine the effect of precipitation, vegetation, and soil moisture distributions on the thermal properties of the surface including the heat and moisture fluxes, and the corresponding response in the boundary-layer circulation. Mean and variance properties of the surface variables are first documented at various time and space scales. These calculations are designed to set the stage for Part 2, a modeling study that will focus on how time-space dependent rainfall distribution influences the intensity of the feedback between a vegetated surface and the atmospheric boundary layer. Further analysis shows strongly demarked vegetation and soil moisture gradients extending across the FIFE experimental site that were developed and maintained by the antecedent and ongoing spatial distribution of rainfall over the region. These gradients are shown to have a pronounced influence on the thermodynamic properties of the surface. Furthermore, perturbation surface wind analysis suggests for both short-term steady-state conditions and long-term averaged conditions that the gradient pattern maintained a diurnally oscillating local direct circulation with perturbation vertical velocities of the same order as developing cumulus clouds. Dynamical and scaling considerations suggest that the embedded perturbation circulation is driven by surface heating/cooling gradients and terrain ef fects rather than the manifestation of an inertial oscillation. The implication is that at even relatively small scales (less than 30 km), the differential evolution in vegetation density and soil moisture distribution over a relatively homogenous ecotone can give rise to preferential boundary-layer circulations capable of modifying local-scale horizontal and vertical motions.

Investigating Summer Thermal Stratification in Lake Ontario

NASA Astrophysics Data System (ADS)

James, S. C.; Arifin, R. R.; Craig, P. M.; Hamlet, A. F.

2017-12-01

Seasonal temperature variations establish strong vertical density gradients (thermoclines) between the epilimnion and hypolimnion. Accurate simulation of vertical mixing and seasonal stratification of large lakes is a crucial element of the thermodynamic coupling between lakes and the atmosphere in integrated models. Time-varying thermal stratification patterns can be accurately simulated with the versatile Environmental Fluid Dynamics Code (EFDC). Lake Ontario bathymetry was interpolated onto a 2-km-resolution curvilinear grid with vertical layering using a new approach in EFDC+, the so-called "sigma-zed" coordinate system which allows the number of vertical layers to be varied based on water depth. Inflow from the Niagara River and outflow to the St. Lawrence River in conjunction with hourly meteorological data from seven local weather stations plus three-hourly data from the North American Regional Reanalysis govern the hydrodynamic and thermodynamic responses of the Lake. EFDC+'s evaporation algorithm was updated to more accurately simulate net surface heat fluxes. A new vertical mixing scheme from Vinçon-Leite that implements different eddy diffusivity formulations above and below the thermocline was compared to results from the original Mellor-Yamada vertical mixing scheme. The model was calibrated by adjusting solar-radiation absorption coefficients in addition to background horizontal and vertical mixing parameters. Model skill was evaluated by comparing measured and simulated vertical temperature profiles at shallow (20 m) and deep (180 m) locations on the Lake. These model improvements, especially the new sigma-zed vertical discretization, accurately capture thermal-stratification patterns with low root-mean-squared errors when using the Vinçon-Leite vertical mixing scheme.

Implementation of Two-Component Time-Dependent Density Functional Theory in TURBOMOLE.

PubMed

Kühn, Michael; Weigend, Florian

2013-12-10

We report the efficient implementation of a two-component time-dependent density functional theory proposed by Wang et al. (Wang, F.; Ziegler, T.; van Lenthe, E.; van Gisbergen, S.; Baerends, E. J. J. Chem. Phys. 2005, 122, 204103) that accounts for spin-orbit effects on excitations of closed-shell systems by employing a noncollinear exchange-correlation kernel. In contrast to the aforementioned implementation, our method is based on two-component effective core potentials as well as Gaussian-type basis functions. It is implemented in the TURBOMOLE program suite for functionals of the local density approximation and the generalized gradient approximation. Accuracy is assessed by comparison of two-component vertical excitation energies of heavy atoms and ions (Cd, Hg, Au(+)) and small molecules (I2, TlH) to other two- and four-component approaches. Efficiency is demonstrated by calculating the electronic spectrum of Au20.

A meridional structure of static stability and ozone vertical gradient around the tropopause in the Southern Hemisphere extratropics

NASA Astrophysics Data System (ADS)

Tomikawa, Y.; Yamanouchi, T.

2010-08-01

An analysis of the static stability and ozone vertical gradient in the ozone tropopause based (OTB) coordinate is applied to the ozonesonde data at 10 stations in the Southern Hemisphere (SH) extratropics. The tropopause inversion layer (TIL) with a static stability maximum just above the tropopause shows similar seasonal variations at two Antarctic stations, which are latitudinally far from each other. Since the sunshine hour varies with time in a quite different way between these two stations, it implies that the radiative heating due to solar ultraviolet absorption of ozone does not contribute to the seasonal variation of the TIL. A meridional section of the static stability in the OTB coordinate shows that the static stability just above the tropopause has a large latitudinal gradient between 60° S and 70° S in austral winter because of the absence of the TIL over the Antarctic. It is accompanied by an increase of westerly shear with height above the tropopause, so that the polar-night jet is formed above this latitude region. This result suggests a close relationship between the absence of the TIL and the stratospheric polar vortex in the Antarctic winter. A vertical gradient of ozone mixing ratio, referred to as ozone vertical gradient, around the tropopause shows similar latitudinal and seasonal variations with the static stability in the SH extratropics. In a height region above the TIL, a small ozone vertical gradient in the midlatitudes associated with the Antarctic ozone hole is observed in a height region of the subvortex but not around the polar vortex. This is a clear evidence of active latitudinal mixing between the midlatitudes and subvortex.

The role of the meridional sea surface temperature gradient in controlling the Caribbean low-level jet

NASA Astrophysics Data System (ADS)

Maldonado, Tito; Rutgersson, Anna; Caballero, Rodrigo; Pausata, Francesco S. R.; Alfaro, Eric; Amador, Jorge

2017-06-01

The Caribbean low-level jet (CLLJ) is an important modulator of regional climate, especially precipitation, in the Caribbean and Central America. Previous work has inferred, due to their semiannual cycle, an association between CLLJ strength and meridional sea surface temperature (SST) gradients in the Caribbean Sea, suggesting that the SST gradients may control the intensity and vertical shear of the CLLJ. In addition, both the horizontal and vertical structure of the jet have been related to topographic effects via interaction with the mountains in Northern South America (NSA), including funneling effects and changes in the meridional geopotential gradient. Here we test these hypotheses, using an atmospheric general circulation model to perform a set of sensitivity experiments to examine the impact of both SST gradients and topography on the CLLJ. In one sensitivity experiment, we remove the meridional SST gradient over the Caribbean Sea and in the other, we flatten the mountains over NSA. Our results show that the SST gradient and topography have little or no impact on the jet intensity, vertical, and horizontal wind shears, contrary to previous works. However, our findings do not discount a possible one-way coupling between the SST and the wind over the Caribbean Sea through friction force. We also examined an alternative approach based on barotropic instability to understand the CLLJ intensity, vertical, and horizontal wind shears. Our results show that the current hypothesis about the CLLJ must be reviewed in order to fully understand the atmospheric dynamics governing the Caribbean region.

Oval gradient coils for an open magnetic resonance imaging system with a vertical magnetic field.

PubMed

Matsuzawa, Koki; Abe, Mitsushi; Kose, Katsumi; Terada, Yasuhiko

2017-05-01

Existing open magnetic resonance imaging (MRI) systems use biplanar gradient coils for the spatial encoding of signals. We propose using novel oval gradient coils for an open vertical-field MRI. We designed oval gradients for a 0.3T open MRI system and showed that such a system could outperform a traditional biplanar gradient system while maintaining adequate gradient homogeneity and subject accessibility. Such oval gradient coils would exhibit high efficiency, low inductance and resistance, and high switching capability. Although the designed oval Y and Z coils showed more heat dissipation and less cooling capability than biplanar coils with the same gap, they showed an efficient heat-dissipation path to the surrounding air, which would alleviate the heat problem. The performance of the designed oval-coil system was demonstrated experimentally by imaging a human hand. Copyright © 2017 Elsevier Inc. All rights reserved.

Demonstration of an Enhanced Vertical Magnetic Gradient System for UXO

DTIC Science & Technology

2008-12-01

fluxgate magnetometers , data recording console, laser altimeter, and acoustic altimeters were tested to ensure proper operation and performance. The VG...Simultaneous Electromagnetic Induction and Magnetometer System WAA wide area assessment ACKNOWLEDGEMENTS We wish to express our sincere...sensors. The benefits of vertical gradient (VG) configurations in magnetometer systems are common knowledge, and these configurations are routinely

Airship stresses due to vertical velocity gradients and atmospheric turbulence

NASA Technical Reports Server (NTRS)

Sheldon, D.

1975-01-01

Munk's potential flow method is used to calculate the resultant moment experienced by an ellipsoidal airship. This method is first used to calculate the moment arising from basic maneuvers considered by early designers, and then expended to calculate the moment arising from vertical velocity gradients and atmospheric turbulence. This resultant moment must be neutralized by the transverse force of the fins. The results show that vertical velocity gradients at a height of 6000 feet in thunderstorms produce a resultant moment approximately three to four times greater than the moment produced in still air by realistic values of pitch angle or steady turning. Realistic values of atmospheric turbulence produce a moment which is significantly less than the moment produced by maneuvers in still air.

Combining Step Gradients and Linear Gradients in Density.

PubMed

Kumar, Ashok A; Walz, Jenna A; Gonidec, Mathieu; Mace, Charles R; Whitesides, George M

2015-06-16

Combining aqueous multiphase systems (AMPS) and magnetic levitation (MagLev) provides a method to produce hybrid gradients in apparent density. AMPS—solutions of different polymers, salts, or surfactants that spontaneously separate into immiscible but predominantly aqueous phases—offer thermodynamically stable steps in density that can be tuned by the concentration of solutes. MagLev—the levitation of diamagnetic objects in a paramagnetic fluid within a magnetic field gradient—can be arranged to provide a near-linear gradient in effective density where the height of a levitating object above the surface of the magnet corresponds to its density; the strength of the gradient in effective density can be tuned by the choice of paramagnetic salt and its concentrations and by the strength and gradient in the magnetic field. Including paramagnetic salts (e.g., MnSO4 or MnCl2) in AMPS, and placing them in a magnetic field gradient, enables their use as media for MagLev. The potential to create large steps in density with AMPS allows separations of objects across a range of densities. The gradients produced by MagLev provide resolution over a continuous range of densities. By combining these approaches, mixtures of objects with large differences in density can be separated and analyzed simultaneously. Using MagLev to add an effective gradient in density also enables tuning the range of densities captured at an interface of an AMPS by simply changing the position of the container in the magnetic field. Further, by creating AMPS in which phases have different concentrations of paramagnetic ions, the phases can provide different resolutions in density. These results suggest that combining steps in density with gradients in density can enable new classes of separations based on density.

The First Results About Earthquake Study with FORMOSAT-3/COSMIC

NASA Astrophysics Data System (ADS)

Liu, J. Y.; Oyama, K.; Jhuang, H. K.; Istep, M.; Hsiao, C. C.; Wang, Y. H.

2007-12-01

To improve the global weather prediction and space weather monitoring, six microsatellites termed the Formosa Satellite 3 - Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) were launched into a circular low-Earth orbit (LEO) from Vandenberg Air Force Base, California, at 0140 UTC on 15 April 2006. Each microsatellite of the joint Taiwan-US satellite constellation mission has a GPS occultation experiment (GOX) payload to operate the atmospheric and ionospheric radio occultation, a tiny ionospheric photometer (TIP) to observe the nighttime ionospheric airglow OI 135.6 nm emission, and a tri-band beacon (TBB) to tomographically estimate fine structures of ionospheric electron density on the satellite-to-receiver plane. While the GOX daily observes about 2500 vertical electron density profiles up to the satellite altitude, the TIP provides accurate horizontal gradients of nighttime electron density. In this study, anomalies in the ionospheric electron density structure and dynamics concurrently observed by FORMOSAT-3/COSMIC and co-located ground- based GPS receivers before recent large earthquakes are presented and discussed.

The Ocean`s Thermohaline Circulation in a Fish Tank

NASA Astrophysics Data System (ADS)

Lavender, K.; Joyce, P.; Graziano, L.; Harris, S.; Jaroslow, G.; Lea, C.; Schell, J.; Witting, J.

2005-12-01

This demonstration develops intuition about density stratification, a concept critical to understanding the ocean`s thermohaline circulation. In addition, students learn how temperature and salinity affect density, how these characteristics may be density-compensating, and students gain practice in graphing and interpreting vertical profiles and temperature-salinity (T-S) diagrams. The demonstration requires a rectangular fish tank (5-10 gallons) with a plexiglass partition, preparation of three colored ''water masses'' representing surface water (warm and fresh), ''mystery'' Mediterranean Water (warm and salty), and North Atlantic Deep Water (NADW; cold and salty), a kitchen sponge, and a temperature and salinity probe. Density may be computed using an Equation of State calculator (e.g. online version at http://fermi.jhuapl.edu/denscalc.html). The larger side of the fish tank is filled halfway with NADW, then surface water is layered on top by carefully pouring it on a floating sponge. A student volunteer measures the temperature and salinity of the two water masses, while another computes the densities. Students draw vertical profiles and T-S diagrams representing the temperature, salinity, and density of the water column. The properties of the ''mystery'' water are measured and students predict what will happen when the water is poured on the opposite side of the partition and is allowed to overflow into the layered water. If the density gradients are sufficiently large, a beautiful internal wave develops as the mystery water overflows the sill and becomes intermediate Mediterranean Water. If time permits, having a student blow on the surface illustrates the limited influence of ''wind'' with depth; an internal wave may by forced by depressing the thermocline with a large, flat spoon; and pouring extra NADW on the sponge floating at the surface may illustrate deep convection.

Predicting nitrogen flux along a vertical canopy gradient in a mixed conifer forest stand of the San Bernardino Mountains in California

Treesearch

Michael J. Arbaugh; Andrzej Bytnerowicz; Mark E. Fenn

1998-01-01

A 3-year study of nitrogenous (N) air pollution deposition to ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) seedlings along a mature tree vertical canopy gradient was conducted in the mixed conifer forest of the San Bernardino Mountains of southern California. Concentrations of nitric acid vapor (HNO3), particulate nitrate...

Environment vs. Plant Ontogeny: Arthropod Herbivory Patterns on European Beech Leaves along the Vertical Gradient of Temperate Forests in Central Germany

PubMed Central

Mantilla-Contreras, Jasmin

2018-01-01

Environmental and leaf trait effects on herbivory are supposed to vary among different feeding guilds. Herbivores also show variability in their preferences for plant ontogenetic stages. Along the vertical forest gradient, environmental conditions change, and trees represent juvenile and adult individuals in the understorey and canopy, respectively. This study was conducted in ten forests sites in Central Germany for the enrichment of canopy research in temperate forests. Arthropod herbivory of different feeding traces was surveyed on leaves of Fagus sylvatica Linnaeus (European beech; Fagaceae) in three strata. Effects of microclimate, leaf traits, and plant ontogenetic stage were analyzed as determining parameters for herbivory. The highest herbivory was caused by exophagous feeding traces. Herbivore attack levels varied along the vertical forest gradient for most feeding traces with distinct patterns. If differences of herbivory levels were present, they only occurred between juvenile and adult F. sylvatica individuals, but not between the lower and upper canopy. In contrast, differences of microclimate and important leaf traits were present between the lower and upper canopy. In conclusion, the plant ontogenetic stage had a stronger effect on herbivory than microclimate or leaf traits along the vertical forest gradient. PMID:29373542

Near-surface Stratification and Submesoscale Fronts in the north Bay of Bengal during Summer Monsoon of 2014 and 2015.

NASA Astrophysics Data System (ADS)

Sengupta, D.; Jarugula, S. L.; D'Asaro, E. A.; Chaudhuri, D.; S, S.; Tandon, A.; M, R.; Lucas, A.; Simmons, H. L.

2016-02-01

The north bay of Bengal is characterised by a shallow layer of fresh water from monsoon rainfall and river discharge, with very strong stratification at its base, and a warm subsurface layer. The thermodynamic structure of the ocean has significant influence on air-sea interaction. We conducted two research cruises of ORV Sagar Nidhi in August-September 2014 and 2015, to study the physical processes that maintain the shallow fresh layer. We collected a total of about 4000 kilometers of underway Conductivity-Temperature-Depth (uCTD) and Acoustic Doppler Current Profiler (ADCP) data. The vertical resolution of the data is 1-2 m; at ship speeds of 4-5 knots, the horizontal resolution is 300-1500 m, sufficient to resolve submesoscale (1-20 km) features. It is known that dynamical instability of submesoscale fronts can lead to slumping of heavier water under lighter water, enhancing vertical stratification. We identified 35 major salinity-dominated near-surface density fronts along the ship track, with surface density gradient exceeding 0.03 kg/m3 per kilometer, and density difference exceeding 0.3 kg/m3. The largest gradients in the open ocean, between fresh water of riverine origin and ambient seawater, exceeded 10 psu in 40 km and 6 psu in 50 km; the spatial scales of the other fronts range from 1 to 25 km. At several submesoscale fronts, the surface mixed layer is shallower directly under the front than on either side, suggesting active restratification. ADCP observations reveal a region of confluence and narrow jets associated with some fronts, consistent with frontal slumping. In addition, wind-driven Ekman transport can enhance near-surface stratification by carrying lighter water over denser water. We discuss the relevance of these two mechanisms in observations and model simulations.

Kinematics and dynamics of Nubia-Somalia divergence along the East African rift

NASA Astrophysics Data System (ADS)

Stamps, Dorothy Sarah

Continental rifting is fundamental to the theory of plate tectonics, yet the force balance driving Earth's largest continental rift system, the East African Rift (EAR), remains debated. The EAR actively diverges the Nubian and Somalian plates spanning ˜5000 km N-S from the Red Sea to the Southwest Indian Ridge and ˜3000 km NW-SE from eastern Congo to eastern Madagascar. Previous studies suggest either lithospheric buoyancy forces or horizontal tractions dominate the force balance acting to rupture East Africa. In this work, we investigate the large-scale dynamics of Nubia-Somalia divergence along the EAR driving present-day kinematics. Because Africa is largely surrounded by spreading ridges, we assume plate-plate interactions are minimal and that the major driving forces are gradients in gravitational potential energy (GPE), which includes the effect of vertical mantle tractions, and horizontal basal tractions arising from viscous coupling to horizontal mantle flow. We quantify a continuous strain rate and velocity field based on kinematic models, an updated GPS velocity solution, and the style of earthquake focal mechanisms, which we use as an observational constraint on surface deformation. We solve the 3D force balance equations and calculate vertically averaged deviatoric stress for a 100 km thick lithosphere constrained by the CRUST2.0 crustal density and thickness model. By comparing vertically integrated deviatoric stress with integrated lithospheric strength we demonstrate forces arising from gradients in gravitational potential energy are insufficient to rupture strong lithosphere, hence weakening mechanisms are required to initiate continental rupture. The next step involves inverting for a stress field boundary condition that is the long-wavelength minimum energy deviatoric stress field required to best-fit the style of our continuous strain rate field in addition to deviatoric stress from gradients in GPE. We infer the stress field boundary condition is an estimate of basal shear stress from viscous coupling to horizontal mantle flow. The stress field boundary condition is small (˜1.6 MPa) compared to deviatoric stress from GPE gradients (8-20 MPa) and does not improve the fit to surface deformation indicators more than 8% when combined with deviatoric stress from GPE gradients. Hence we suggest the style of deformation across the EAR can be explained by forces derived from gradients in GPE. We then calculate dynamic velocities using two types of forward models to solve the instantaneous momentum equations. One method is regional and requires vertically averaged effective viscosity to define lithospheric structure with velocity boundary conditions and a free-slip basal boundary condition. The second is a global model that accounts for a brittle upper crust and viscous mantle lithosphere with velocity boundary conditions imposed at the base of the lithosphere from 5 mantle flow models. With both methods we find deformation driven by internal lithospheric buoyancy forces provides the best-fit to GPS observations of surface velocities on the Somalian plate. We find that any additional contribution from horizontal tractions results in overpredicting surface velocities. This work indicates horizontal mantle flow plays a minimal role in Nubia-Somalia divergence and the EAR is driven largely by gradients in GPE.

On the metallicity gradients of the Galactic disk as revealed by LSS-GAC red clump stars

NASA Astrophysics Data System (ADS)

Huang, Yang; Liu, Xiao-Wei; Zhang, Hua-Wei; Yuan, Hai-Bo; Xiang, Mao-Sheng; Chen, Bing-Qiu; Ren, Juan-Juan; Sun, Ning-Chen; Wang, Chun; Zhang, Yong; Hou, Yong-Hui; Wang, Yue-Fei; Yang, Ming

2015-08-01

Using a sample of over 70 000 red clump (RC) stars with 5%-10% distance accuracy selected from the LAMOST Spectroscopic Survey of the Galactic Anti-center (LSS-GAC), we study the radial and vertical gradients of the Galactic disk(s) mainly in the anti-center direction, covering a significant volume of the disk in the range of projected Galactocentric radius 7 ≤ RGC ≤ 14 kpc and height from the Galactic midplane 0 ≤ |Z| ≤ 3 kpc. Our analysis shows that both the radial and vertical metallicity gradients are negative across much of the volume of the disk that is probed, and they exhibit significant spatial variations. Near the solar circle (7 ≤ RGC ≤ 115 kpc), the radial gradient has a moderately steep, negative slope of -0.08 dex kpc-1 near the midplane (|Z| < 0.1 kpc), and the slope flattens with increasing |Z|. In the outer disk (11.5 < RGC ≤ 14 kpc), the radial gradients have an essentially constant, much less steep slope of -0.01 dex kpc-1 at all heights above the plane, suggesting that the outer disk may have experienced an evolutionary path different from that of the inner disk. The vertical gradients are found to flatten largely with increasing RGC. However, the vertical gradient of the lower disk (0 ≤ |Z| ≤ 1 kpc) is found to flatten with RGC quicker than that of the upper disk (1 < |Z| ≤ 3 kpc). Our results should provide strong constraints on the theory of disk formation and evolution, as well as the underlying physical processes that shape the disk (e.g. gas flows, radial migration, and internal and external perturbations).

Spontaneous Emergence of Order in Vibrated Sand*

NASA Astrophysics Data System (ADS)

Swinney, Harry L.

2004-05-01

Granular media such as sand, pharmaceuticals, cereal, cosmetics, and asteroids are involved in many processes, yet granular media remain less well understood than fluids and solids. Vertically oscillating granular layers provide a test bed for theory and modeling of granular dynamics. Experiments on vertically oscillating granular layers have revealed a variety of spatial patterns that emerge spontaneously as a function of the container acceleration amplitude and frequency: stripes, squares, hexagons, spirals, and oscillons (localized structures). Molecular dynamics simulations yield results in quantitative accord with laboratory observations. Since the gradients of density and velocity are large over a particle mean free path, the applicability of continuum theory has been questionable. However, hydrodynamic equations proposed for dissipative particles yield results in surprising qualitative accord with the laboratory observations. *Work in collaboration with C. Bizon, D. Goldman, W.D. McCormick, S.J. Moon, E. Rericha, M. Shattuck, and J. Swift. Supported by DOE.

Vertical profile of tritium concentration in air during a chronic atmospheric HT release.

PubMed

Noguchi, Hiroshi; Yokoyama, Sumi

2003-03-01

The vertical profiles of tritium gas and tritiated water concentrations in air, which would have an influence on the assessment of tritium doses as well as on the environmental monitoring of tritium, were measured in a chronic tritium gas release experiment performed in Canada in 1994. While both of the profiles were rather uniform during the day because of atmospheric mixing, large gradients of the profiles were observed at night. The gradient coefficients of the profiles were derived from the measurements. Correlations were analyzed between the gradient coefficients and meteorological conditions: solar radiation, wind speed, and turbulent diffusivity. It was found that the solar radiation was highly correlated with the gradient coefficients of tritium gas and tritiated water profiles and that the wind speed and turbulent diffusivity showed weaker correlations with those of tritiated water profiles. A one-dimensional tritium transport model was developed to analyze the vertical diffusion of tritiated water re-emitted from the ground into the atmosphere. The model consists of processes of tritium gas deposition to soil including oxidation into tritiated water, reemission of tritiated water, dilution of tritiated water in soil by rain, and vertical diffusion of tritiated water in the atmosphere. The model accurately represents the accumulation of tritiated water in soil water and the time variations and vertical profiles of tritiated water concentrations in air.

On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

NASA Astrophysics Data System (ADS)

Yellin-Bergovoy, Ron; Heifetz, Eyal; Umurhan, Orkan M.

2016-05-01

The dynamical response of edge waves under the influence of self-gravity is examined in an idealised two-dimensional model of a proto-stellar disc, characterised in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius ?. The fluid in basic state is prescribed to rotate with a Keplerian profile ? modified by some additional azimuthal sheared flow. A linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar Rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. Instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabiliser irrespective of the stratification of the system. The propagation properties of the waves are discussed in detail in the language of vorticity edge waves. The roles of both Boussinesq and non-Boussinesq effects upon the stability and propagation of these waves with and without the inclusion of self-gravity are then quantified. The dynamics involved with self-gravity non-Boussinesq effect is shown to be a source of vorticity production where there is a jump in the basic state density In addition, self-gravity also alters the dynamics via the radial main pressure gradient, which is a Boussinesq effect. Further applications of these mechanical insights are presented in the conclusion including the ways in which multiple density jumps or gaps may or may not be stable.

Method of making a functionally graded material

DOEpatents

Lauf, Robert J.; Menchhofer, Paul A.; Walls, Claudia A.; Moorhead, Arthur J.

2002-01-01

A gelcasting method of making an internally graded article alternatively includes the steps of: preparing a slurry including a least two different phases suspended in a gelcasting solution, the phases characterized by having different settling characteristics; casting the slurry into a mold having a selected shape; allowing the slurry to stand for a sufficient period of time to permit desired gravitational fractionation in order to achieve a vertical compositional gradient in the molded slurry; gelling the slurry to form a solid gel while preserving the vertical compositional gradient in the molded slurry; drying the gel to form a dried green body; and sintering the dry green body to form a solid object, at least one property thereof varying along the vertical direction because of the compositional gradient in the molded slurry.

The effect of bio-irrigation by the polychaete Lanice conchilega on active denitrifiers: Distribution, diversity and composition of nosZ gene

PubMed Central

Yazdani Foshtomi, Maryam; Leliaert, Frederik; Derycke, Sofie; Willems, Anne; Vincx, Magda

2018-01-01

The presence of large densities of the piston-pumping polychaete Lanice conchilega can have important consequences for the functioning of marine sediments. It is considered both an allogenic and an autogenic ecosystem engineer, affecting spatial and temporal biogeochemical gradients (oxygen concentrations, oxygen penetration depth and nutrient concentrations) and physical properties (grain size) of marine sediments, which could affect functional properties of sediment-inhabiting microbial communities. Here we investigated whether density-dependent effects of L. conchilega affected horizontal (m-scale) and vertical (cm-scale) patterns in the distribution, diversity and composition of the typical nosZ gene in the active denitrifying organisms. This gene plays a major role in N2O reduction in coastal ecosystems as the last step completing the denitrification pathway. We showed that both vertical and horizontal composition and richness of nosZ gene were indeed significantly affected when large densities of the bio-irrigator were present. This could be directly related to allogenic ecosystem engineering effects on the environment, reflected in increased oxygen penetration depth and oxygen concentrations in the upper cm of the sediment in high densities of L. conchilega. A higher diversity (Shannon diversity and inverse Simpson) of nosZ observed in patches with high L. conchilega densities (3,185–3,440 ind. m-2) at deeper sediment layers could suggest a downward transport of NO3− to deeper layers resulting from bio-irrigation as well. Hence, our results show the effect of L. conchilega bio-irrigation activity on denitrifying organisms in L. conchilega reefs. PMID:29408934

Seasonal Variation of Submesoscale Flow Features in a Mesoscale Eddy-dominant Region in the East Sea

NASA Astrophysics Data System (ADS)

Chang, Yeon S.; Choi, Byoung-Ju; Park, Young-Gyu

2018-03-01

Seasonal changes in the distribution of submesoscale (SM) flow features were examined using a fine-resolution numerical simulation. The SM flows are expected to be strong where mesoscale (MS) eddies actively develop and also when the mixed layer depth (MLD) is deep due to enhanced baroclinic instability. In the East Sea (ES), MS eddies more actively develop in summer while the MLD is deeper in winter, which provided the motivation to conduct this study to test the effects of MLD and MS eddies on the SM activity in this region. Finite-scale Liapunov exponents and the vertical velocity components were employed to analyze the SM activities. It was found that the SM intensity was marked by seasonality: it is stronger in winter when the mixed layer is deep but weaker in summer - despite the greater eddy kinetic energy. This is because in summer the mixed layer is so thin that there is not enough available potential energy. When the SM activity was quantified based on parameterization, (MLD × density gradient), it was determined that the seasonal variation of MLD plays a more important role than the lateral density gradient variation on SM flow motion in the ES.

Turbulent Mixing in Gravity Currents with Transverse Shear

NASA Astrophysics Data System (ADS)

White, Brian; Helfrich, Karl; Scotti, Alberto

2010-11-01

A parallel flow with horizontal shear and horizontal density gradient undergoes an intensification of the shear by gravitational tilting and stretching, rapidly breaking down into turbulence. Such flows have the potential for substantial mixing in estuaries and the coastal ocean. We present high-resolution numerical results for the mixing efficiency of these flows, which can be viewed as gravity currents with transverse shear, and contrast them with the well-studied case of stably stratified, homogeneous turbulence (uniform vertical density and velocity gradients). For a sheared gravity current, the buoyancy flux, turbulent Reynolds stress, and dissipation are well out of equilibrium. The total kinetic energy first increases as potential energy is transferred to the gravity current, but rapidly decays once turbulence sets in. Despite the non-equilibrium character, mixing efficiencies are slightly higher but qualitatively similar to homogeneous stratified turbulence. Efficiency decreases in the highly energetic regime where the dissipation rate is large compared with viscosity and stratification, ɛ/(νN^2)>100, further declining as turbulence decays and kinetic energy dissipation dominates the buoyancy flux. In general, the mixing rate, parameterized by a turbulent eddy diffusivity, increases with the strength of the transverse shear.

Estimation of the depth to the fresh-water/salt-water interface from vertical head gradients in wells in coastal and island aquifers

NASA Astrophysics Data System (ADS)

Izuka, Scot K.; Gingerich, Stephen B.

An accurate estimate of the depth to the theoretical interface between fresh, water and salt water is critical to estimates of well yields in coastal and island aquifers. The Ghyben-Herzberg relation, which is commonly used to estimate interface depth, can greatly underestimate or overestimate the fresh-water thickness, because it assumes no vertical head gradients and no vertical flow. Estimation of the interface depth needs to consider the vertical head gradients and aquifer anisotropy that may be present. This paper presents a method to calculate vertical head gradients using water-level measurements made during drilling of a partially penetrating well; the gradient is then used to estimate interface depth. Application of the method to a numerically simulated fresh-water/salt-water system shows that the method is most accurate when the gradient is measured in a deeply penetrating well. Even using a shallow well, the method more accurately estimates the interface position than does the Ghyben-Herzberg relation where substantial vertical head gradients exist. Application of the method to field data shows that drilling, collection methods of water-level data, and aquifer inhomogeneities can cause difficulties, but the effects of these difficulties can be minimized. Résumé Une estimation précise de la profondeur de l'interface théorique entre l'eau douce et l'eau salée est un élément critique dans les estimations de rendement des puits dans les aquifères insulaires et littoraux. La relation de Ghyben-Herzberg, qui est habituellement utilisée pour estimer la profondeur de cette interface, peut fortement sous-estimer ou surestimer l'épaisseur de l'eau douce, parce qu'elle suppose l'absence de gradient vertical de charge et d'écoulement vertical. L'estimation de la profondeur de l'interface requiert de prendre en considération les gradients verticaux de charge et l'éventuelle anisotropie de l'aquifère. Cet article propose une méthode de calcul des gradients verticaux de charge à partir des mesures de niveau piézométrique faites en cours de foration d'un puits incomplet; le gradient est alors utilisé pour estimer la profondeur de l'interface. L'application de cette méthode à un système eau douce - eau salée simulé numériquement montre que la méthode est la plus précise lorsque le gradient est mesuré dans un puits pénétrant profondément dans l'aquifère. Même en utilisant un puits peu profond, la méthode estime la position de l'interface avec plus de précision que ne le fait la relation de Ghyben-Herzberg lorsqu'il existe un gradient vertical de charge bien marqué. L'application de la méthode à des données de terrain montre que la foration, les méthodes de mesure de niveau et les hétérogénéités au sein de l'aquifère peuvent être la cause de difficultés, mais que les effets de ces difficultés peuvent être réduits. Resumen Para la estimación de la productividad de pozos en acuíferos costeros y en islas es necesaria una estimación precisa de la profundidad de la interfaz teórica entre agua dulce y agua salada. La relación de Ghyben-Herzberg, usada habitualmente para estimar la profundidad de la interfaz, puede subestimar o sobrestimar el espesor de agua dulce, al asumir la ausencia de flujos y gradientes verticales. La estimación de la profundidad de la interfaz debe considerar tanto estos gradientes verticales, como la posible anisotropía del acuífero. En este artículo se presenta un método para calcular los gradientes verticales de niveles a partir de las medidas obtenidas durante la perforación de un pozo parcialmente penetrante para, a partir de este gradiente, estimar la profundidad de la interfaz. La aplicación del método a un sistema de agua dulce/agua salada simulado numéricamente muestra que el método es más preciso cuando el gradiente se mide en un pozo profundo. Incluso en el caso de un pozo superficial, el método permite una estimación más precisa de la profundidad de la interfaz que la aplicación de la fórmula de Ghyben-Herzberg, en los casos en los que existen gradientes verticales significativos. La aplicación del método a datos reales muestra que la perforación, la recogida de datos de niveles y la heterogeneidad en el acuífero pueden causar dificultades en la aplicación del método, pero que estas pueden minimizarse.

Stabilization of electron-scale turbulence by electron density gradient in national spherical torus experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ruiz Ruiz, J.; White, A. E.; Ren, Y.

2015-12-15

Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k{sub ⊥}ρ{sub e} ≲ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed in NSTX with a high-k microwave scattering system [D. R. Smith et al., Rev. Sci. Instrum. 79, 123501 (2008)]. Here we report on stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, which ismore » shown to be consistent with linear stabilization of ETG modes due to the density gradient using the analytical ETG linear threshold in F. Jenko et al. [Phys. Plasmas 8, 4096 (2001)] and linear gyrokinetic simulations with GS2 [M. Kotschenreuther et al., Comput. Phys. Commun. 88, 128 (1995)]. We also found that the observed power of electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient, suggesting density gradient as a nonlinear stabilizing mechanism. Higher density gradients give rise to lower values of the plasma frame frequency, calculated based on the Doppler shift of the measured density fluctuations. Linear gyrokinetic simulations show that higher values of the electron density gradient reduce the value of the real frequency, in agreement with experimental observation. Nonlinear electron-scale gyrokinetic simulations show that high electron density gradient reduces electron heat flux and stiffness, and increases the ETG nonlinear threshold, consistent with experimental observations.« less

The thermal structure of Titan’s upper atmosphere, I: Temperature profiles from Cassini INMS observations

NASA Astrophysics Data System (ADS)

Snowden, D.; Yelle, R. V.; Cui, J.; Wahlund, J.-E.; Edberg, N. J. T.; Ågren, K.

2013-09-01

We derive vertical temperature profiles from Ion Neutral Mass Spectrometer (INMS) N2 density measurements from 32 Cassini passes. We find that the average temperature of Titan’s thermosphere varies significantly from pass-to-pass between 112 and 175 K. The temperatures from individual temperature profiles also varies considerably, with many passes exhibiting wave-like temperature perturbations and large temperature gradients. Wave-like temperature perturbations have wavelengths between 150 and 420 km and amplitudes between 3% and 22% and vertical wave power spectra of the INMS data and HASI data have a slope between -2 and -3, which is consistent with vertically propagating atmospheric waves. The lack of a strong correlation between temperature and latitude, longitude, solar zenith angle, or local solar time indicates that the thermal structure of Titan’s thermosphere is not primarily determined by the absorption of solar EUV flux. At N2 densities greater than 108 cm-3, Titan’s thermosphere is colder when Titan is observed in Saturn’s magnetospheric lobes compared to Saturn’s plasma sheet as proposed by Westlake et al. (Westlake, J.H. et al. [2011]. J. Geophys. Res. 116, A03318. http://dx.doi.org/10.1029/2010JA016251). This apparent correlation suggests that magnetospheric particle precipitation causes the temperature variability in Titan’s thermosphere; however, at densities smaller than 108 cm-3 the lobe passes are hotter than the plasma sheet passes and we find no correlation between the temperature of Titan’s thermosphere and ionospheric signatures of enhanced particle precipitation, which suggests that the correlation is not indicative of a physical connection. The temperature of Titan’s thermosphere also may have decreased by ∼10 K around mid-2007. Finally, we classify the vertical temperature profiles to show which passes are hot and cold and which passes have the largest temperature variations. In a companion paper (Part II), we estimate the strength of energy sources and sinks in Titan’s thermosphere.

Bed failure induced by internal solitary waves

NASA Astrophysics Data System (ADS)

Rivera-Rosario, Gustavo A.; Diamessis, Peter J.; Jenkins, James T.

2017-07-01

The pressure field inside a porous bed induced by the passage of an Internal Solitary Wave (ISW) of depression is examined using high-accuracy numerical simulations. The velocity and density fields are obtained by solving the Dubreil-Jacotin-Long Equation, for a two-layer, continuously stratified water column. The total wave-induced pressure across the surface of the bed is computed by vertically integrating for the hydrostatic and nonhydrostatic contributions. The bed is assumed to be a continuum composed of either sand or silt, with a small amount of trapped gas. Results show variations in pore-water pressure penetrating deeper into more conductive materials and remaining for a prolonged period after the wave has passed. In order to quantify the potential for failure, the vertical pressure gradient is compared against the buoyant weight of the bed. The pressure gradient exceeds this weight for weakly conductive materials. Failure is further enhanced by a decrease in bed saturation, consistent with studies in surface-wave induced failure. In deeper water, the ISW-induced pressure is stronger, causing failure only for weakly conductive materials. The pressure associated with the free-surface displacement that accompanies ISWs is significant, when the water depth is less than 100 m, but has little influence when it is greater than 100 m, where the hydrostatic pressure due to the pycnocline displacement is much larger. Since the pore-pressure gradient reduces the specific weight of the bed, results show that particles are easier for the flow to suspend, suggesting that pressure contributes to the powerful resuspension events observed in the field.

Observational and numerical evidence for ocean frontogenesis inducing submesoscale processes and impacting biochemistry

NASA Astrophysics Data System (ADS)

Claret, M.; Ruiz, S.; Pascual, A.; Olita, A.; Mahadevan, A.; Tovar, A.; Troupin, C.; Tintore, J.; Capet, A.

2016-02-01

We present the results of ALBOREX, a multi-platform and multi-disciplinary experiment completed in May 2014 as a part of PERSEUS EU funded project. This unique process-oriented experiment in the eastern Alboran Sea (Western Mediterranean) examined mesoscale and submesoscale dynamics at an intense front. The field campaign, conducted during 8 days, included 25 drifters, 2 gliders, 3 Argo floats and one ship (66 CTDs and 500 biochemical samples). The drifters followed coherently an anticyclonic gyre. ADCP data showed consistent patterns with currents up to 1 m/s in the southern part of the domain and Rossby numbers up to 1.5 suggesting significant ageostrophic motion. We show observational evidence for mesoscale frontogenesis produced by the confluence of (fresh) Atlantic Water and the resident (more saline) Mediterranean Water. This confluence resulted in lateral density gradients of the order of 1 kg/m3 in 10 km and associated vertical velocities of about ±20 m/day, diagnosed using the QG Omega equation. However, the vertical velocity is likely underestimated due to unresolved submesoscale processes (<10 km), which are induced by intense mesoscale frontogenesis. In order to assess the role of these submesoscale processes in the frontal vertical transport, a high-resolution Process Ocean Model Study is initialized with hydrographic data (0.5-1 km resolution) from underwater gliders. Numerical results show that observed lateral buoyancy gradients are large enough to trigger submesoscale mixed layer instabilities. The coupling between mesoscale and submesoscale phenomena can explain remarkable subduction events of chlorophyll and oxygen captured by ocean gliders, as well as local increases of primary production.

Density Gradients in Chemistry Teaching

ERIC Educational Resources Information Center

Miller, P. J.

1972-01-01

Outlines experiments in which a density gradient might be used to advantage. A density gradient consists of a column of liquid, the composition and density of which varies along its length. The procedure can be used in analysis of solutions and mixtures and in density measures of solids. (Author/TS)

Biophysical Processes at the Boundary between the Arctic and Subarctic in the Barents Sea

NASA Astrophysics Data System (ADS)

Drinkwater, K.

2016-02-01

The boundary between the Arctic and Subarctic is often in the form of a strong front. Results from field studies on the Polar Front in the Barents Sea separating Arctic and Atlantic waters are presented. The focus is on the physical dynamics of the front and their influence on the structure and function of the associated marine biology from biogeochemistry and nutrient dynamics through plankton ecology up to fish. Data were collected using CTDs, autonomous gliders, microstructure profilers, a Fast Repetition Rate Flourometer (FRRF) and conventional nets for capturing plankton and fish. The Polar Front exhibits strong horizontal gradients in temperature and salinity but weak density gradients owing to density compensation of the water mass characteristics. Intense interleaving of the water masses occurs at the front along isopycnals resulting in large variability in the vertical profiles of the temperature-salinity characteristics. Although there are elevated turbulence levels in the vicinity of the front owing to both current shear and double diffusion, turbulence levels are still relatively weak and not strong enough to create strong vertical mixing. As a result nutrient levels in the near surface layers remain low through the summer following the spring bloom and there is no evidence of greatly enhanced primary production or high phytoplankton biomass in the front. Small zooplankton appear to be more prominent at the front and large zooplankton away from the front. Capelin show a similar distribution with small individuals in the front and larger capelin away from the front, mainly in the Arctic waters. Hypotheses on the relationship between the frontal dynamics and fish distributions are presented. Changes in the frontal position and intensity under climate change will also be discussed.

Effect of the Earth's inner structure on the gravity in definitions of height systems

NASA Astrophysics Data System (ADS)

Tenzer, Robert; Foroughi, Ismael; Pitoňák, Martin; Šprlák, Michal

2017-04-01

In context of the vertical datum unification, the geoid-to-quasi-geoid separation has been of significant interest in recent years, because most of existing local vertical datums are realized in the system of either normal or orthometric heights. Nevertheless, the normal-orthometric heights are still used in many other countries where the normal gravity values along leveling lines were adopted instead of the observed gravity. Whereas the conversion between the orthometric and normal heights is defined by means of the mean gravity disturbances (i.e. differences between the mean values of the actual and normal gravity) along the plumbline within the topography, differences between the normal and normal-orthometric heights can be described by means of the surface gravity disturbances. Since the normal gravity field does not reflect the topographic masses and actual mass density distribution inside the Earth, the definition of gravity represents a principal aspect for a realization of particular vertical datum. To address this issue in this study, we investigate effects of the Earth's inner density structure on the surface and mean gravity disturbances, and discuss their impact on the vertical datum realization. These two gravity field quantities are computed globally with a spectral resolution complete to a spherical harmonic degree 2160 using the global gravity, terrain, ice-thickness, inland bathymetry and crustal structure models. Our results reveal that both, the surface and mean gravity disturbances mostly comprise the gravitational signal of topography and masses distributed below the geoid surface. Moreover, in polar areas, a significant contribution comes from large glaciers. In contrast, the contributions of anomalous density distribution within the topography attributed to major lakes, sediments and bedrock density variations are much less pronounced. We also demonstrate that the mean gravity disturbances within the topography are significantly modified compared to the corresponding surface values mainly due to topographic elevation and terrain geometry as well as the presence of large glaciers in polar regions. Changes of the vertical gravity gradient within the topography attributed to the masses distributed below the geoid (dominated mainly by the isostatic signature and the long-wavelength gravitational signature of deep mantle density heterogeneities) are, on the other hand, relatively small. Despite differences between the normal and normal-orthometric heights could directly be assessed from the surface gravity disturbances only when taken along leveling lines with information about the spirit leveling height differences, our results indicate that differences between these two height systems can be significant.

The effect of vocal fold vertical stiffness gradient on sound production

NASA Astrophysics Data System (ADS)

Geng, Biao; Xue, Qian; Zheng, Xudong

2015-11-01

It is observed in some experimental studies on canine vocal folds (VFs) that the inferior aspect of the vocal fold (VF) is much stiffer than the superior aspect under relatively large strain. Such vertical difference is supposed to promote the convergent-divergent shape during VF vibration and consequently facilitate the production of sound. In this study, we investigate the effect of vertical variation of VF stiffness on sound production using a numerical model. The vertical variation of stiffness is produced by linearly increasing the Young's modulus and shear modulus from the superior to inferior aspects in the cover layer, and its effect on phonation is examined in terms of aerodynamic and acoustic quantities such as flow rate, open quotient, skewness of flow wave form, sound intensity and vocal efficiency. The flow-induced vibration of the VF is solved with a finite element solver coupled with 1D Bernoulli equation, which is further coupled with a digital waveguide model. This study is designed to find out whether it's beneficial to artificially induce the vertical stiffness gradient by certain implanting material in VF restoring surgery, and if it is beneficial, what gradient is the most favorable.

A novel technique for real-time estimation of edge pedestal density gradients via reflectometer time delay data

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zeng, L., E-mail: zeng@fusion.gat.com; Doyle, E. J.; Rhodes, T. L.

2016-11-15

A new model-based technique for fast estimation of the pedestal electron density gradient has been developed. The technique uses ordinary mode polarization profile reflectometer time delay data and does not require direct profile inversion. Because of its simple data processing, the technique can be readily implemented via a Field-Programmable Gate Array, so as to provide a real-time density gradient estimate, suitable for use in plasma control systems such as envisioned for ITER, and possibly for DIII-D and Experimental Advanced Superconducting Tokamak. The method is based on a simple edge plasma model with a linear pedestal density gradient and low scrape-off-layermore » density. By measuring reflectometer time delays for three adjacent frequencies, the pedestal density gradient can be estimated analytically via the new approach. Using existing DIII-D profile reflectometer data, the estimated density gradients obtained from the new technique are found to be in good agreement with the actual density gradients for a number of dynamic DIII-D plasma conditions.« less

Geocoronal imaging with Dynamics Explorer

NASA Technical Reports Server (NTRS)

Rairden, R. L.; Frank, L. A.; Craven, J. D.

1986-01-01

The ultraviolet photometer of the University of Iowa spin-scan auroral imaging instrumentation on board Dynamics Explorer-1 has returned numerous hydrogen Lyman alpha images of the geocorona from altitudes of 570 km to 23,300 km (1.09 R sub E to 4.66 R sub E geocentric radial distance). The hydrogen density gradient is shown by a plot of the zenith intensities throughout this range, which decrease to near celestial background values as the spacecraft approaches apogee. Characterizing the upper geocorona as optically thin (single-scattering), the zenith intensity is converted directly to vertical column density. This approximation loses its validity deeper in the geocorona, where the hydrogen is demonstrated to be optically thick in that there is no Lyman alpha limb brightening. Further study of the geocoronal hydrogen distribution will require computer modeling of the radiative transfer.

Sporadic-E As Observed with Rockets

NASA Technical Reports Server (NTRS)

Seddon, J. Carl

1961-01-01

Data obtained with rockets flown over New Mexico, U.S.A. and Manitoba, Canada have always shown the sporadic-E layer to be a thin layer with a large electron density gradient. The vertical electron density profiles and the horizontal uniformity of the sporadic-E layer are discussed herein. These layers have a strong tendency to form at preferential altitudes separated by approximately 6 km, and a striking correlation exists with wind- shears and magnetic field variations. In two cases where comparisons with ionograms were possible, the minimum frequency of the F-region echoes was found approximately equal to the plasma frequency of the sporadic-E layer reduced by half the gyrofrequency. On the other hand, the maximum frequency of the sporadic-E echoes as noted on ionograms was sometimes as much as 1 to 2 Mc greater than the plasma frequency.

Implications of the nonlinear equation of state for upwelling in the ocean interior

NASA Astrophysics Data System (ADS)

McDougall, Trevor J.; You, Yuzhu

1990-08-01

The nonlinear nature of the in situ density of seawater as a function of the potential temperature, pressure and salinity causes two vertical advection processes (thermobaricity and cabbeling) and also complicates the use of microstructure data to deduce upwelling velocities. Cabbeling and thermobaricity are evaluated and mapped on some neutral surfaces in each of the world's oceans by taking the lateral flux of scalars to be parameterized by a lateral diffusivity. In most of the ocean, these two processes are weak, but where there is a significant epineutral gradient of potential temperature, the downwelling due to cabbeling is quite large. In the Southern ocean, where there is a large slope of the neutral surfaces, thermobaricity causes a larger downwelling velocity than cabbeling, and the two processes together cause a dianeutral velocity of about -2×10-7 m s-1. The complementary roles of vertical mixing and vertical advection in achieving water-mass conversion are demonstrated, since maps of the dianeutral motion caused by vertical mixing are quite different to maps of water-mass conversion caused by the same process. This emphasizes the need to include both vertical advection and vertical mixing in ocean models. The method that is used to infer the upwelling velocity from microstructure dissipation measurements is also significantly affected by the nonlinear nature of the equation of state. The extra term that needs to be included in this method is a strong function of depth, changing sign at a depth of about 1500m.

Tracer Lamination in the Stratosphere: A Global Climatology

NASA Technical Reports Server (NTRS)

Appenzeller, Christof; Holton, James R.

1997-01-01

Vertical soundings of stratospheric ozone often exhibit laminated tracer structures characterized by strong vertical tracer gradients. The change in time of these gradients is used to define a tracer lamination rate. It is shown that this quantity can be calculated by the cross product of the horizontal temperature and horizontal tracer gradients. A climatology based on UARS satellite-borne ozone data and on ozone-like pseudotracer data is presented. Three stratospheric regions with high lamination rates were found: the part of the stratospheric overworld which is influenced by the polar vortex, the part of the lowermost stratosphere which is influenced by the tropopause and a third region in the subtropical lower stratosphere mainly characterized with strong vertical shear. High lamination rates in the stratospheric overworld were absent during summer, whereas in the lowermost stratosphere high lamination rates were found year-round. This is consistent with the occurrence and seasonal variation of the horizontal tracer gradient and vertical shear necessary for tilting the tracer surfaces. During winter, high lamination rates associated with the stratospheric polar vortex are present down to approximately 100 hPa. Several features of the derived climatology are roughly consistent with earlier balloon-borne studies. The patterns in the southern and northern hemisphere are comparable, but details differ as anticipated from a less disturbed and more symmetric southern polar vortex.

Dip and anisotropy effects on flow using a vertically skewed model grid.

PubMed

Hoaglund, John R; Pollard, David

2003-01-01

Darcy flow equations relating vertical and bedding-parallel flow to vertical and bedding-parallel gradient components are derived for a skewed Cartesian grid in a vertical plane, correcting for structural dip given the principal hydraulic conductivities in bedding-parallel and bedding-orthogonal directions. Incorrect-minus-correct flow error results are presented for ranges of structural dip (0 < or = theta < or = 90) and gradient directions (0 < or = phi < or = 360). The equations can be coded into ground water models (e.g., MODFLOW) that can use a skewed Cartesian coordinate system to simulate flow in structural terrain with deformed bedding planes. Models modified with these equations will require input arrays of strike and dip, and a solver that can handle off-diagonal hydraulic conductivity terms.

The impact of spherical symmetry assumption on radio occultation data inversion in the ionosphere: An assessment study

NASA Astrophysics Data System (ADS)

Shaikh, M. M.; Notarpietro, R.; Nava, B.

2014-02-01

'Onion-peeling' is a very common technique used to invert Radio Occultation (RO) data in the ionosphere. Because of the implicit assumption of spherical symmetry for the electron density (N(e)) distribution in the ionosphere, the standard Onion-peeling algorithm could give erroneous concentration values in the retrieved electron density profile. In particular, this happens when strong horizontal ionospheric electron density gradients are present, like for example in the Equatorial Ionization Anomaly (EIA) region during high solar activity periods. In this work, using simulated RO Total Electron Content (TEC) data computed by means of the NeQuick2 ionospheric electron density model and ideal RO geometries, we tried to formulate and evaluate an asymmetry level index for quasi-horizontal TEC observations. The asymmetry index is based on the electron density variation that a signal may experience along its path (satellite to satellite link) in a RO event and is strictly dependent on the occultation geometry (e.g. azimuth of the occultation plane). A very good correlation has been found between the asymmetry index and errors related to the inversion products, in particular those concerning the peak electron density NmF2 estimate and the Vertical TEC (VTEC) evaluation.

Observations of wave-induced pore pressure gradients and bed level response on a surf zone sandbar

NASA Astrophysics Data System (ADS)

Anderson, Dylan; Cox, Dan; Mieras, Ryan; Puleo, Jack A.; Hsu, Tian-Jian

2017-06-01

Horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep, near-breaking waves in the surf zone. A barred beach was constructed in a large-scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar. Horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed. Colocated observations of erosion depth were made during asymmetric wave trials with wave heights between 0.10 and 0.98 m, consistently resulting in onshore sheet flow sediment transport. The pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle, directed predominantly upward under the trough and then rapidly rotating onshore and downward as the wavefront passed. The magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth. Momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore-directed pore pressure gradients, but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds. An expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at nonnegligible values of both forces.