Secular trends and climate drift in coupled ocean-atmosphere general circulation models

NASA Astrophysics Data System (ADS)

Covey, Curt; Gleckler, Peter J.; Phillips, Thomas J.; Bader, David C.

2006-02-01

Coupled ocean-atmosphere general circulation models (coupled GCMs) with interactive sea ice are the primary tool for investigating possible future global warming and numerous other issues in climate science. A long-standing problem with such models is that when different components of the physical climate system are linked together, the simulated climate can drift away from observation unless constrained by ad hoc adjustments to interface fluxes. However, 11 modern coupled GCMs, including three that do not employ flux adjustments, behave much better in this respect than the older generation of models. Surface temperature trends in control run simulations (with external climate forcing such as solar brightness and atmospheric carbon dioxide held constant) are small compared with observed trends, which include 20th century climate change due to both anthropogenic and natural factors. Sea ice changes in the models are dominated by interannual variations. Deep ocean temperature and salinity trends are small enough for model control runs to extend over 1000 simulated years or more, but trends in some regions, most notably the Arctic, differ substantially among the models and may be problematic. Methods used to initialize coupled GCMs can mitigate climate drift but cannot eliminate it. Lengthy "spin-ups" of models, made possible by increasing computer power, are one reason for the improvements this paper documents.

Two-fluid equilibrium transition during multi-pulsing CHI in spherical torus

NASA Astrophysics Data System (ADS)

Kanki, T.; Nagata, M.

2015-11-01

Two-fluid dynamo current drive has been studied to achieve a quasi-steady sustainment and good confinement of spherical torus (ST) plasmas by multi-pulsing CHI (M-CHI) in the HIST device. The density gradient, poloidal flow shear, and radial electric shear enhanced by applying the second CHI pulse is observed around the separatrix in the high field side to cause not only the ExB drift but also the ion diamagnetic drift, leading the two-fluid dynamo. The two-fluid equilibrium transition during the M-CHI in the ST is investigated by modelling the M-CHI in the two-fluid equilibrium calculations. The toroidal magnetic field becomes from a diamagnetic to a paramagnetic profile in the closed flux region due to the increase of the poloidal electron flow velocity in the central open flux column (OFC) region, while the diamagnetic profile is kept in the OFC region. The toroidal ion flow velocity is increased from negative to positive values in the closed flux region due to the increase in the drift velocity and the Hall effect. As the ion diamagnetic drift velocity is changed in the same direction as the ExB drift velocity around the separatrix in the high field side through the negative ion pressure gradient there, the poloidal ion flow velocity is increased in the OFC region, enhancing the flow shear. The radial electric field shear around the separatrix is enhanced due to the strong dependence on the magnetic force through the interaction of toroidal ion flow velocity and axial magnetic field. The density is decreased in the closed flux region according to the generalized Bernoulli law and its negative gradient around the separatrix steepens.

Neoclassical toroidal plasma viscosity in the vicinity of the magnetic axis in tokamaks with broken symmetry

NASA Astrophysics Data System (ADS)

Shaing, K. C.; Lee, H.; Seol, J.; Aydemir, A. Y.

2015-08-01

Theory for neoclassical toroidal plasma viscosity in the low collisionality regime is extended to the vicinity of the magnetic axis in tokamaks with broken symmetry. The toroidal viscosity is induced by particles drifting off the perturbed magnetic surface under the influence of the symmetry breaking magnetic field. In the region away from the magnetic axis, the drift orbit dynamics is governed by the bounce averaged drift kinetic equation in the low collisionality regimes. In the vicinity of the magnetic axis, it is the drift kinetic equation, averaged over the trapped particle orbits, i.e., potato orbits, that governs the drift dynamics. The orbit averaged drift kinetic equation is derived when collision frequency is low enough for trapped particles to complete their potato trajectories. The resultant equation is solved in the 1 /ν regime to obtain transport fluxes and, thus, toroidal plasma viscosity through flux-force relation. Here, ν is the collision frequency. The viscosity does not vanish on the magnetic axis, and has the same scalings as that in the region away from magnetic axis, except that the fraction of bananas is replaced by the fraction of potatoes. It also has a weak radial dependence. Modeling of plasma flow velocity V for the case where the magnetic surfaces are broken is also discussed.

Quantifying Particle Numbers and Mass Flux in Drifting Snow

NASA Astrophysics Data System (ADS)

Crivelli, Philip; Paterna, Enrico; Horender, Stefan; Lehning, Michael

2016-12-01

We compare two of the most common methods of quantifying mass flux, particle numbers and particle-size distribution for drifting snow events, the snow-particle counter (SPC), a laser-diode-based particle detector, and particle tracking velocimetry based on digital shadowgraphic imaging. The two methods were correlated for mass flux and particle number flux. For the SPC measurements, the device was calibrated by the manufacturer beforehand. The shadowgrapic imaging method measures particle size and velocity directly from consecutive images, and before each new test the image pixel length is newly calibrated. A calibration study with artificially scattered sand particles and glass beads provides suitable settings for the shadowgraphical imaging as well as obtaining a first correlation of the two methods in a controlled environment. In addition, using snow collected in trays during snowfall, several experiments were performed to observe drifting snow events in a cold wind tunnel. The results demonstrate a high correlation between the mass flux obtained for the calibration studies (r ≥slant 0.93) and good correlation for the drifting snow experiments (r ≥slant 0.81). The impact of measurement settings is discussed in order to reliably quantify particle numbers and mass flux in drifting snow. The study was designed and performed to optimize the settings of the digital shadowgraphic imaging system for both the acquisition and the processing of particles in a drifting snow event. Our results suggest that these optimal settings can be transferred to different imaging set-ups to investigate sediment transport processes.

Three-dimensional ring current decay model

NASA Technical Reports Server (NTRS)

Fok, Mei-Ching; Moore, Thomas E.; Kozyra, Janet U.; Ho, George C.; Hamilton, Douglas C.

1995-01-01

This work is an extension of a previous ring current decay model. In the previous work, a two-dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three-dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawnside and duskside of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H(+) fluxes at tens of keV, which are always overestimated. A newly invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm time ring current, such as day-night asymmetry during injection and drift hole on the dayside at low energies (less than 10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, J(sub o)(1 + Ay(sup n)), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (less than 30 keV), both drift dispersion and charge exchange are important in determining n.

A three-dimensional ring current decay model

NASA Technical Reports Server (NTRS)

Fok, Mei-Ching; Moore, Thomas E.; Kozyra, Janet U.; Ho, George C.; Hamilton, Douglas C.

1994-01-01

This work is an extension of a previous ring current decay model. In the previous work, a two-dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three-dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawn and dusk sides of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H+ fluxes at tens of keV, which are always over-estimated. A newly-invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm-time ring current, such as day-night asymmetry during injection and drift hole on the dayside at low energies (less than 10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, j(sub o)(1+Ay(exp n)), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (less than 30 keV), both drift dispersion and charge exchange are important in determining n.

Energetic-particle drift motions in the outer dayside magnetosphere

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

Buck, R.C.

1987-01-01

Models of the geomagnetic field predict that within a distance of approximately one earth radius inside the dayside magnetopause, magnetic fields produced by the Chapman-Ferraro magnetopause currents create high-latitude minimum-B pockets in the geomagnetic field. These pockets are theoretically capable of temporarily trapping azimuthally-drifting electrons and modifying electron directional distributions. The Lawrence Livermore National Laboratory's scanning electron spectrometer aboard the OGO-5 satellite provided detailed energetic (E > 70 keV) electron pitch-angle distributions throughout the magnetosphere. Distributions obtained in the outer dayside magnetosphere over a wide range of longitudes show unusual flux features. This study analyzes drift-shell branching caused by themore » minimum-B pockets, and interprets the observed flux features in terms of an adiabatic-shell branching and rejoining process. The author examines the shell-branching process for a static field in detail, using the Choe-Beard 1974 magnetospheric magnetic field mode. He finds that shell branching and rejoining conserves the particle mirror field B/sub M/, the fieldline integral invariant I, and the directional electron flux j. He also finds a good correlation between the itch angles that mark the transition from branched to unbranched shells in the model and the distinctive features of the OGO-5 distributions.« less

Spray droplet size, drift potential, and risks to nontarget organisms from aerially applied glyphosate for coca control in Colombia.

PubMed

Hewitt, Andrew J; Solomon, Keith R; Marshall, E J P

2009-01-01

A wind tunnel atomization study was conducted to measure the emission droplet size spectra for water and Glyphos (a glyphosate formulation sold in Colombia) + Cosmo-flux sprays for aerial application to control coca and poppy crops in Colombia. The droplet size spectra were measured in a wind tunnel for an Accu-Flo nozzle (with 16 size 0.085 [2.16 mm] orifices), under appropriate simulated aircraft speeds (up to 333 km/h), using a laser diffraction instrument covering a dynamic size range for droplets of 0.5 to 3,500 microm. The spray drift potential of the glyphosate was modeled using the AGDISP spray application and drift model, using input parameters representative of those occurring in Colombia for typical aerial application operations. The droplet size spectra for tank mixes containing glyphosate and Cosmo-Flux were considerably finer than water and became finer with higher aircraft speeds. The tank mix with 44% glyphosate had a D(v0.5) of 128 microm, while the value at the 4.9% glyphosate rate was 140 microm. These are classified as very fine to fine sprays. Despite being relatively fine, modeling showed that the droplets would not evaporate as rapidly as most similarly sized agricultural sprays because the nonvolatile proportion of the tank mix (active and inert adjuvant ingredients) was large. Thus, longer range drift is small and most drift that does occur will deposit relatively close to the application area. Drift will only occur downwind and, with winds of velocity less than the modeled maximum of 9 km/h, the drift distance would be substantially reduced. Spray drift potential might be additionally reduced through various practices such as the selection of nozzles, tank mix adjuvants, aircraft speeds, and spray pressures that would produce coarser sprays. Species sensitivity distributions to glyphosate were constructed for plants and amphibians. Based on modeled drift and 5th centile concentrations, appropriate no-spray buffer zones (distance from the end of the spray boom as recorded electronically +/-5%) for protection of sensitive plants were 50-120 m for coca spray scenarios and considerably lower for poppy spray scenarios. The equivalent buffer zone for amphibia was 5 m. The low toxicity of glyphosate to humans suggests that these aerial applications are not a concern for human health.

Drift effects on the tokamak power scrape-off width

NASA Astrophysics Data System (ADS)

Meier, E. T.; Goldston, R. J.; Kaveeva, E. G.; Mordijck, S.; Rozhansky, V. A.; Senichenkov, I. Yu.; Voskoboynikov, S. P.

2015-11-01

Recent experimental analysis suggests that the scrape-off layer (SOL) heat flux width (λq) for ITER will be near 1 mm, sharply narrowing the planned operating window. In this work, motivated by the heuristic drift (HD) model, which predicts the observed inverse plasma current scaling, SOLPS-ITER is used to explore drift effects on λq. Modeling focuses on an H-mode DIII-D discharge. In initial results, target recycling is set to 90%, resulting in sheath-limited SOL conditions. SOL particle diffusivity (DSOL) is varied from 0.1 to 1 m2/s. When drifts are included, λq is insensitive to DSOL, consistent with the HD model, with λq near 3 mm; in no-drift cases, λq varies from 2 to 5 mm. Drift effects depress near-separatrix potential, generating a channel of strong electron heat convection that is insensitive to DSOL. Sensitivities to thermal diffusivities, plasma current, toroidal magnetic field, and device size are also assessed. These initial results will be discussed in detail, and progress toward modeling experimentally relevant high-recycling conditions will be reported. Supported by U.S. DOE Contract DE-SC0010434.

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC)

NASA Astrophysics Data System (ADS)

Nicolaus, M.; Rex, M.; Dethloff, K.; Shupe, M.; Sommerfeld, A.

2016-12-01

The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) is a key international flagship initiative under the auspices of the International Arctic Science Committee (IASC). The main aim of MOSAiC is to improve our understanding of the functioning of the Arctic coupled system with a complex interplay between processes in the atmosphere, ocean, sea ice and ecosystem coupled through bio-geochemical interactions. The main objective of MOSAiC is to develop a better understanding of these important coupled-system processes so they can be more accurately represented in regional- and global-scale weather- and climate models. Observations covering a full annual cycle over the Arctic Ocean of many critical parameters such as cloud properties, surface energy fluxes, atmospheric aerosols, small-scale sea-ice and oceanic processes, biological feedbacks with the sea-ice ice and ocean, and others have never been made in the central Arctic in all seasons, and certainly not in a coupled system fashion. The main scientific goals focus on data assimilation for numerical weather prediction models, improved sea ice forecasts and climate models, ground truth for satellite remote sensing, energy budget and fluxes through interfaces, sources, sinks and cycles of chemical species, boundary layer processes, habitat conditions and primary productivity and stakeholder services. The MOSAiC Observatory will be deployed in, and drift with, the Arctic sea-ice pack for a full annual cycle, starting in fall 2019 and ending in fall 2020. Initial drift plans are to start in the newly forming fall sea-ice in the East Siberian Sea and follow the Transpolar Drift. The German Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research will made a huge contribution with the icebreaker Polarstern to serve as the central drifting observatory for this year long drift, and the US Department of Energy has committed a comprehensive atmospheric measurement suite. Many other nations and agencies have expressed interest in participation and in gaining access to this unprecedented observational dataset.

Anomalous pinch of turbulent plasmas driven by the magnetic-drift-induced Lorentz force through the Stokes-Einstein relation

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

Wang, Shaojie, E-mail: wangsj@ustc.edu.cn

It is found that the Lorentz force generated by the magnetic drift drives a generic plasma pinch flux of particle, energy and momentum through the Stokes-Einstein relation. The proposed theoretical model applies for both electrons and ions, trapped particles, and passing particles. An anomalous parallel current pinch due to the electrostatic turbulence with long parallel wave-length is predicted.

Investigation of surface boundary conditions for continuum modeling of RF plasmas

NASA Astrophysics Data System (ADS)

Wilson, A.; Shotorban, B.

2018-05-01

This work was motivated by a lacking general consensus in the exact form of the boundary conditions (BCs) required on the solid surfaces for the continuum modeling of Radiofrequency (RF) plasmas. Various kinds of number and energy density BCs on solid surfaces were surveyed, and how they interacted with the electric potential BC to affect the plasma was examined in two fundamental RF plasma reactor configurations. A second-order local mean energy approximation with equations governing the electron and ion number densities and the electron energy density was used to model the plasmas. Zero densities and various combinations of drift, diffusion, and thermal fluxes were considered to set up BCs. It was shown that the choice of BC can have a significant impact on the sheath and bulk plasma. The thermal and diffusion fluxes to the surface were found to be important. A pure drift BC for dielectric walls failed to produce a sheath.

A tale of two theories: How the adiabatic response and ULF waves affect relativistic electrons

NASA Astrophysics Data System (ADS)

Green, J. C.; Kivelson, M. G.

2001-11-01

Using data from the Comprehensive Energetic Particle and Pitch Angle Distribution (CEPPAD)-High Sensitivity Telescope (HIST) instrument on the Polar spacecraft and ground magnetometer data from the 210 meridian magnetometer chain, we test the ULF wave drift resonance theory proposed to explain relativistic electron phase space density enhancements. We begin by investigating changes in electron flux due to the ``Dst effect.'' The Dst effect refers to the adiabatic response of relativistic electrons to changes in the magnetic field characterized by the Dst index. The Dst effect, assuming no loss or addition of new electrons, produces reversible order of magnitude changes in relativistic electrons flux measured at fixed energy, but it cannot account for the flux enhancement that occurs in the recovery phase of most storms. Liouville's theorem states that phase space density expressed in terms of constant adiabatic invariants is unaffected by adiabatic field changes and thus is insensitive to the Dst effect. It is therefore useful to express flux measurements in terms of phase space densities at constant first, second and third adiabatic invariants. The phase space density is determined from the CEPPAD-HIST electron detector that measures differential directional flux of electrons from 0.7 to 9 MeV and the Tsyganenko 96 field model. The analysis is done for January to June 1997. The ULF wave drift resonance theory that we test proposes that relativistic electrons are accelerated by an m=2 toroidal or poloidal mode wave whose frequency equals the drift frequency of the electron. The theory is tested by comparing the relativistic electron phase space densities to wave power determined at three ground stations with L* values of 4.0, 5.7 and 6.2. Comparison of the wave data to the phase space densities shows that five out of nine storm events are consistent with the ULF wave drift resonance mechanism, three out of nine give ambiguous support to the model, and one event has high ULF wave power at the drift frequency of the electrons but no corresponding phase space density enhancement suggesting that ULF wave power alone is not sufficient to cause an electron response. Two explanations of the anomalous event are investigated including excessive loss of electrons to the magnetopause and wave duration.

Simulations of Plasmasheet Electrons in a Model Magnetosphere with AMIE Potentials: Implications for Diffuse Aurora

NASA Astrophysics Data System (ADS)

Chen, M. W.; Schulz, M.; Lu, G.

2001-12-01

We obtain distributions of precipitating electrons by tracing drift shells of plasmasheet electrons in the limit of strong pitch angle diffusion in Dungey's model magnetosphere, which consists of a dipolar magnetic field plus a uniform southward field. Under strong pitch-angle diffusion particles drift so as to conserve an adiabatic invariant Λ equal to the enclosed phase-space volume (i.e., the cube of the particle momentum p times the occupied flux-tube volume per unit magnetic flux). In the past we applied a quiescent Stern-Volland electric-field model with a cross-tail potential drop of 25 kV and added to it a storm-associated Brice-Nishida cross-magnetospheric electric field with impulses to represent substorm effects. For the present study we use the more realistic Assimilative Model of Ionospheric Electrodynamics (AMIE). We use an analytical expansion to express the AMIE ionospheric potential as a function of latitude and magnetic local time. We map this AMIE potential to latitudes >= 50^o to magnetospheric field lines with (L \\ge 2.5) in Dungey's magnetic field model. We trace the bounce-averaged drift motion of representative plasmasheet electrons for values of \\Lambda corresponding to energies of 0.25-64 keV on field lines of equatorial radial distance r = 6 R_E (L = 5.7), which maps to \\approx 65^o$ latitude in the ionosphere. We use the simulation results to map stormtime phase space distributions taking into account loss due to precipitation. We consider 2 models of electron scattering: (1) the limit of strong scattering everywhere, and (2) an MLT-dependent scattering that is less than everywhere strong in the plasma sheet. From the phase space distributions we calculate the total precipitating electron energy flux into the ionosphere. For this study we focus on the October 19, 1998, storm. We compare qualitatively the simulated energy flux with X-ray intensity from Polar/PIXIE images during this storm.

Progress towards modeling tokamak boundary plasma turbulence and understanding its role in setting divertor heat flux widths

NASA Astrophysics Data System (ADS)

Chen, Bin

2017-10-01

QCMs (quasi-coherent modes) are well characterized in the edge of Alcator C-Mod, when operating in the Enhanced Dα (EDA) H-mode, a promising alternative regime for ELM (edge localized modes) suppressed operation. To improve the understanding of the physics behind the QCMs, three typical C-Mod EDA H-Mode discharges are simulated by BOUT + + using a six-field two-fluid model (based on the Braginskii equations). The simulated characteristics of the frequency versus wave number spectra of the modes is in reasonable agreement with phase contrast imaging data. The key simulation results are: 1) Linear spectrum analysis and the nonlinear phase relationship indicate the dominance of resistive-ballooning modes and drift-Alfven wave instabilities; 2) QCMs originate inside the separatrix; (3) magnetic flutter causes the mode spreading into the SOL; 4) the boundary electric field Er changes the turbulent characteristics of the QCMs and controls edge transport and the divertor heat flux width; 5) the magnitude of the divertor heat flux depends on the physics models, such as sources and sinks, sheath boundary conditions, and parallel heat flux limiting coefficient. The BOUT + + simulations have also been performed for inter-ELM periods of DIII-D and EAST discharges, and similar quasi-coherent modes have been found. The parallel electron heat fluxes projected onto the target from these BOUT + + simulations follow the experimental heat flux width scaling, in particular the inverse dependence of the width on the poloidal magnetic field with an outlier. Further turbulence statistics analysis shows that the blobs are generated near the pedestal peak gradient region inside the separatrix and contribute to the transport of the particle and heat in the SOL region. To understand the Goldston heuristic drift-based model, results will also be presented from self-consistent transport simulations with the electric and magnetic drifts in BOUT + + and with the sheath potential included in the SOL. Work supported by LLNL under Contract DE-AC52-07NA27344. This work was also supported by US DOE Grant DE-FC02-99ER54512, using Alcator C-Mod, a DOE Office of Science User Facility, and under the auspices of the CSC (No. 201506340019).

Short-Term Forecasting of Radiation Belt and Ring Current

NASA Technical Reports Server (NTRS)

Fok, Mei-Ching

2007-01-01

A computer program implements a mathematical model of the radiation-belt and ring-current plasmas resulting from interactions between the solar wind and the Earth s magnetic field, for the purpose of predicting fluxes of energetic electrons (10 keV to 5 MeV) and protons (10 keV to 1 MeV), which are hazardous to humans and spacecraft. Given solar-wind and interplanetary-magnetic-field data as inputs, the program solves the convection-diffusion equations of plasma distribution functions in the range of 2 to 10 Earth radii. Phenomena represented in the model include particle drifts resulting from the gradient and curvature of the magnetic field; electric fields associated with the rotation of the Earth, convection, and temporal variation of the magnetic field; and losses along particle-drift paths. The model can readily accommodate new magnetic- and electric-field submodels and new information regarding physical processes that drive the radiation-belt and ring-current plasmas. Despite the complexity of the model, the program can be run in real time on ordinary computers. At present, the program can calculate present electron and proton fluxes; after further development, it should be able to predict the fluxes 24 hours in advance

Analysis of drift effects on the tokamak power scrape-off width using SOLPS-ITER

NASA Astrophysics Data System (ADS)

Meier, E. T.; Goldston, R. J.; Kaveeva, E. G.; Makowski, M. A.; Mordijck, S.; Rozhansky, V. A.; Senichenkov, I. Yu; Voskoboynikov, S. P.

2016-12-01

SOLPS-ITER, a comprehensive 2D scrape-off layer modeling package, is used to examine the physical mechanisms that set the scrape-off width ({λq} ) for inter-ELM power exhaust. Guided by Goldston’s heuristic drift (HD) model, which shows remarkable quantitative agreement with experimental data, this research examines drift effects on {λq} in a DIII-D H-mode magnetic equilibrium. As a numerical expedient, a low target recycling coefficient of 0.9 is used in the simulations, resulting in outer target plasma that is sheath limited instead of conduction limited as in the experiment. Scrape-off layer (SOL) particle diffusivity (D SOL) is scanned from 1 to 0.1 m2 s-1. Across this diffusivity range, outer divertor heat flux is dominated by a narrow (˜3-4 mm when mapped to the outer midplane) electron convection channel associated with thermoelectric current through the SOL from outer to inner divertor. An order-unity up-down ion pressure asymmetry allows net ion drift flux across the separatrix, facilitated by an artificial mechanism that mimics the anomalous electron transport required for overall ambipolarity in the HD model. At {{D}\\text{SOL}}=0.1 m2 s-1, the density fall-off length is similar to the electron temperature fall-off length, as predicted by the HD model and as seen experimentally. This research represents a step toward a deeper understanding of the power scrape-off width, and serves as a basis for extending fluid modeling to more experimentally relevant, high-collisionality regimes.

Analysis of drift effects on the tokamak power scrape-off width using SOLPS-ITER

DOE PAGES

Meier, E. T.; Goldston, R. J.; Kaveeva, E. G.; ...

2016-11-02

SOLPS-ITER, a comprehensive 2D scrape-off layer modeling package, is used to examine the physical mechanisms that set the scrape-off width (more » $${{\\lambda}_{q}}$$ ) for inter-ELM power exhaust. Guided by Goldston's heuristic drift (HD) model, which shows remarkable quantitative agreement with experimental data, this research examines drift effects on $${{\\lambda}_{q}}$$ in a DIII-D H-mode magnetic equilibrium. As a numerical expedient, a low target recycling coefficient of 0.9 is used in the simulations, resulting in outer target plasma that is sheath limited instead of conduction limited as in the experiment. Scrape-off layer (SOL) particle diffusivity (D SOL) is scanned from 1 to 0.1 m2 s –1. Across this diffusivity range, outer divertor heat flux is dominated by a narrow (~3–4mm when mapped to the outer midplane) electron convection channel associated with thermoelectric current through the SOL from outer to inner divertor. An order-unity up–down ion pressure asymmetry allows net ion drift flux across the separatrix, facilitated by an artificial mechanism that mimics the anomalous electron transport required for overall ambipolarity in the HD model. At $${{D}_{\\text{SOL}}}=0.1$$ m2 s –1, the density fall-off length is similar to the electron temperature fall-off length, as predicted by the HD model and as seen experimentally. Furthermore, this research represents a step toward a deeper understanding of the power scrape-off width, and serves as a basis for extending fluid modeling to more experimentally relevant, high-collisionality regimes.« less

Large Eddy Simulation of Heat Entrainment Under Arctic Sea Ice

NASA Astrophysics Data System (ADS)

Ramudu, Eshwan; Gelderloos, Renske; Yang, Di; Meneveau, Charles; Gnanadesikan, Anand

2018-01-01

Arctic sea ice has declined rapidly in recent decades. The faster than projected retreat suggests that free-running large-scale climate models may not be accurately representing some key processes. The small-scale turbulent entrainment of heat from the mixed layer could be one such process. To better understand this mechanism, we model the Arctic Ocean's Canada Basin, which is characterized by a perennial anomalously warm Pacific Summer Water (PSW) layer residing at the base of the mixed layer and a summertime Near-Surface Temperature Maximum (NSTM) within the mixed layer trapping heat from solar radiation. We use large eddy simulation (LES) to investigate heat entrainment for different ice-drift velocities and different initial temperature profiles. The value of LES is that the resolved turbulent fluxes are greater than the subgrid-scale fluxes for most of our parameter space. The results show that the presence of the NSTM enhances heat entrainment from the mixed layer. Additionally there is no PSW heat entrained under the parameter space considered. We propose a scaling law for the ocean-to-ice heat flux which depends on the initial temperature anomaly in the NSTM layer and the ice-drift velocity. A case study of "The Great Arctic Cyclone of 2012" gives a turbulent heat flux from the mixed layer that is approximately 70% of the total ocean-to-ice heat flux estimated from the PIOMAS model often used for short-term predictions. Present results highlight the need for large-scale climate models to account for the NSTM layer.

Combined effects of drift waves and neoclassical transport on density profiles in tokamaks

NASA Astrophysics Data System (ADS)

Houlberg, W. A.; Strand, P.

2005-10-01

The relative importance of neoclassical and anomalous particle transport depends on the charge number of the species being studied. The detailed particle balance including the EDWM [1] drift wave model for anomalous transport that includes ITG, TEM and in some cases ETG modes, and the neoclassical model NCLASS [2], are illustrated by simulations with the DEA particle transport code. DEA models the evolution of all ion species, and can be run in a mode to evaluate dynamic responses to perturbations or to conditions far from equilibrium by perturbing the profiles from the experimental measurements. The perturbations allow the fluxes to be decomposed into diffusive and convective (pinch) terms. The different scaling with charge number between drift wave and neoclassical models favors a stronger component of neoclassical transport for higher Z impurities through the effective pinch term. Although trace impurities illustrate a simple Ficks Law form, the main ions as well as higher concentrations of intrinsic impurities exhibit non-linear responses to the density gradients as well as off-diagonal gradient dependencies, leading to a more complicated response for the particle fluxes.[1] H. Nordman, et al., Plasma Phys. Control. Fusion 47 (2005) L11. [2] W.A. Houlberg, et al., Phys. Plasmas 4 (1997) 3230.

Scaling for the SOL/separatrix χ ⊥ following from the heuristic drift model for the power scrape-off layer width

NASA Astrophysics Data System (ADS)

Huber, A.; Chankin, A. V.

2017-06-01

A simple two-point representation of the tokamak scrape-off layer (SOL) in the conduction limited regime, based on the parallel and perpendicular energy balance equations in combination with the heat flux width predicted by a heuristic drift-based model, was used to derive a scaling for the cross-field thermal diffusivity {χ }\\perp . For fixed plasma shape and neglecting weak power dependence indexes 1/8, the scaling {χ }\\perp \\propto {P}{{S}{{O}}{{L}}}/(n{B}θ {R}2) is derived.

Well logging interpretation of production profile in horizontal oil-water two phase flow pipes

NASA Astrophysics Data System (ADS)

Zhai, Lu-Sheng; Jin, Ning-De; Gao, Zhong-Ke; Zheng, Xi-Ke

2012-03-01

Due to the complicated distribution of local velocity and local phase hold up along the radial direction of pipe in horizontal oil-water two phase flow, it is difficult to measure the total flow rate and phase volume fraction. In this study, we carried out dynamic experiment in horizontal oil-water two phases flow simulation well by using combination measurement system including turbine flowmeter with petal type concentrating diverter, conductance sensor and flowpassing capacitance sensor. According to the response resolution ability of the conductance and capacitance sensor in different range of total flow rate and water-cut, we use drift flux model and statistical model to predict the partial phase flow rate, respectively. The results indicate that the variable coefficient drift flux model can self-adaptively tone the model parameter according to the oil-water two phase flow characteristic, and the prediction result of partial phase flow rate of oil-water two phase flow is of high accuracy.

An unstaggered central scheme on nonuniform grids for the simulation of a compressible two-phase flow model

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

Touma, Rony; Zeidan, Dia

In this paper we extend a central finite volume method on nonuniform grids to the case of drift-flux two-phase flow problems. The numerical base scheme is an unstaggered, non oscillatory, second-order accurate finite volume scheme that evolves a piecewise linear numerical solution on a single grid and uses dual cells intermediately while updating the numerical solution to avoid the resolution of the Riemann problems arising at the cell interfaces. We then apply the numerical scheme and solve a classical drift-flux problem. The obtained results are in good agreement with corresponding ones appearing in the recent literature, thus confirming the potentialmore » of the proposed scheme.« less

Electron transport fluxes in potato plateau regime

NASA Astrophysics Data System (ADS)

Shaing, K. C.; Hazeltine, R. D.

1997-12-01

Electron transport fluxes in the potato plateau regime are calculated from the solutions of the drift kinetic equation and fluid equations. It is found that the bootstrap current density remains finite in the region close to the magnetic axis, although it decreases with increasing collision frequency. This finite amount of the bootstrap current in the relatively collisional regime is important in modeling tokamak startup with 100% bootstrap current.

Anomalous fluxes in the plateau regime for a weakly turbulent, magnetically confined plasma

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

Balescu, R.

1990-09-01

The anomalous particle and heat fluxes, together with the parallel electric current, are determined for a confined plasma in the plateau regime in the presence of weak electrostatic drift-wave turbulence. Proper account is taken of nonstationarity and of the finite ion Larmor radius (FLR). The quasineutrality of the drift-wave fluctuations imposes a consistency condition, by which the evaluation of the anomalous fluxes is closely related to the drift-wave dispersion equation. On the other hand, these fluxes are related to the thermodynamic forces via the poloidal fluxes. For the weak turbulence approximation considered here, a unified formulation of the anomalous transportmore » problem has been obtained, including all aspects of neoclassical theory. The complete set of transport coefficients is calculated and various relations between them are exhibited. It clearly appears, for instance, that the anomalous ion heat flux is a pure FLR effect that vanishes as the Larmor radius goes to zero. The Onsager symmetry is broken for anomalous transport. The Appendix is devoted to a general discussion of the concept of heat flux in turbulent plasmas.« less

Unusual behavior of quiet-time zonal and vertical plasma drift velocities over Jicamarca during the recent extended solar minimum of 2008

NASA Astrophysics Data System (ADS)

Santos, Ângela M.; Abdu, Mangalathayil A.; Souza, Jonas R.; Batista, Inez S.; Sobral, José H. A.

2017-11-01

The influence of the recent deep and prolonged solar minimum on the daytime zonal and vertical plasma drift velocities during quiet time is investigated in this work. Analyzing the data obtained from incoherent scatter radar from Jicamarca (11.95° S, 76.87° W) we observe an anomalous behavior of the zonal plasma drift during June 2008 characterized by lower than usual daytime westward drift and its early afternoon reversal to eastward. As a case study the zonal drift observed on 24 June 2008 is modeled using a realistic low-latitude ionosphere simulated by the Sheffield University Plasmasphere-Ionosphere Model-INPE (SUPIM-INPE). The results show that an anomalously low zonal wind was mainly responsible for the observed anomalous behavior in the zonal drift. A comparative study of the vertical plasma drifts obtained from magnetometer data for some periods of maximum (2000-2002) and minimum solar activity (1998, 2008, 2010) phases reveal a considerable decrease on the E-region conductivity and the dynamo electric field during 2008. However, we believe that the contribution of these characteristics to the unusual behavior of the zonal plasma drift is significantly smaller than that arising from the anomalously low zonal wind. The SUPIM-INPE result of the critical frequency of the F layer (foF2) over Jicamarca suggested a lower radiation flux than that predicted by solar irradiance model (SOLAR2000) for June 2008.

Modeling the effect of lithium-induced pedestal profiles on scrape-off-layer turbulence and the heat flux width

DOE PAGES

Russell, David A.; D'Ippolito, Daniel A.; Myra, James R.; ...

2015-09-01

The effect of lithium (Li) wall coatings on scrape-off-layer (SOL) turbulence in the National Spherical Torus Experiment (NSTX) is modeled with the Lodestar SOLT (“SOL Turbulence”) code. Specifically, the implications for the SOL heat flux width of experimentally observed, Li-induced changes in the pedestal profiles are considered. The SOLT code used in the modeling has been expanded recently to include ion temperature evolution and ion diamagnetic drift effects. This work focuses on two NSTX discharges occurring pre- and with-Li deposition. The simulation density and temperature profiles are constrained, inside the last closed flux surface only, to match those measured inmore » the two experiments, and the resulting drift-interchange-driven turbulence is explored. The effect of Li enters the simulation only through the pedestal profile constraint: Li modifies the experimental density and temperature profiles in the pedestal, and these profiles affect the simulated SOL turbulence. The power entering the SOL measured in the experiments is matched in the simulations by adjusting “free” dissipation parameters (e.g., diffusion coefficients) that are not measured directly in the experiments. With power-matching, (a) the heat flux SOL width is smaller, as observed experimentally by infra-red thermography, and (b) the simulated density fluctuation amplitudes are reduced with Li, as inferred for the experiments as well from reflectometry analysis. The instabilities and saturation mechanisms that underlie the SOLT model equilibria are also discussed.« less

Relationships of models of the inner magnetosphere to the Rice Convection Model

NASA Astrophysics Data System (ADS)

Heinemann, M.; Wolf, R. A.

2001-08-01

Ideal magnetohydrodynamics is known to be inaccurate for the Earth's inner magnetosphere, where transport by gradient-curvature drift is nonnegligible compared to E×B drift. Most theoretical treatments of the inner plasma sheet and ring current, including the Rice Convection Model (RCM), treat the inner magnetospheric plasma in terms of guiding center drifts. The RCM assumes that the distribution function is isotropic, but particles with different energy invariants are treated as separate guiding center fluids. However, Peymirat and Fontaine [1994] developed a two-fluid picture of the inner magnetosphere, which utilizes modified forms of the conventional fluid equations, not guiding center drift equations. Heinemann [1999] argued theoretically that for inner magnetospheric conditions the fluid energy equation should include a heat flux term, which, in the case of Maxwellian plasma, was derived by Braginskii [1965]. We have now reconciled the Heinemann [1999] fluid approach with the RCM. The fluid equations, including the Braginskii heat flux, can be derived by taking appropriate moments of the RCM equations for the case of the Maxwellian distribution. The physical difference between the RCM formalism and the Heinemann [1999] fluid approach is that the RCM pretends that particles suffer elastic collisions that maintain the isotropy of the distribution function but do not change particle energies. The Heinemann [1999] fluid treatment makes a different physical approximation, namely that the collisions maintain local thermal equilibrium among the ions and separately among the electrons. For some simple cases, numerical results are presented that illustrate the differences in the predictions of the two formalisms, along with those of MHD, guiding center theory, and Peymirat and Fontaine [1994].

Magnetically Controlled Upper Ionosphere of Mars

NASA Astrophysics Data System (ADS)

Majeed, T.; Al Aryani, O.; Al Mutawa, S.; Bougher, S. W.; Haider, S. A.

2017-12-01

The electron density (Ne) profiles measured by the Mars Express spacecraft over regions of strong crustal magnetic fields have shown anomalous characteristics of the topside plasma distribution with variable scale heights. One of such Ne profiles is located at 82oS and 180oE whose topside ionosphere is extended up to an altitude of 700 km. The crustal magnetic field at this southern site is nearly vertical and open to the access of solar wind plasma through magnetic reconnection with the interplanetary magnetic field. This can lead to the acceleration of electrons and ions during the daytime ionosphere. The downward accelerated electrons with energies >200 eV can penetrate deep into the Martian upper ionosphere along vertical magnetic field lines and cause heating, excitation and ionization of the background atmosphere. The upward acceleration of ions resulting from energy input by precipitating electrons can lead to enhance ion escape rate and modify scale heights of the topside ionosphere. We have developed a 1-D chemical diffusive model from 100 km to 400 km to interpret the Martian ionospheric structure at 82oS latitude. The primary source of ionization in the model is due to solar EUV radiation. An extra ionization source due to precipitating electrons of 0.25 keV, peaking near an altitude of 145 km is added in the model to reasonably reproduce the measured ionospheric structure below an altitude of 180 km. The behavior of the topside ionosphere can be interpreted by the vertical plasma transport caused by precipitating electrons. The vertical transport of plasma in our model is simulated by vertical ion velocities, whose values can be interpreted as drift velocities along magnetic field lines. We find that the variation of the topside Ne scale heights is sensitive to the magnitudes of upward and downward drifts with an imposed outward flux boundary condition at the top of the model. The model requires an upward flux of more than 107 ions cm-2 s-1 for both O2+ and O+, and drift speeds of 200 m/s to interpret the measured topside ionospheric structure for altitudes >180 km. The magnitudes of outward ion fluxes and drift velocities are compared with those simulated by existing models. The model results will be presented in comparison with the measured electron density profile. This work is supported by MBRSC, Dubai, UAE.

GEM-CEDAR Challenge: Poynting Flux at DMSP and Modeled Joule Heat

NASA Technical Reports Server (NTRS)

Rastaetter, Lutz; Shim, Ja Soon; Kuznetsova, Maria M.; Kilcommons, Liam M.; Knipp, Delores J.; Codrescu, Mihail; Fuller-Rowell, Tim; Emery, Barbara; Weimer, Daniel R.; Cosgrove, Russell;

Escape of asteroids from the main belt

NASA Astrophysics Data System (ADS)

Granvik, Mikael; Morbidelli, Alessandro; Vokrouhlický, David; Bottke, William F.; Nesvorný, David; Jedicke, Robert

2017-02-01

Aims: We locate escape routes from the main asteroid belt, particularly into the near-Earth-object (NEO) region, and estimate the relative fluxes for different escape routes as a function of object size under the influence of the Yarkovsky semimajor-axis drift. Methods: We integrated the orbits of 78 355 known and 14 094 cloned main-belt objects and Cybele and Hilda asteroids (hereafter collectively called MBOs) for 100 Myr and recorded the characteristics of the escaping objects. The selected sample of MBOs with perihelion distance q > 1.3 au and semimajor axis a < 4.1 au is essentially complete, with an absolute magnitude limit ranging from HV < 15.9 in the inner belt (a < 2.5 au) to HV < 14.4 in the outer belt (2.5 au < a < 4.1 au). We modeled the semimajor-axis drift caused by the Yarkovsky force and assigned four different sizes (diameters of 0.1, 0.3, 1.0, and 3.0 km) and random spin obliquities (either 0 deg or 180 deg) for each test asteroid. Results: We find more than ten obvious escape routes from the asteroid belt to the NEO region, and they typically coincide with low-order mean-motion resonances with Jupiter and secular resonances. The locations of the escape routes are independent of the semimajor-axis drift rate and thus are also independent of the asteroid diameter. The locations of the escape routes are likewise unaffected when we added a model for Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) cycles coupled with secular evolution of the rotation pole as a result of the solar gravitational torque. A Yarkovsky-only model predicts a flux of asteroids entering the NEO region that is too high compared to the observationally constrained flux, and the discrepancy grows larger for smaller asteroids. A combined Yarkovsky and YORP model predicts a flux of small NEOs that is approximately a factor of 5 too low compared to an observationally constrained estimate. This suggests that the characteristic timescale of the YORP cycle is longer than our canonical YORP model predicts.

Analysis of edge stability for models of heat flux width

DOE PAGES

Makowski, Michael A.; Lasnier, Charles J.; Leonard, Anthony W.; ...

2017-05-12

Detailed measurements of the n e, and T e, and T i profiles in the vicinity of the separatrix of ELMing H-mode discharges have been used to examine plasma stability at the extreme edge of the plasma and assess stability dependent models of the heat flux width. The results are strongly contrary to the critical gradient model, which posits that a ballooning instability determines a gradient scale length related to the heat flux width. The results of this analysis are not sensitive to the choice of location to evaluate stability. Significantly, it is also found that the results are completelymore » consistent with the heuristic drift model for the heat flux width. Here the edge pressure gradient scales with plasma density and is proportional to the pressure gradient inferred from the equilibrium in accordance with the predictions of that theory.« less

Effects of magnetic drift tangential to magnetic surfaces on neoclassical transport in non-axisymmetric plasmas

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

Matsuoka, Seikichi, E-mail: matsuoka@rist.or.jp; Satake, Shinsuke; Kanno, Ryutaro

2015-07-15

In evaluating neoclassical transport by radially local simulations, the magnetic drift tangential to a flux surface is usually ignored in order to keep the phase-space volume conservation. In this paper, effect of the tangential magnetic drift on the local neoclassical transport is investigated. To retain the effect of the tangential magnetic drift in the local treatment of neoclassical transport, a new local formulation for the drift kinetic simulation is developed. The compressibility of the phase-space volume caused by the tangential magnetic drift is regarded as a source term for the drift kinetic equation, which is solved by using a two-weightmore » δf Monte Carlo method for non-Hamiltonian system [G. Hu and J. A. Krommes, Phys. Plasmas 1, 863 (1994)]. It is demonstrated that the effect of the drift is negligible for the neoclassical transport in tokamaks. In non-axisymmetric systems, however, the tangential magnetic drift substantially changes the dependence of the neoclassical transport on the radial electric field E{sub r}. The peaked behavior of the neoclassical radial fluxes around E{sub r }={sub  }0 observed in conventional local neoclassical transport simulations is removed by taking the tangential magnetic drift into account.« less

Guiding center model to interpret neutral particle analyzer results

NASA Technical Reports Server (NTRS)

Englert, G. W.; Reinmann, J. J.; Lauver, M. R.

1974-01-01

The theoretical model is discussed, which accounts for drift and cyclotron components of ion motion in a partially ionized plasma. Density and velocity distributions are systematically precribed. The flux into the neutral particle analyzer (NPA) from this plasma is determined by summing over all charge exchange neutrals in phase space which are directed into apertures. Especially detailed data, obtained by sweeping the line of sight of the apertures across the plasma of the NASA Lewis HIP-1 burnout device, are presented. Selection of randomized cyclotron velocity distributions about mean azimuthal drift yield energy distributions which compared well with experiment. Use of data obtained with a bending magnet on the NPA showed that separation between energy distribution curves of various mass species correlate well with a drift divided by mean cyclotron energy parameter of the theory. Use of the guiding center model in conjunction with NPA scans across the plasma aid in estimates of ion density and E field variation with plasma radius.

Heavy-ion dominance near Cluster perigees

NASA Astrophysics Data System (ADS)

Ferradas, C. P.; Zhang, J.-C.; Kistler, L. M.; Spence, H. E.

2015-12-01

Time periods in which heavy ions dominate over H+ in the energy range of 1-40 keV were observed by the Cluster Ion Spectrometry (CIS)/COmposition DIstribution Function (CODIF) instrument onboard Cluster Spacecraft 4 at L values less than 4. The characteristic feature is a narrow flux peak at around 10 keV that extends into low L values, with He+ and/or O+ dominating. In the present work we perform a statistical study of these events and examine their temporal occurrence and spatial distribution. The observed features, both the narrow energy range and the heavy-ion dominance, can be interpreted using a model of ion drift from the plasma sheet, subject to charge exchange losses. The narrow energy range corresponds to the only energy range that has direct drift access from the plasma sheet during quiet times. The drift time to these locations from the plasma sheet is > 30 h, so that charge exchange has a significant impact on the population. We show that a simple drift/loss model can explain the dependence on L shell and MLT of these heavy-ion-dominant time periods.

Impact of resonant magnetic perturbations on nonlinearly driven modes in drift-wave turbulence

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

Leconte, M.; Diamond, P. H.; CMTFO and CASS, UCSD, California 92093

2012-05-15

In this work, we study the effects of resonant magnetic perturbations (RMPs) on turbulence, flows, and confinement in the framework of resistive drift wave turbulence. We extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large ({delta}B{sub r}/B{sub 0}). Both the vorticity flux (Reynolds stress) and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolutionmore » of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. We find states that are similar to the ZF-dominated state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude, the energy of zonal flows decreases and the turbulence energy increases with ({delta}B{sub r}/B{sub 0}), corresponding to a damping of zonal flows.« less

Threshold flux-controlled memristor model and its equivalent circuit implementation

NASA Astrophysics Data System (ADS)

Wu, Hua-Gan; Bao, Bo-Cheng; Chen, Mo

2014-11-01

Modeling a memristor is an effective way to explore the memristor properties due to the fact that the memristor devices are still not commercially available for common researchers. In this paper, a physical memristive device is assumed to exist whose ionic drift direction is perpendicular to the direction of the applied voltage, upon which, corresponding to the HP charge-controlled memristor model, a novel threshold flux-controlled memristor model with a window function is proposed. The fingerprints of the proposed model are analyzed. Especially, a practical equivalent circuit of the proposed model is realized, from which the corresponding experimental fingerprints are captured. The equivalent circuit of the threshold memristor model is appropriate for various memristors based breadboard experiments.

Snow drift: acoustic sensors for avalanche warning and research

NASA Astrophysics Data System (ADS)

Lehning, M.; Naaim, F.; Naaim, M.; Brabec, B.; Doorschot, J.; Durand, Y.; Guyomarc'h, G.; Michaux, J.-L.; Zimmerli, M.

Based on wind tunnel measurements at the CSTB (Jules Verne) facility in Nantes and based on field observations at the SLF experimental site Versuchsfeld Weissfluhjoch, two acoustic wind drift sensors are evaluated against different mechanical snow traps and one optical snow particle counter. The focus of the work is the suitability of the acoustic sensors for applications such as avalanche warning and research. Although the acoustic sensors have not yet reached the accuracy required for typical research applications, they can, however, be useful for snow drift monitoring to help avalanche forecasters. The main problem of the acoustic sensors is a difficult calibration that has to take into account the variable snow properties. Further difficulties arise from snow fall and high wind speeds. However, the sensor is robust and can be operated remotely under harsh conditions. It is emphasized that due to the lack of an accurate reference method for snow drift measurements, all sensors play a role in improving and evaluating snow drift models. Finally, current operational snow drift models and snow drift sensors are compared with respect to their usefulness as an aid for avalanche warning. While drift sensors always make a point measurement, the models are able to give a more representative drift index that is valid for a larger area. Therefore, models have the potential to replace difficult observations such as snow drift in operational applications. Current models on snow drift are either only applicable in flat terrain, are still too complex for an operational application (Lehning et al., 2000b), or offer only limited information on snow drift, such as the SNOWPACK drift index (Lehning et al., 2000a). On the other hand, snow drift is also difficult to measure. While mechanical traps (Mellor 1960; Budd et al., 1966) are probably still the best reference, they require more or less continuous manual operation and are thus not suitable for remote locations or long-term monitoring. Optical sensors (Schmidt, 1977; Brown and Pomeroy, 1989; Sato and Kimura, 1993) have been very successful for research applications, but suffer from the fact that they give a single flux value at one specific height. In addition, they have not been used, to our knowledge, for long-term monitoring applications or at remote sites. New developments of acoustic sensors have taken place recently (Chritin et al., 1999; Font et al., 1998). Jaedicke (2001) gives examples of possible applications of acoustic snow drift sensors. He emphasizes the advantages of acoustic sensors for snow drift monitoring at remote locations, but could not present any evaluation of the accuracy of the measurements. We present a complete evaluation of the new acoustic sensors for snow drift and discuss their applications for research or avalanche warning. We compare the suitability of sensors for operational applications.

Ocean circulation drifts in multi-millennial climate simulations: the role of salinity corrections and climate feedbacks

NASA Astrophysics Data System (ADS)

Dentith, Jennifer E.; Ivanovic, Ruza F.; Gregoire, Lauren J.; Tindall, Julia C.; Smith, Robin S.

2018-05-01

Low-resolution, complex general circulation models (GCMs) are valuable tools for studying the Earth system on multi-millennial timescales. However, slowly evolving salinity drifts can cause large shifts in climatic and oceanic regimes over thousands of years. We test two different schemes for neutralising unforced salinity drifts in the FAMOUS GCM: surface flux correction and volumetric flux correction. Although both methods successfully maintain a steady global mean salinity, local drifts and subsequent feedbacks promote cooling (≈ 4 °C over 6000 years) and freshening (≈ 2 psu over 6000 years) in the North Atlantic Ocean, and gradual warming (≈ 0.2 °C per millennium) and salinification (≈ 0.15 psu per millennium) in the North Pacific Ocean. Changes in the surface density in these regions affect the meridional overturning circulation (MOC), such that, after several millennia, the Atlantic MOC (AMOC) is in a collapsed state, and there is a strong, deep Pacific MOC (PMOC). Furthermore, the AMOC exhibits a period of metastability, which is only identifiable with run lengths in excess of 1500 years. We also compare simulations with two different land surface schemes, demonstrating that small biases in the surface climate may cause regional salinity drifts and significant shifts in the MOC (weakening of the AMOC and the initiation then invigoration of PMOC), even when the global hydrological cycle has been forcibly closed. Although there is no specific precursor to the simulated AMOC collapse, the northwest North Pacific and northeast North Atlantic are important areas that should be closely monitored for trends arising from such biases.

Extensive electron transport and energization via multiple, localized dipolarizing flux bundles

NASA Astrophysics Data System (ADS)

Gabrielse, Christine; Angelopoulos, Vassilis; Harris, Camilla; Artemyev, Anton; Kepko, Larry; Runov, Andrei

2017-05-01

Using an analytical model of multiple dipolarizing flux bundles (DFBs) embedded in earthward traveling bursty bulk flows, we demonstrate how equatorially mirroring electrons can travel long distances and gain hundreds of keV from betatron acceleration. The model parameters are constrained by four Time History of Events and Macroscale Interactions during Substorms satellite observations, putting limits on the DFBs' speed, location, and magnetic and electric field magnitudes. We find that the sharp, localized peaks in magnetic field have such strong spatial gradients that energetic electrons ∇B drift in closed paths around the peaks as those peaks travel earthward. This is understood in terms of the third adiabatic invariant, which remains constant when the field changes on timescales longer than the electron's drift timescale: An energetic electron encircles a sharp peak in magnetic field in a closed path subtending an area of approximately constant flux. As the flux bundle magnetic field increases the electron's drift path area shrinks and the electron is prevented from escaping to the ambient plasma sheet, while it continues to gain energy via betatron acceleration. When the flux bundles arrive at and merge with the inner magnetosphere, where the background field is strong, the electrons suddenly gain access to previously closed drift paths around the Earth. DFBs are therefore instrumental in transporting and energizing energetic electrons over long distances along the magnetotail, bringing them to the inner magnetosphere and energizing them by hundreds of keV.