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NASA NTRS1992

Results of a zonally truncated three-dimensional model of the Venus middle atmosphere

Although the equatorial rotational speed of the solid surface of Venus is only 4 m s(exp-1), the atmospheric rotational speed reaches a maximum of approximately 100 m s(exp-1) near the equatorial cloud top level (65 to 70 km). This phenomenon, known as superrotation, is the central dynamical problem of the Venus atmosphere. We report here the results of numerical simulations aimed at clarifying the mechanism for maintaining the equatorial cloud top rotation. Maintenance of an equatorial rotational speed maximum above the surface requires waves or eddies that systematically transport angular momentum against its zonal mean gradient. The zonally symmetric Hadley circulation is driven thermally and acts to reduce the rotational speed at the equatorial cloud top level; thus wave or eddy transport must counter this tendency as well as friction. Planetary waves arising from horizontal shear instability of the zonal flow (barotropic instability) could maintain the equatorial rotation by transporting angular momentum horizontally from midlatitudes toward the equator. Alternatively, vertically propagating waves could provide the required momentum source. The relative motion between the rotating atmosphere and the pattern of solar heating, which as a maximum where solar radiation is absorbed near the cloud tops, drives diurnal and semidiurnal thermal tides that propagate vertically away from the cloud top level. The effect of this wave propagation is to transport momentum toward the cloud top level at low latitudes and accelerate the mean zonal flow there. We employ a semispectral primitive equation model with a zonal mean flow and zonal wavenumbers 1 and 2. These waves correspond to the diurnal and semidiurnal tides, but they can also be excited by barotropic or baroclinic instability. Waves of higher wavenumbers and interactions between the waves are neglected. Symmetry about the equator is assumed, so the model applies to one hemisphere and covers the altitude range 30 to 110 km. Horizontal resolution is 1.5 deg latitude, and vertical resolution is 1.5 km. Solar and thermal infrared heating, based on Venus observations and calculations drive the model flow. Dissipation is accomplished mainly by Rayleigh friction, chosen to produce strong dissipation above 85 km in order to absorb upward propagating waves and limit extreme flow velocities there, yet to give very weak Rayleigh friction below 70 km; results in the cloud layer do not appear to be sensitive to the Rayleigh friction. The model also has weak vertical diffusion, and very weak horizontal diffusion, which has a smoothing effect on the flow only at the two grid points nearest the pole.

Newman, M.↗
NASA NTRS1980

A simple approximate result for the maximum growth rate of baroclinic instabilities

The Charney problem for baroclinic instability involves the quasi-geostrophic instability of a zonal flow on a beta plane where the zonal flow is characterized by a constant vertical shear. The atmosphere is non-Boussinesq and continuous. The solution of this problem involves confluent hypergeometric functions, and the mathematical difficulty of the problem has precluded extracting simple results of generality. It is shown that there exists very simple, powerful approximate result for the growth rate of the most rapidly growing instability, viz., that this growth rate is linearly proportional to the surface meridional temperature gradient. The coefficient of proportionality is also easily determined. The result extends to substantially more general profiles than those in the Charney problem.

Lindzen, R. S.↗
DOE OSTI2021

Electric field and turbulence in global Braginskii simulations across the ASDEX Upgrade edge and scrape-off layer

Turbulence simulations in diverted geometry across the edge and scrape-off layer (SOL) of ASDEX Upgrade are performed with the GRILLIX code (Stegmeir et al 2019 Phys. Plasmas 26 052517). The underlying global (full-f) drift-reduced Braginskii model allows to concurrently study the self-consistent dynamics of the turbulence and the background as well as the evolution of toroidal and zonal flows. Different contributions to the radial electric field are identified. The dominant contribution on closed flux surfaces comes from the ion pressure gradient, due to the diamagnetic drift in the curved magnetic field. Large deviations can be induced, in particular, by the polarization particle flux, leading to zonal flows. The latter are driven by small-scale eddies, but do not exhibit much impact on the overall transport which is driven by ballooning modes at larger scales. Ion viscosity is found to be important in damping poloidal rotation through adjusting of the parallel velocity profile, but not via direct vorticity damping. The zonal flow drive peaks at the separatrix, where a strong shear layer forms due to the sheath-induced counter-propagating SOL flow, allowing for the formation of a transport barrier. The temperature profile across the separatrix is determined by the competition between cross-field transport and outflow in the SOL, the latter being largely controlled by the parallel heat conductivity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Effects of resonant magnetic perturbations on radial electric fields in DIII-D tokamak

Here, gyrokinetic simulations of DIII-D tokamak equilibrium find that resonant magnetic perturbation (RMP) drives a neoclassical non-ambipolar electron particle flux, which causes a rapid change of equilibrium radial electric fields consistent with experimental observations during the suppression of the edge localized mode (ELM). The simulation results provide a support for the conjecture that RMP-induced changes of radial electric fields lead to the enhanced turbulent transport at the pedestal top during the ELM suppression (Taimourzadeh et al 2019 Nucl. Fusion 59 046005). Furthermore, gyrokinetic simulations of collisionless damping of zonal flows show that resonant responses to the RMP decrease the residual level of the zonal flows and damp the geodesic acoustic mode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2000

Wave Driven Non-linear Flow Oscillator for the 22-Year Solar Cycle

In the Earth's atmosphere, a zonal flow oscillation is observed with periods between 20 and 32 months, the Quasi Biennial Oscillation. This oscillation does not require external time dependent forcing but is maintained by non-linear wave momentum deposition. It is proposed that such a mechanism also drives long-period oscillations in planetary and stellar interiors. We apply this mechanism to generate a flow oscillation for the 22-year solar cycle. The oscillation would occur just below the convective envelope where waves can propagate. Using scale analysis, we present results from a simplified model that incorporates Hines' gravity wave parameterization. Wave amplitudes less than 10 m/s can produce reversing zonal flows of 25 m/s that should be sufficient to generate a corresponding oscillation in the poloidal magnetic field. Low buoyancy frequency and the associated increase in turbulence help to produce the desired oscillation period of the flow.

Mayr, Hans G.↗
NASA NTRS2001

Model of Wave Driven Flow Oscillation for Solar Cycle

At low latitudes in the Earth's atmosphere, the observed zonal flow velocities are dominated by the semi-annual and quasi-biennial oscillations with periods of 6 months and 20 to 32 months respectively. These terrestrial oscillations, the SAO and QBO respectively, are driven by wave-mean flow interactions due to upward propagating planetary-scale waves (periods of days) and small-scale gravity waves (periods of hours). We are proposing (see also Mayr et al., GRL, 2001) that such a mechanism may drive long period oscillations (reversing flows) in stellar and planetary interiors, and we apply it to the Sun. The reversing flows would occur below the convective envelope where waves can propagate. We apply a simplified, one dimensional, analytical flow model that incorporates a gravity wave parameterization due to Hines (1997). Based on this analysis, our estimates show that relatively small wave amplitudes less than 10 m/s can produce zonal flow amplitudes of 20 m/s, which should be sufficient to generate the observed variations in the magnetic field. To produce the 22-year period of oscillation, a low buoyancy frequency must be chosen, and this places the proposed flow in a region that is close to (and below) the base of the convective envelope. Enhanced turbulence associated with this low stability should help to generate the dynamo currents. With larger stability at deeper levels in the solar interior, the model can readily produce also oscillations with much longer periods. To provide an understanding of the fluid dynamics involved, we present numerical results from a 2D model for the terrestrial atmosphere that exemplify the non-linear nature of the wave interaction for which a mechanical analog is the escapement mechanism of the clock.

Mayr, Hans G.↗
NASA NTRS1983

Daily earth radiation budget results from the Nimbus-7 scanning radiometer

Nimbus-7 broadband narrow-field-of-view scanning-radiometer data obtained in the Earth Radiation Budget Experiment on three days in 1979 (one with a blocking situation over the Northern Hemisphere, one with normal zonal flow, and one with two mature hurricanes over the Caribbean), are presented graphically and discussed. Both blocking and hurricane situations are characterized by anomalies (relative to the zonal-flow condition) in the zonally averaged emitted flux and albedo, with decreased tropical and midlatitude gradients in the case of blocking.

Randel, D. L.↗
NASA NTRS1982

The role of gravity wave induced drag and diffusion in the momentum budget of the mesosphere

A slight modification of the parameterization suggested by Lindzen (1981) for the zonal drag and eddy diffusion effects generated by breaking internal gravity waves in the mesosphere is tested using a severely truncated midlatitude beta-plane channel model. It is found that realistic mean zonal flow profiles with zonal wind reversals above the mesopause can be simulated for both winter and summer radiative heating conditions provided that a gravity-wave spectrum is assumed which includes both stationary waves and waves of relatively large phase speeds. These results contrast greatly with the unrealistic mean wind profiles produced when Rayleigh friction is used to parameterize the effects of small scale motions on the mean flow.

Holton, J. R.↗
NASA NTRS1980

Sudden stratospheric warmings forced by mountains

A global, quasigeostrophic model of planetary wave interaction with the mean zonal flow has been used to simulate sudden stratospheric warmings forced by mean tropospheric zonal winds interacting with orography. Planetary waves m = 1 and 2 are simultaneously computed and interact with each other only through the mean zonal flow. An increase in the jet stream maximum from 36 to 48 m/sec is sufficient to initiate repeated major warmings with an approximate 80 day cycle. These results are in agreement with the observed anticorrelation of the annual variation of the winter monthly averaged jet stream strength and polar night jet strength.

Schoeberl, M. R.↗
NASA NTRS2011

C/NOFS Satellite Electric Field and Plasma Density Observations of Plasma Instabilities Below the Equatorial F-Peak -- Evidence for Approximately 500 km-Scale Spread-F "Precursor" Waves Driven by Zonal Shear Flow and km-Scale, Narrow-Banded Irregularities

As solar activity has increased, the ionosphere F-peak has been elevated on numerous occasions above the C/NOFS satellite perigee of 400km. In particular, during the month of April, 2011, the satellite consistently journeyed below the F-peak whenever the orbit was in the region of the South Atlantic anomaly after sunset. During these passes, data from the electric field and plasma density probes on the satellite have revealed two types of instabilities which had not previously been observed in the C/NOFS data set (to our knowledge): The first is evidence for 400-500km-scale bottomside "undulations" that appear in the density and electric field data. In one case, these large scale waves are associated with a strong shear in the zonal E x B flow, as evidenced by variations in the meridional (outward) electric fields observed above and below the F-peak. These undulations are devoid of smaller scale structures in the early evening, yet appear at later local times along the same orbit associated with fully-developed spread-F with smaller scale structures. This suggests that they may be precursor waves for spread-F, driven by a collisional shear instability, following ideas advanced previously by researchers using data from the Jicamarca radar. A second new result (for C/NOFS) is the appearance of km-scale irregularities that are a common feature in the electric field and plasma density data that also appear when the satellite is below the F -peak at night. The vector electric field instrument on C/NOFS clearly shows that the electric field component of these waves is strongest in the zonal direction. These waves are strongly correlated with simultaneous observations of plasma density oscillations and appear both with, and without, evidence of larger-scale spread-F depletions. These km-scale, quasi-coherent waves strongly resemble the bottomside, sinusoidal irregularities reported in the Atmosphere Explorer satellite data set by Valladares et al. [JGR, 88, 8025, 1983]. We interpret these new observations in terms of fundamental plasma instabilities associated with the unstable, nighttime equatorial ionosphere.

Pfaff, R.↗
NASA NTRS1993

The effects of zonal atmospheric currents on the spectra of rotating early-type stars

We suggest the existence of zonal currents in the atmospheres of rapidly rotating stars analogous to those found in planetary atmospheres. The zonal flow is assumed to be characterized by 'thin' atmospheric, nearly geostrophic flow which does not change the gravity darkening and stellar shape determined by the underlying uniformly rotating model. The contribution that such flows make to the continuum spectra of such stars is investigated. The additional rotationally induced Doppler displacement resulting from such zonal wind belts can distort the rotationally broadened stellar lines leading to significant departures from the line profiles predicted by the classical model of rotating stars. Our estimates of the zonal flow velocity stem from the assumption of a relation between it and the latitudinal wavenumber of the zonal velocity field. It is thus possible to create barotropic atmosphere models which, in turn, enable the modeling of the stellar spectrum including important spectral lines. In addition, the radiative transfer equations for the Stokes parameters I and Q are solved for the locally plane-parallel atmospheres so that the polarization structure of the radiation field is determined. We find that the presence of zonal wind belts leads to significant changes in the photospheric polarization from those characteristic of a uniformly rotating model.

Cranmer, Steven R.↗
NASA NTRS1985

Persistent anomalies, blocking and variations in atmospheric predictability

The fully nonlinear, equivalent-barotropic vorticity equation on the sphere, with simplified forcing, dissipation and topography, is the model used in the present consideration of low frequency variability regimes in large scale atmospheric dynamics. The solutions obtained are studied as a function of the nondimensional intensity of the forcing and dissipation. The number of modes retained in the analysis allows the multiple equilibria thus obtained, which exhibit blocked and zonal flow patterns very similar to synoptically defined zonal and blocked Northern Hemisphere midlatitude flows, to appear for realistic values of the forcing. The number of episodes of blocked or zonal flow decreases monotonically as their duration increases, in agreement with observations.

Legras, B.↗
NASA NTRS1980

Numerical simulation of stratospheric sudden warmings with a primitive equation spectral model

A 26-level primitive equation spherical harmonic spectral model allowing for wave-wave and wave-zonal flow interactions is presented for the study of stratospheric sudden warmings. The warmings are simulated by the forcing of a single planetary wave at the tropopause. Four numerical experiments were performed. Nonlinear wave-wave interactions appear to play an important role in the evolution of the flow and temperature fields in the middle to upper stratosphere. In the case involving both wave-wave and wave-zonal flow interactions, this was manifested by the split in the initial polar vortex into a quasi-wave number 2 pattern. In the cases at 60 deg N, easterlies develop first in the upper mesosphere and descend gradually. About the same time or a little later, easterlies also develop in the mid-stratosphere. The linear cases exhibit warmings which are more shallow and more intense at 30 km than the nonlinear cases.

Lordi, N. J.↗
DOE OSTI2022

Global gyrokinetic simulations of electrostatic microturbulent transport using kinetic electrons in LHD stellarator

Global gyrokinetic simulations of ion temperature gradient (ITG) and trapped electron mode (TEM) in the LHD stellarator are carried out using the gyrokinetic toroidal code (GTC) with kinetic electrons. ITG simulations show that kinetic electron effects increase the growth rate by more than 50% and more than double the turbulent transport levels compared with simulations using adiabatic electrons. Zonal flow dominates the saturation mechanism in the ITG turbulence. Nonlinear simulations of the TEM turbulence show that the main saturation mechanism is not the zonal flow but the inverse cascade of high to low toroidal harmonics. Further nonlinear simulations with various pressure profiles indicate that the ITG turbulence is more effective in driving heat conductivity whereas the TEM turbulence is more effective for particle diffusivity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1986

Dynamic characteristics of observed sudden warmings

The planetary wave dynamics of stratospheric sudden warmings in the Northern Hemisphere for a large number of observed events that occurred during winters from 1970 to 1975 and 1978 to 1981 are investigated. The analysis describes wave propagation and zonal flow interaction from the troposphere upwards to near 50 km, and in some years to near 80 km. Three primary topics are covered here: (1) the interaction of zonally propagating and quasi-stationary planetary waves during warming events; (2) planetary wave influence on zonal flow near the stratopause; and (3) planetary wave propagation to near 80 km as seen from Stratospheric and Mesospheric Sounder (SAMS) data.

Dartt, D. G.↗
DOE OSTI2020

Theoretical study of various nonlinear phenomena in plasma systems and scaling of magneto-inertial-fusion targets

Plasma physics is an exciting field of study with a wide variety of nonlinear processes that come into play. Examples of such processes include the interaction of small-scale turbulence with large-scale plasma structures and the nonlinear saturation of plasma instabilities, for example those of magneto-hydrodynamical nature. During this Truman LDRD project, I studied a collection of nonlinear problems that are of interest to the field of plasma physics. This LDRD report summarizes four main research accomplishments. First, a new statistical model for describing inhomogeneous drift-wave turbulence inter- acting with zonal flows was developed. This new model includes the effects of nonlinear wave-wave collisions, which are expected to change the spectrum of the underlying DW turbulence and therefore the generation of zonal flows. Second, a new mathematical formalism was proposed to systematically apply the non- linear WKB approximation to general field theories, including those often used in fluid dynamics. This formalism represents an interesting tool for studying physical systems that show an explicit scale separation. Third, a weakly nonlinear model was developed to describe the magneto-Rayleigh-Taylor instability. This instability is of paramount importance to understand as it can reduce the performance of magnetic-inertial-fusion (MIF) platforms. The developed models captures the effects of harmonic generation and saturation of the linear growth of the instability. Finally, a framework was proposed for scaling magneto-inertial fusion (MIF) targets to larger pulsed-power drivers. From this framework, a set of scaling rules were derived that conserve the physical regimes of MIF systems when scaling up in peak current. By doing so, deleterious nonlinear processes that affect MIF performance may be kept at bay.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Threshold Heat-Flux Reduction by Near-Resonant Energy Transfer

Near-resonant energy transfer to large-scale stable modes is shown to reduce transport above the linear critical gradient, contributing to the onset of transport at higher gradients. This is demonstrated for a threshold fluid theory of ion temperature gradient turbulence based on zonal-flow-catalyzed transfer. Furthermore, the heat flux is suppressed above the critical gradient by resonance in the triplet correlation time, a condition enforced by the wave numbers of the interaction of the unstable mode, zonal flow, and stable mode.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1979

Two-dimensional zonal mean flow model

The interactions of dynamics, photochemistry and radiation in the stratosphere can be described by the continuity, momentum, and energy equations. A more comprehensive model for stratospheric transport theory was developed for the purpose of aiding predictions of changes in the stratospheric ozone content as a consequence of natural and anthropogenic processes. This model is time dependent and the dependent variables are zonal means of the relevant meteorological quantities which are functions of latitude and height. The detailed formulation of a numerical model both in physics and mathematics is given. A set of fundamental dynamical equations, and the numerical method used in the integration are described.

Chen, H. C.↗