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NASA NTRS1982

The steady global corona

The formation of the steady coronal structure which consists of coronal streamers and holes is seen in terms of a model employing numerical solutions of time-dependent, dissipationless, magnetohydrodynamic equations of motion applicable to the meridional plane. A coronal streamer consists of closed magnetic field lines near the solar surface with overlying and adjacent open field lines, and the atmosphere, which is stationary in the closed region, flows outward in the open region, or coronal hole. The steady coronal structure is obtained by starting the numerical calculation with a state comprising a polytropic, hydrodynamical solution to the steady-state radial equation of motion coupled with a dipole magnetic field. Global coronal configurations are calculated for values of the plasma beta which vary from 0.1 to 100. It is found that the height and lateral extension of the closed region are only weakly dependent on the data.

Steinolfson, R. S.↗

NASA NTRS1986

Interaction of the Asian summer monsoon and the Southern Hemisphere

The possible role that divergent circulations emanating from regions of large scale organized convection may have on the rotational motions of the subsiding branch of the resulting overturnings is considered. Correspondence is found between the cross-equatorial flow emanating from regions of organized convection in the Northern Hemisphere, and the subtropical jet in the vicinity of Australia. The difference field also reveals a good correspondence between changes in the meridional gravity wave component and changes in the intensity of maximum zonal wind. The results are consistent with the picture of a local ageostrophic Hadley overturning in the entrance region of subtropical jets, and they show the importance of adequate resolution of tropical divergent modes.

Paegle, Julia N.↗

NASA NTRS1991

Phase speed spectra of transient eddy fluxes and critical layer absorption

Tropospheric zonal mean eddy fluxes of heat and momentum, and the divergence of the Eliassen-Palm flux, are decomposed into contributions from different zonal phase speeds. Data analyzed are the European Center for Medium Range Weather Forecasts operational global analyses covering 1980-1987. Eastward moving medium-scale waves (zonal waves 4-7) dominate the spectra of lower tropospheric heat fluxes in both hemispheres and all seasons. Upper tropospheric wave flux spectra are similar to the low level spectra in midlatitudes, but shift to slower zonal phase speeds as low latitudes are approached. The cause of this shift is the selective absorption of faster moving components in midlatitudes as the waves propagate meridionally. Latitude-phase speed distributions of eddy fluxes are constructed and compared to the zonal mean wind structure. These results demonstrate that upper tropospheric eddies break and decelerate the zonal mean flow approximately 10-20 deg in latitude away from their critical line (where phase speed equals zonal wind speed). Comparisons are also made with results from the middle stratosphere.

Randel, William J.↗

NASA NTRS1973

General atmospheric circulation driven by polar and diurnal surface temperature variations.

Described is a global circulation model for the Venus atmosphere that includes the effects of both polar cooling and diurnal temperature variation. It is based on a linearized Boussinesq approximation and boundary conditions derived from theoretical and empirical considerations. The time-dependent, three-dimensional flow field is deduced without any a priori assumptions about its configuration. Results show that the mean atmospheric motions are essentially zonal in a narrow belt near the equator and change to become meridional over most of the globe. The circulation pattern is not symmetrical and rotates about the polar axis of the planet with the period of the solar day.

Bohachevsky, I. O.↗

NASA NTRS1979

Thermal contrast in the atmosphere of Venus - Initial appraisal from Pioneer Venus probe data

The altitude profiles of temperature and pressure were measured during the descent of four Pioneer Venus probes, showing small contrasts below the clouds, but significant differences within the clouds at altitudes from 45 to 61 km. Measurements of pressure differences were found to be consistent with the cyclostrophic balance of zonal winds ranging from 110 to 150 m/sec at 60 km and from 43 to 77 m/sec at 40 km. The clouds were 10 to 20 K warmer than the extended profiles of the lower atmosphere and the middle cloud is convectively unstable. Both phenomena are due to thermal radiation from below. Meridional wind velocities were studied, concluding that significant planetary scale non-axisymmetric motions were present at latitudes below 30 degrees. This result was consistent with the day-night pressure difference. Indications of flow oscillations in the lower atmosphere were noted and the inference of wave motions in the lower atmosphere was supported by analysis of oscillations in the Doppler residuals.

Seiff, A.↗

NASA NTRS1985

Transient convection over the Amazon-Bolivia region and the dynamics of drought over Northeast Brazil

A two layer, nonlinear, equatorial beta-plane model, in p-coordinates is used to study the atmospheric response to a large scale prescribed heat source varying in time. The heat source is meant to represent a convective burst with total duration of approximately 48 hours over the Amazon/Bolivia region. The boundary conditions used are meridional velocity zero at 60 deg S, omega w = 0 at the top and zero geometric velocity at the lower boundary. Sensitivity study was done which includes initial state at rest, compared with realistic initial flow. The scale of the heat source is 1500 km in latitude and longitude and it is centered at 10 deg S. Special attention is paid to the distribution and intensity of the induced vertical motion. The model is integrated for two days and the preliminary results show agreement with the observed 200 mb flow. Of interest is the establishment of a trough and descending motion to the northeast of the heat source. A conjucture is thus made that the Amazon heat source and its fluctuations bear some relationship with the drought problem over Northeast Brazil.

Buchmann, J.↗

NASA NTRS1992

High resolution observations of the magnetic and velocity field of simple sunspots

We have observed the disk passage of relatively simple round sunspots using a narrowband filter and a large format CCD detector and have created magnetograms, Dopplergrams, and continuum images nearly simultaneously. In addition the spectral resolution of the filter allows the construction of 'spectra' for all points in the field of view. The mean inclination of the magnetic field increases from 45-50 deg to 70-75 deg across the penumbra and there is a fluctuation of the inclination angle about the mean of about 4 +/- 18 deg. The variation in inclination is large enough that substantial amounts of magnetic field are parallel to the solar surface from the mid to outer penumbra. The Evershed flow tends to occur in the regions where the magnetic field is horizontal. This suggests that the Evershed flow is confined to the regions of horizontal fields. We show that a simple sunspot model with azimuthal variations in inclination but no azimuthal variations of field strength is free from azimuthal Lorentz forces. The meridional component of the currents which arise from the azimuthal variation in inclination are parallel to the field lines. This suggests that a cylindrically symmetric magnetostatic sunspot model can be perturbed into one with azimuthal variations in inclination with some adjustment in the meridional force balance.

Title, Alan M.↗

NASA NTRS1984

Midstratospheric circulations in the southern hemisphere from super pressure balloon trajectories

Three instrumented super pressure balloons were flown throughout the southern hemisphere at 18 mbar, over a period of five months. The balloon trajectories, reconstructed from NIMBUS 6 satellite location data, have proved to be valuable indicators of midstratospheric structure and dynamics in the Lagrangian frame of reference. Several examples of high zonal and meridional winds are reported. Also shown in the analysis is a feature encountered between 30 S-40 S during a two-week period. The feature appears to be a breakdown of large-scale zonal flow and its replacement by weak cyclonic eddies. During the observational period the balloon motions document the spring reversal. Mean zonal wind speed analysis from balloon positions were obtained covering the area 15 S-65 S. The results presented here are in substantial agreement with those of Hartmann (1977).

Olivero, J. J.↗

NASA NTRS1986

Polar cap observations of thermospheric winds and temperatures at Sondre Stromfjord, Greenland

An agreement of averaged temperatures with mass spectrometer incoherent scatter radar looked reasonable for several nights, but for many nights there are differences: (1) midnight period of cooling, and (2) temperature increases associated with overhead crossings of the auroral belt. The observed rise of the temperature before dawn in conjunction with the high 6300A intensities suggests a connection between the two effects: soft particle precipitation most likely candidate but frictional heating perhaps also a possibility. A comparison with the thermospheric general circulation model calculations also needed. The technique for formulating neutral wind vectors performs well in most cases. The observed patterns show evidence for abatement in the midnight sector in the meridional wind component at the separatix between the two cells with a frequency of the order of 20 to 25%, also observed in radar observations at Sondre Stromfjord. The observed patterns for magnetically quiet conditions show flow characteristic of the auroral belt, westward in evening followed by the midnight surge. The observed patterns for active conditions show dominance either by the evening cell or the morning cell, but most often the former.

Meriwether, J. W., Jr.↗

NASA NTRS1976

Characteristics of Southern Hemisphere 200 mb flow as determined from satellite data

Characteristics of Southern Hemisphere 200 mb flow are examined using geopotential height fields constructed with the aid of satellite based thermal structure. Similar Northern Hemisphere, satellite based fields are developed in order to make interhemispheric comparisons. Results indicate that both the zonal and meridional components of the S.H. eddy kinetic energy are as large as their N.H. counterparts. In winter the principal interhemispheric difference with respect to eddy kinetic energy is that the S.H. standing eddies are much less important only to the meridional component. Zonal component standing energy is about equal in the two hemispheres. In summer the S.H. has larger zonal eddy kinetic energy than the N.H. and smaller standing eddy contributions in both components. The meridional spectra show a preference for intermediate size transient waves.

Adler, R. F.↗

NASA NTRS1985

A fully implicit scheme for global numerical weather prediction

A fast-slow factored scheme is presented for use with shallow-water primitive equation numerical weather prediction models. The technique was developed to reduce the rotational mode errors which arise when the fast and slow terms of the governing differential equations are treated simultaneously. The method factors out the fast and slow terms along the coordinate directions by means of a modified Crank-Nicolson scheme. A finite-difference spatial discretization is carried out in the zonal and meridional directions to reduce the factorization error to near-zero, and that time steps of 60-90 min can be used to obtain acceptably accurate results, even in the presence of fine spatial structures in the flow.

Augenbaum, J. M.↗

NASA NTRS1990

Low-latitude thermospheric neutral winds determined from AE-E measurements of the 6300-A nightglow at solar maximum

Atmosphere Explorer E (AE-E) measurements of the O(1D) 6300-A emission in the nighttime equatorial thermosphere are used to infer the height of the F2 layer peak as a function of latitude and local time. The investigation is conducted both for northern hemisphere winter solstice and for spring equinox, under solar maximum conditions. The layer heights are used to derive magnetic meridional components of the transequatorial neutral wind, in conjunction with the MSIS-86 model and previous Jicamarca incoherent scatter measurements of the zonal electric field. The AE-E wind estimates indicate a predominant summer to winter flow for the winter solstice case. Comparisons are made with the empirical horizontal wind model HWM87 and with winds generated by the thermospheric general circulation model. The model predictions and experimental results are generally in good agreement, confirming the applicability of visible airglow data to studies of the global neutral wind pattern.

Burrage, M. D.↗

NASA NTRS1992

Remote sensing of mesospheric winds with the High-Resolution Doppler Imager

Observations of the winds in the upper atmosphere obtained with the High-Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) are discussed. This instrument is a very stable high-resolution triple-etalon Fabry-Perot interferometer, which is used to observe the slight Doppler shifts of absorption and emission lines in the O2 Atmospheric bands induced by atmospheric motions. Preliminary observations indicate that the winds in the mesosphere and lower thermosphere are a mixture of migrating and non-migrating tides, and planetary-scale waves. The mean meridional winds are dominated by the 1,1 diurnal tide which is easily extracted from the daily zonal means of the satellite observations. The daily mean zonal winds are a mixture of the diurnal tide and a zonal flow which is consistent with theoretical expectations.

Hays, Paul B.↗

NASA NTRS2017

Large-Scale Traveling Weather Systems in Mars Southern Extratropics

Between late fall and early spring, Mars' middle- and high-latitude atmosphere supports strong mean equator-to-pole temperature contrasts and an accompanying mean westerly polar vortex. Observations from both the MGS Thermal Emission Spectrometer (TES) and the MRO Mars Climate Sounder (MCS) indicate that a mean baroclinicity-barotropicity supports intense, large-scale eastward traveling weather systems (i.e., transient synoptic-period waves). Such extratropical weather disturbances are critical components of the global circulation as they serve as agents in the transport of heat and momentum, and generalized scalar/tracer quantities (e.g., atmospheric dust, water-vapor and ice clouds). The character of such traveling extratropical synoptic disturbances in Mars' southern hemisphere during late winter through early spring is investigated using a moderately high-resolution Mars global climate model (Mars GCM). This Mars GCM imposes interactively-lifted and radiatively-active dust based on a threshold value of the surface stress. The model exhibits a reasonable "dust cycle" (i.e., globally averaged, a dustier atmosphere during southern spring and summer occurs). Compared to the northern-hemisphere counterparts, the southern synoptic-period weather disturbances and accompanying frontal waves have smaller meridional and zonal scales, and are far less intense. Influences of the zonally asymmetric (i.e., east-west varying) topography on southern large-scale weather are investigated, in addition to large-scale up-slope/down-slope flows and the diurnal cycle. A southern storm zone in late winter and early spring presents in the western hemisphere via orographic influences from the Tharsis highlands, and the Argyre and Hellas impact basins. Geographically localized transient-wave activity diagnostics are constructed that illuminate dynamical differences amongst the simulations and these are presented.

Hollingsworth, Jeffery L.↗

NASA NTRS1986

Mesospheric gravity-wave climatology at Adelaide

The MF Adelaide partial-reflection radar has been operating continuously since November 1983. This has enabled a climatology of gravity-wave activity to be constructed for the mesosphere. The data have been analyzed for a medium-period range of 1 to 8 hr. and a longer period range between 8 and 24 hr. covering the inertio-period waves. The tidal motions have been filtered out prior to analysis. For the data analyses so far (Nov. 1983 to Dec. 1984), a number of interesting features emerged. Firstly, the wave activity at heights above 80 km shows a small seimannual variation with season with the activity being strongest in summer and winter. At heights below 80 km however, there is a similar but more marked variation with the weakest amplitudes occurring at the time of the changeovers in the prevailing circulation. If breaking gravity waves are responsible for much of the turbulence in the mesosphere, then the periods March to April and September to October might also be expected to be periods of weak turbulence. The wave field appears to be partially polarized. The meridional amplitudes are larger than the zonal amplitudes, especially in water. It is found that the degree of polarization is about 15% in summer and 30% in winter. The polarized component is found to propagate in the opposite direction to the background flow in the stratosphere, which suggests that the polarization arises through directional filtering of the waves as they propagate up from below.

Vincent, R. A.↗

NASA NTRS1982

Transport of aurorally produced N/2D/ by winds in the high latitude thermosphere

A time-dependent, two-dimensional model is developed for describing the meridional circulation of thermospheric odd nitrogen species produced in the auroral zone. The model is based on a previous model by Roble and Gary (1979) extended to upper altitude transport of the nitrogen species. Assumptions made include the existence of a steady neutral wind flowing from low to high latitudes, and an initial background due to scattered Lyman-beta and nightglow emissions. The aurora is also assumed as steady, along with a constant ion production. Predictions made using the model are compared with observations with the Atmosphere Explorer C spacecraft and rocket sounding measurements of the 5200 A distribution near the day-side polar cusp. The model requires thermospheric winds of 100-200 m/sec, flowing from day to nightside. Convective velocities near 1000 m/sec were detected by the Explorer spacecraft, as well as a day-to-nightside flow at the cusp.

Gerard, J.-C.↗

NASA NTRS1987

Resonant excitation of hemispheric barotropic instability in the winter mesosphere

In the present consideration of the subtropical mesospheric jet observed by the Nimbus 7 Limb IR Monitor of the Stratosphere in late 1978, where enhanced planetary wave activity propagating upward into the mesosphere in mid-December led to visible overreflection from the low-latitude reversed gradient region and rapid deceleration of the jet, the possible Rossby wave and mean meridional circulation causes for these observations are considered and rejected as unsuitable. The wave 1 mode is for practical purposes a radiating mode, excitable by sources in the far field. Since the phase speed of the eigenmodes can be made exactly zero when the mean flow vanishes within this region, as observed in mid-December 1978, resonant excitation is possible.

Dunkerton, Timothy J.↗

NASA NTRS1985

Three-dimensional inviscid analysis of radial turbine flow and a limited comparison with experimental data

The three-dimensional inviscid DENTON code is used to analyze flow through a radial-inflow turbine rotor. Experimental data from the rotor are compared with analytical results obtained by using the code. The experimental data available for comparison are the radial distributions of circumferentially averaged values of absolute flow angle and total pressure downstream of the rotor exit. The computed rotor-exit flow angles are generally underturned relative to the experimental values, which reflect the boundary-layer separation at the trailing edge and the development of wakes downstream of the rotor. The experimental rotor is designed for a higher-than-optimum work factor of 1.126 resulting in a nonoptimum positive incidence and causing a region of rapid flow adjustment and large velocity gradients. For this experimental rotor, the computed radial distribution of rotor-exit to turbine-inlet total pressure ratios are underpredicted due to the errors in the finite-difference approximations in the regions of rapid flow adjustment, and due to using the relatively coarser grids in the middle of the blade region where the flow passage is highly three-dimensional. Additional results obtained from the three-dimensional inviscid computation are also presented, but without comparison due to the lack of experimental data. These include quasi-secondary velocity vectors on cross-channel surfaces, velocity components on the meridional and blade-to-blade surfaces, and blade surface loading diagrams. Computed results show the evolution of a passage vortex and large streamline deviations from the computational streamwise grid lines. Experience gained from applying the code to a radial turbine geometry is also discussed.

Choo, Y. K.↗