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At least 145 records · Page 8

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NASA NTRS1984

The zonally averaged circulation, temperature, and compositional structure of the lower thermosphere and variations with geomagnetic activity

A zonally averaged chemical-dynamical model of the thermosphere is used to examine the effect of high-latitude particle and Joule heating on the neutral composition, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating alone is a summer-to-winter flow, with a winter enhancement in atomic oxygen. The high-latitude heat sources drive mean circulation cells that reinforce the solar-driven circulation in the summer hemisphere and oppose this circulation in the winter hemisphere. The changes in wind and temperature caused by the high-latitude heat sources increase the relative concentration of N2 and O2 in the high-latitude upper thermosphere and decrease the O concentration in the high-latitude lower thermosphere. For prolonged moderate levels of geomagnetic activity the peak atomic oxygen density in the polar regions can decrease by factors of 2-3 from geomagnetic quiet conditions.

Roble, R. G.↗

NASA NTRS1984

The effects of physical processes on the Hadley circulation

The Hadley cell is involved in the energy, momentum and moisture budgets in the atmosphere; it may be expected to change as sources and sinks of these quantities are altered due to climate perturbations. The nature of the Hadley cell change is complicated since alterations in one budget generally result in alterations in the others. Thus, Hadley cell sensitivity needs to be explored in an interactive system. In the GISS GCM (model I), a number of experiments are performed in which physical processes in each of the three budgets are omitted, the system adjusts, and the resultant circulation is compared to that of the control run. This procedure highlights which effects are most important and reveals the nature of the various interactions. The results emphasize the wide variety of processes that appear capable of influencing the mean circulation. The intensity of the circulation is related to the coherence of the thermal forcing, and to the thermal opacity of the atmosphere. When all frictional forcing is removed, the circulation is restricted to the equatorial region. The latitudinal extent appears to be controlled primarily by eddy processes (Ferrel cell intensity). The implications for climate modeling and climate projections (e.g. rainfall changes) are discussed.

Rind, D.↗

NASA NTRS1984

Interannual Variability in the Observed Circulation

An observational study of the relative roles of transient eddies and the time-mean flow in maintaining the momentum balance and in forcing vertical motion illuminated the role of time-mean secondary circulations forced by transient eddies. These secondary circulations transport westerly momentum from the upper to the lowr troposphere in the storm tracks, accelerating surface westerlies and making the time-mean flow more barotropic. An investigation of teleconnections in the Southern Hemisphere revealed a zonal wavenumber 3 pattern of low-frequency variability in the winterime midlatitudes and a land-ocean seesaw during summer. An observational comparison of the Northern Hemisphere winters found substantial differences in circulation patterns. The circulation changes may have been associated with substantial changes in low-frequency transient eddies in the Pacific, perhaps related to barotropic instability in the exit region of the Asian jet. Changes in the baroclinicity of the low-level flow appeared to be associated with changes in the storm tracks, while changes in the transient eddies appeared to play a minor role in interannual differences in the zonal momentum balance or in the forcing of vertical motion.

White, G. H.↗

NASA NTRS1984

Transverse ageostrophic circulations associated with elevated mixed layers

The nature of the frontogenetically forced transverse ageostrophic circulations connected with elevated mixed layer structure is investigated as a first step toward diagnosing the complex vertical circulation patterns occurring in the vicinity of elevated mixed layers within a severe storm environment. The Sawyer-Eliassen ageostrophic circulation equation is reviewed and applied to the elevated mixed layer detected in the SESAME IV data set at 2100 GMT of May 9, 1979. The results of the ageostrophic circulation diagnosis are confirmed and refined by considering an analytic specification for the elevated mixed layer structure.

Keyser, D.↗

NASA NTRS1985

Mars atmospheric circulation - Aspects from Viking Landers

Winds measured by the two Viking Landers have been filtered and then compared with predictions from the general circulation model and to Orbiter observations of clouds and surface phenomena that indicate wind direction. This was done to determine the degree to which filtered winds may represent aspects of the general circulation. Excellent agreement was found between wind direction data from Lander 1 and the model predictions and Orbiter observations. For Lander 2, agreement was generally good, but there were periods of disagreement which indicate that the filtering did not remove other extraneous effects. It is concluded that Lander 1 gives a good representation of the general circulation at 22.5 deg N latitude but that Lander 2 is suspect. Most wind data from Lander 1 have yet to be analyzed. It appears that when analyzed these Lander 1 data (covering 3.5 Mars years) can provide information about interannual variations in the general circulation at the Lander latitude.

Ryan, J. A.↗

NASA NTRS1985

Diagnosis of ageostrophic circulations in a two-dimensional primitive equation model of frontogenesis

A two-dimensional primitive equation frontogenesis model is applied to the identification of dynamical mechanisms which contribute to transverse ageostrophic circulation in warm and cold advection situations. Attention is focused on conditions conducive to significant alongfront ageostrophic flow with divergence confined to the transverse plane. The circulation is partitioned as a function of forcing mechanisms accompanied by confluence, horizontal shear and the alongfront component of the ageostrophic wind. A series of diagnostic equations is defined for the transverse geostrophic circulation and instantaneous contribution from each partitioned component of the circulation are quantified. In the cold advection case, results are obtained which are consistent with positive feedback between the subsidence pattern and forcing by horizontal shear.

Keyser, D.↗

NASA NTRS1985

Large-scale circulation departures related to wet episodes in northeast Brazil

Large scale circulation features are presented as related to wet spells over northeast Brazil (Nordeste) during the rainy season (March and April) of 1979. The rainy season season is devided into dry and wet periods, the FGGE and geostationary satellite data was averaged and mean and departure fields of basic variables and cloudiness were studied. Analysis of seasonal mean circulation features show: lowest sea level easterlies beneath upper level westerlies; weak meridional winds; high relative humidity over the Amazon basin and relatively dry conditions over the South Atlantic Ocean. A fluctuation was found in the large scale circulation features on time scales of a few weeks or so over Nordeste and the South Atlantic sector. Even the subtropical High SLP's have large departures during wet episodes, implying a short period oscillation in the Southern Hemisphere Hadley circulation.

Sikdar, D. N.↗

NASA NTRS1985

Impact of FGGE on diagnoses of the general circulation

Several studies have pointed out that a number of significant differences exist in circulation fields derived from the various Level IIIb analyses. For example, differences in the strength of the January Hadley cell between the GFDL and ECMWF analyses are as large as 35 percent, and there are some locations over the Northern Hemisphere where the 200 mb zonal wind in January differs by 10 m/s between these two analyses. Calculations of the global energy cycle based on the GFDL and ECMWF analyses are significantly different as well. In addition, the uncertain quality of some of the FGGE IIIb moisture and vertical motion fields raises doubts about whether new insights into the role of these fields in the general circulation will be gained from the FGGE data. Overall, therefore, a sense of disappointment emerges that the FGGE analyses have thus far failed to live up to the expectation that they would yield more definitive general circulation statistics. The shortcomings in the Level IIIb analyses need to be rectified before full confidence can be placed in results pertaining to the general circulation that are based on such modern data assimilation approaches.

Rosen, R. D.↗

NASA NTRS1986

General circulation model simulations of superrotation in slowly rotating atmospheres - Implications for Venus

The results of three-dimensional general circulation model experiments simulating the earth atmosphere with planetary rotation rates between 1 and 1/64th times the earth's and diurnally averaged thermal forcing are used to evaluate Venus atmospheric superrotation theories invoking the upward transport of angular momentum from the solid planet by the zonal mean circulation. While the theories predict that superrotation is a common feature of slowly rotating planetary atmospheres, suggesting that superrotation should appear in the present, slowly rotating cases, the steady state circulations are noted to include only weak globally averaged superrotation, consistent with previously reported results from lower resolution models. The thermally driven equator-to-pole circulation rapidly generates zonal mean winds during spinup, near the planetary surface, precluding vertical angular momentum transport and suppressing further superrotation development.

Covey, C.↗

NASA NTRS1986

Tracer transport by the diabatic circulation deduced from satellite observations

Nimbus-7 sensor data were used to track the diabatic circulation in the stratosphere to study the advective transport of CH4 and N2O as tracer species. Advective transport by the mean circulation was found to be a function of the temperature field and associated deviations from radiative equilibrium. A photochemical model was applied to account for the disappearance of the tracer species from the stratosphere. Comparisons between the SAMS data and modeling on the basis of the chemical loss rates of the tracers and the LIMS circulation data showed that the model predictions underestimated the resident abundances, although the global distributions and circulations exhibited a good match.

Solomon, S.↗

NASA NTRS1987

Large-scale circulation departures related to wet episodes in north-east Brazil

Large scale circulation features are presented as related to wet spells over northeast Brazil (Nordeste) during the rainy season (March and April) of 1979. The rainy season is divided into dry and wet periods; the FGGE and geostationary satellite data was averaged; and mean and departure fields of basic variables and cloudiness were studied. Analysis of seasonal mean circulation features show: lowest sea level easterlies beneath upper level westerlies; weak meridional winds; high relative humidity over the Amazon basin and relatively dry conditions over the South Atlantic Ocean. A fluctuation was found in the large scale circulation features on time scales of a few weeks or so over Nordeste and the South Atlantic sector. Even the subtropical High SLPs have large departures during wet episodes, implying a short period oscillation in the Southern Hemisphere Hadley circulation.

Sikdar, Dhirendra N.↗

NASA NTRS1987

Navier-Stokes calculations and turbulence modeling in the trailing edge region of a circulation control airfoil

The accurate prediction of turbulent flows over curved surfaces in general and over the trailing edge region of circulation control airfoils in particular requires the coupled efforts of turbulence modelers, numerical analysts and experimentalists. The purpose of the research program in this area is described. Then, the influence on turbulence modeling of the flow characteristics over a typical circulation control wing is discussed. Next, the scope of this effort to study turbulence in the trailing edge region of a circulation control airfoil is presented. This is followed by a brief overview of the computation scheme, including the grid, governing equations, numerical method, boundary conditions and turbulence models applied to date. Then, examples of applications of two algebraic eddy viscosity models to the trailing edge region of a circulation control airfoil is presented. The results from the calculations is summarized, and conclusions drawn based on examples. Finally, the future directions of the program is outlined.

Viegas, John R.↗

NASA NTRS1987

The existence and stability of steady circulations in a conditionally symmetrically unstable basic flow

The existence of steady, nonlinear circulations in a flow susceptible to conditionally symmetric instability is studied, treating the latent heating as an energy source which is implicitly related to the motion field. The viscous nonlinear circulations of symmetrical instability are briefly discussed, and an existence theorem for steady, nonlinear symmetric circulations with bounded rates of latent heat release is given. The uniqueness and stability of these circulations are discussed, and some physical interpretations are given.

Xu, Qin↗

NASA NTRS1987

A simplified model of the modified Hadley circulation

In this paper, a simplified model of modified Hadley circulation that is amenable to analysis is developed to find the low-latitude upper-tropospheric circulation which would exist in the absence of transient eddies. The model is developed as a perturbation of the ideal Hadley circulation solution due to zonally assymmetric thermal forcing and is solved semianalytically. The modified Hadley circulation, found using forcing derived from observed outgoing long-wave radiation, exhibits some similarities to observed features of the tropical and subtropical time-mean upper-tropospheric flow, particularly with regard to the velocity potential.

Schneider, Edwin K.↗

NASA NTRS1988

Active and inactive phases of the South Pacific Convergence Zone and changes in global circulation patterns - A case study

A set of FGGE Level III-b analyses produced at the NASA Goddard Laboratory for Atmospheres (GLA) is used to examine changes that occur in the global-scale circulation features during the period, January 10 - February 13,1979. In the first two weeks of this period, the South Pacific Convergence Zone (SPCZ) and its convective cloud band were observed to be dominant features of the circulation. Subsequent to January 24, there were marked changes in the global-scale circulation, particularly in the Southern Hemisphere tropics. Concomitant with these changes was the disappearance of the SPCZ and its cloud band. The primary purpose of this study is to compare some general circulation parameters, which frequently correspond to deep convection, for two 15-day periods: January 10-24, when the SPCZ was very convectively active, and January 28 - February 11 when it was inactive. Daily variations of some parameters are also shown. It is seen that distinct changes occur in each parameter by the end of the first period, particularly in the vicinity of the SPCZ. Suggestions are offered regarding mechanisms which might be responsible for the observed changes.

Huang, H.-J.↗

NASA NTRS1989

Minimal modeling of the extratropical general circulation

The ability of low-order, two-layer models to reproduce basic features of the mid-latitude general circulation is investigated. Changes in model behavior with increased spectral resolution are examined in detail. Qualitatively correct time-mean heat and momentum balances are achieved in a beta-plane channel model which includes the first and third meridional modes. This minimal resolution also reproduces qualitatively realistic surface and upper-level winds and mean meridional circulations. Higher meridional resolution does not result in substantial changes in the latitudinal structure of the circulation. A qualitatively correct kinetic energy spectrum is produced when the resolution is high enough to include several linearly stable modes. A model with three zonal waves and the first three meridional modes has a reasonable energy spectrum and energy conversion cycle, while also satisfying heat and momentum budget requirements. This truncation reproduces the basic mechanisms and zonal circulation features that are obtained at higher resolution. The model performance improves gradually with higher resolution and is smoothly dependent on changes in external parameters.

O'Brien, Enda↗

NASA NTRS1991

South Pacific convergence zone and global-scale circulations

The major goals of the research can be classified under three categories: (1) heat and moisture budget studies in the tropics; (2) the role of tropical forcing on subtropical wind maxima in the Southern Hemisphere; and (3) general circulation modeling of low latitude circulations. Research related to the first goal included budget estimates of precipitation, as well as a comparison to rainfall rates observed at numerous island stations. It also included a comparison between observed and satellite estimates of precipitable water. Results and methods used to achieve this goal are given. Included in the second research goal were diagnostic studies of the intraseasonal oscillation which is known to be an important component of organized tropical convection. Also, the role of extratropical forcing on subtropical wind maxima was examined through the use of Eliassen-Palm (EP) fluxes. Again, results and procedural details are discussed in section 3. To accomplish the third research goal, the GLA general circulation model (GCM) was ported to Purdue University and successfully used in a number of control and experimental runs to investigate the significance of tropical heating on subtropical and extratropical circulation features. As with the first two research goals, results and methodology are detailed. Documentation of publications, theses, conference preprint papers, and other scientific information relevant to the research results and significant accomplishments achieved under the contract are given. Finally, the conclusions of the sponsored research are given.

Vincent, Dayton G.↗

NASA NTRS1991

Global ocean circulation by altimetry

The overall objectives of this project are to determine the general circulation of the oceans and many of its climate and biochemical consequences through the optimum use of altimetry data from TOPEX/POSEIDON and related missions. Emphasis is on the global-scale circulation, as opposed to the regional scale, but some more local studies will be carried out. Because of funding limitations, the primary initial focus will be on the time-dependent global-scale circulation rather than the mean; eventually, the mean circulation must be dealt with as well.

Wunsch, Carl↗