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Gerald Schubert

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Curt Covey
and
Gerald Schubert

Abstract

A numerical model of planetary-scale waves in Venus’ atmosphere is used to simulate observed wave-like cloud features such as the dark horizontal Y. The model is based on the linearized primitive equations. Observed variations of static stability and mean zonal wind as a function of altitude are included in the basic state. Preferred modes of oscillation are found by imposing forcing over a range of frequencies, and determining the frequencies at which atmospheric response is greatly enhanced. Preferred responses exist at frequencies which are observed for the Y and other wave-like features. The Y shape can be produced by a linear combination of two model output waves: a midlatitude Rossby wave and an equatorial Kelvin wave. In order to preserve the relative phase between the waves and maintain the Y, nonlinear coupling between the waves is needed. Both waves are upward propagating, similar to the upward propagating planetary waves in Earth's stratosphere. The Kelvin wave may be forced at any altitude, but the Rossby wave must be forced at cloud heights to avoid absorption at a critical level. The Kelvin wave transports westward momentum upward, and thus can act to maintain the strong westward zonal winds on Venus. The Rossby wave acts to decrease the equator-pole temperature difference and therefore would decelerate the zonal wind.

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Keke Zhang
and
Gerald Schubert

Abstract

The onset of penetrative convection in a rotating, gravitationally unstable spherical fluid layer bounded above or below by a stable spherical corotating layer is investigated. Rapid rotation and spherical geometry produce new phenomena in the context of penetrative convection that are fundamentally different from what has been observed in the well-studied plane-layer model. In a slowly rotating or nonrotating spherical system, the character of penetration is qualitatively similar to that in plane fluid layers. In a rapidly rotating spherical system with a spherical layer of stable fluid bounded above by an unstable spherical layer, the stable fluid prevents penetration of convection across the interface between the stable and unstable spherical layers. In the reciprocal situation, however, when a spherical layer of stable fluid is bounded below by an unstable spherical layer, convective motions penetrate from the unstable layer all the way through the outermost stable layer with nearly the same amplitude as in the underlying unstable layer. The phenomena can be explained as a direct consequence of the combined effects of rapid rotation and spherical layer geometry.

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Gerald Schubert
and
Richard E. Young

Abstract

It has been proposed that the observed 4-day retrograde rotation of the Venus atmosphere is a zonal motion of at least the upper atmosphere driven by periodic solar thermal forcing. We have assessed the relative importance of periodic thermal forcing for the atmospheres of Venus, Earth, Mars, Jupiter, Saturn and Uranus. Periodic thermal forcing is likely to play a significant role only in the dynamics of the Venus atmosphere, principally because of the favorable overhead speed of the sun; this is 3 m sec−1, at least two orders of magnitude slower than the overhead solar speeds for the other planets. For a simplified channel flow problem we have examined the detailed mechanisms by which periodic induced circulations transport the momentum necessary to support a mean shear.

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Gerald Schubert
and
Richard L. Walterscheid

Abstract

The amplification and attenuation of small-scale acoustic-gravity waves in Venus's atmosphere is studied with a plane-wave model that realistically simulates height variations in structure and zonal circulation. Forcing for these waves could be convective activity at cloud heights or close to the surface, or turbulence arising from small-scale shear instability of the zonal flow; the model treats both surface forcing and cloud-level forcing by diabatic heating variations in the low-stability layer near the base of the clouds. Waves are attenuated in this cloud-level, low-static-stability layer. Slowly moving waves with small vertical length scales are attenuated by eddy diffusivity. Westward moving waves can undergo critical level absorption. A net enhancement in wave amplitude is ago possible because waves can be trapped between the surface and the base of the low stability layer at about 50 km. Observations of small-scale wave activity at the cloud tops and above can be used to explore uncertain aspects of atmospheric structure and circulation such as the persistence or decay of the atmospheric superrotation with height above the clouds.

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Siegfried D. Schubert
and
Gerald F. Herman

Abstract

A method is demonstrated for evaluating global and zonally averaged heat balance statistics based on a four-dimensional assimilation with an atmospheric general circulation model (GCM). The procedure, which provides observationally constrained model diagnostics, uses the GCM of NASA's Goddard Laboratory for Atmospheric Sciences to evaluate the atmospheric heat balance for the February 1976 Data Systems Test period. The global distribution of the adiabatic and diabatic components of the heat balance are obtained by sampling the continuous GCM assimilation shortly after the insertion of conventional synoptic observations. Sampling times of 6 and 9 h after data insertion were chosen to provide adequate damping of high-frequency oscillations in the vertical velocity field caused by the data insertion.

Salient features of the February 1976 analysis include the following: Maximum rising motion in the mean vertical velocity field at 500 mb over South America, south-central Africa, Australia and the Indonesian archipelago. These regions also were characterized by large values of diabatic heating due to convective latent heat release. The cyclogenetically active regions over the North Atlantic and North Pacific oceans were characterized by maxima in latent heat release due to supersaturation cloud formation, and also maxima in the upward and northward transient eddy heat fluxes. In contrast, the continental west coasts showed a tendency for large downward and southward transient eddy beat fluxes.

Some differences are obtained between the heating rates calculated with the model parameterizations and through a residual method. Other shortcomings of the procedure include data deficiencies in the Southern Hemisphere, which cause the results to be comparatively more model dependent in the high southern latitudes.

The potential applicability of this method of analysis to the recently acquired FGGE data is noted.

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Curt Covey
,
Richard L. Walterscheid
, and
Gerald Schubert

Abstract

A linearized planetary scale wave model is used to investigate the effects of thermal and mechanical damping on atmospheric tides. When the damping rate β is comparable to the frequency of solar diurnal forcing &Omega (δ≳0.1ω), the circulation consists of three parts: a classical vertically propagating “atmospheric tide ” in the upper atmosphere, a simple thermally direct subsolar-to-antisolar circulation or “Halley cell” in most of the lower atmosphere, and finally, a reversed “anti-Halley cell” near the surface. The near-surface circulation produces horizontal divergence near the subsolar point. While tides are a frequently encountered phenomenon (Venus, Earth and Mars), there is so far no observational evidence of a Halley circulation in any planetary atmosphere. A subsolar-antisolar circulation might be possible in Venus'slowly rotating lower atmosphere if the mechanical dissipation time scale is of the order of or less than a Venusian day. Such a circulation could be a factor in maintaining the superrotation of Venus' upper atmosphere.

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Michael J. S. Belton
,
Gerald R. Smith
,
Gerald Schubert
, and
Anthony D. Del Genio

Abstract

We provide morphological and kinematic desc6ptions of the UV markings seen in the Mariner 10 imagery of Venus: the dark horizontal Y, bow-like waves, circumequatorial belts, subsolar disturbance, spiral streaks and bands, polar ring and polar region. The dark horizontal Y is interpreted as a westward-propagating planetary wave with zonal wavenumber 1 and period ∼4.2 days; it may he the superposition of a Rossby-Haurwitz wave dominant at mid-latitudes and a Kelvin wave dominant in equatorial regions. Bow-like waves may be true bow waves formed by the interaction of the rapid zonal flow with internal gravity waves of lower horizontal phase speeds generated by the subsolar disturbance. Circumequatorial belts are interpreted as internal gravity waves with horizontal wavelength ∼500 km and zonal extent ∼5000 km. They are essentially parallel to latitude circles and propagate southward at about 20 m s−1. Cellular features in the subsolar region undoubtedly imply convection there. The identificatiod of both bright- and dark-rimmed cells, with horizontal scales of about 200 and 500 km, respectively, implies a 15 km deep convective layer, based on an analogy with mesoscale convection in the terrestrial maritime atmosphere. The dark areas of the cells may be regions of downwelling. Variability in the location and intensity of the polar ring may be caused by a zonally propagating disturbance, perhaps related to the planetary wave producing the Y in lower latitudes. Circulation patterns and other atmospheric processes in the polar region may be rather different from elsewhere on the planet; only in the polar region are UV markings also visible in the orange.

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R. David Baker
,
Gerald Schubert
, and
Philip W. Jones

Abstract

This paper is the first of a two-part study that investigates internal gravity wave generation by convection in the lower atmosphere of Venus. A two-dimensional, nonlinear, fully compressible model of a perfect gas is employed. The calculations consider the lower atmosphere from 12- to 60-km altitude, thereby including two convection regions: the lower atmosphere convection layer from roughly 18- to 30-km altitude and the cloud-level convection layer from roughly 48- to 55-km altitude. The gravity waves of interest are located in the stable layer between these two convection regions. Part I of this study considers gravity wave generation and propagation in the absence of mean wind shear.

In the absence of mean wind shear, internal gravity waves are primarily generated by cloud-level convection. Horizontal wavelengths (∼10–15 km) are similar to dominant horizontal scales in the cloud-level penetrative region, and intrinsic horizontal phase speeds are comparable to cloud-level downdraft velocities. Without mean wind shear, there is no effective coupling between the lower atmosphere below 34-km altitude and the overlying stable layer. Simulated wave amplitudes and vertical wavelengths agree well with spacecraft observations, suggesting that gravity waves generated by cloud-level convection through the “mechanical oscillator” effect may be responsible for observed variations in the stable layer.

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R. David Baker
,
Gerald Schubert
, and
Philip W. Jones

Abstract

This paper is the second of a two-part study that numerically investigates internal gravity wave generation by convection in the lower atmosphere of Venus. Part I of this study considers gravity wave generation and propagation in the absence of mean wind shear. In Part II, the Venus westward superrotation is included, and wave–mean flow interaction is assessed.

Both lower-atmosphere convection and cloud-level convection play active roles in the dynamics of the stable layer from 31- to 47-km altitude when mean wind shear is present. This result contrasts with the simulation without mean wind shear presented in Part I where cloud-level convection was primarily responsible for gravity wave generation in the stable layer. In the presence of mean wind shear, upward entrainment from lower-atmosphere convection and downward penetration from cloud-level convection are comparable in magnitude. Convectively generated internal gravity waves have horizontal wavelengths (∼25–30 km) comparable to horizontal scales in both convection layers. Quasi-stationary gravity waves (with respect to the lower convection layer) occur in the lower part of the stable layer, while both eastward- and westward-propagating waves generated by cloud-level convection exist in the upper part of the stable layer. Simulated wave amplitudes and vertical wavelengths agree well with observations. Eastward-propagating waves generated by cloud-level convection experience critical level absorption in the stable layer and thus decelerate the Venus westward superrotation below the clouds. The deceleration is comparable in magnitude to zonal accelerations above the clouds by thermal tides.

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