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Eugenia Kálnay De Rivas

Abstract

The deep circulation of the atmosphere of Venus is simulated by means of two-dimensional numerical models. Two extreme cases are considered: first, rotation is neglected and the subsolar point is assumed to be fixed; second (and more realistically), the solar heating is averaged over a Venus solar day and rotation is included. For each case a Boussinesq model, in which density variations are neglected except when coupled with gravity, and a quasi-Boussinesq model which includes a basic stratification of density and a semi-gray treatment of radiation, are developed. The results obtained with the Boussinesq models are similar to those obtained by Goody and Robinson and by Stone. However, when the stratification of density is included and most of the solar radiation is absorbed near the top, the large-scale circulation is confined to the upper layers of the atmosphere during the 4×107 sec of simulated time. We cannot be sure that on a much longer time scale (109 sec) the circulation will not penetrate the interior, but our results suggest that radiation will tend to make the lower atmosphere highly stable. When solar radiation is allowed to penetrate the atmosphere, so that at the equator 6% of the incoming solar radiation reaches the surface, then the combination of a more deeply driven circulation and a partial greenhouse effect is able to maintain an adiabatic stratification.

The effect of symmetrical solar heating is to produce direct Hadley cells in each hemisphere with small reverse cells near the poles. Poleward angular momentum transport in the upper atmosphere produces a shear in the zonal motion with a maximum retrograde velocity of the order of 10 m sec−1 at the top of the atmosphere.

The numerical integrations were performed using non-uniform grids to allow adequate resolution of the boundary layers.

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Eugenia Kálnay de Rivas

Abstract

No abstract available.

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Eugenia Kálnay de Rivas

Abstract

The results of two-dimensional simulations of the deep circulation of Venus are presented. They prove that the high surface temperature can only be explained by the greenhouse effect, and that Goody and Robinson's dynamical model is not valid. Very long time integrations, up to a time comparable with the radiative relaxation time, confirm these results. Analytical radiative equilibrium solutions for a semi-grey atmosphere, both with and without an internal heat source, are presented. It is shown that the green-house effect is sufficient to produce the high surface temperature if τT * ≫ 100 and S = τS *T * ≲ 0.005. This result is still valid in the presence of an internal heat source of intensity compatible with observations.

A two-dimensional version of a three-dimensional model is used to test the validity of the new mechanism proposed by Gierasch to explain the 4-day circulation. Numerical experiments with horizontal viscosities vH = 1011 – 1012 cm2 s−1 failed to show strong zonal velocities even for the case of large Prandtl numbers. It is observed that the dissipation of angular momentum introduced by the strong horizontal diffusion more than compensates for the upward transport of angular momentum due to the Hadley cell.

Preliminary three-dimensional calculations show a tendency to develop strong small-scale circulations.

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