Radiative-Convective Equilibrium of the Venusian Atmosphere

Abstract

The radiative equilibrium and the radiative-convective equilibrium of the Venusian atmosphere corresponding to the averaged insolation are numerically calculated for the layer up to 76km. The non-grey absorption in infra-red region due to carbon dioxide and water vapour and the grey absorption due to the cloud are included in the present study. The amount of solar radiation reaching the ground is assumed to be 1.5% of the solar constant at the orbit of Venus based on the information by Venera-8, and the rest of the absorbed energy is assumed to be distributed uniformly in the cloud layer. The heat trans-port by convection is included by means of eddy mixing of the potential temperature.
The results of the present calculations on the radiative equilibrium and the radiative- convective equilibrium lead to the conclusion that the high surface temperature can be explained in terms of a green house effect provided that the concentration of water vapour assumed in the present study is not far from the correct value. Namely, water vapour is indispensable to the maintenance of the high surface temperature. The cloud also has an important effect concerning the infra-red region on the thermal structure of the atmosphere, especially on that in the upper layer. However the cloud's contribution to green house effect seems not to be indispensable, as indicated from the result that the high surface temperature can be maintained in spite of a small optical thickness of the cloud assumed in the present study. In the calculations of the radiative-convective equilibrium, convection takes place in most part of the atmosphere. The eddy diffusion coefficient is of the order of 106cm²/sec in the lower layer and 107cm²/sec in the upper layer. However a non-convective layer appears below the cloud base under most conditions assumed in the present study. It suggests that a connection between the lower layer and the cloud layer is rather weak. A calculation on the diurnal variation of the Venusian atmosphere was made by the present model in which lateral mixing is neglected. The result indicates that in the night side strong convection occurs to transport heat upward within the cloud layer and prevents cooling of the cloud top. The maximum diurnal temperature range averaged over a layer between the surface and the 1km level turns out to be 0.3°K.