Effect of meridional circulation on the venusian cloud layer: A model study

The meridional circulation can have a large influence on the cloud formation on Venus. In our model, the observed depression of near-infrared optical thickness at mid-latitude is reproduced as a result of the enhancement of cloud thickness at low and high latitudes. At high latitudes, the poleward transport of H2SO4-H2O droplets in the upper atmosphere makes a thick cloud, since most of the H2SO4-H2O droplets photochemically produced are transported poleward by the meridional circulation. At low latitudes, the large-scale ascent of the concentrated H2SO4 vapor forms a dense lower cloud in accord with observations by entry probes. The equatorward transport of H2SO4 vapor below the cloud by the meridional circulation and the sedimentation of droplets in the lower cloud leads to the accumulation of H2SO4 vapor around the cloud base at low latitudes. The H2SO4 vapor distribution agrees with the radio occultation observations. The middle cloud can be caused by the condensation of H2SO4 vapor carried aloft by convective mixing. (C) 1999 COSPAR. Published by Elsevier Science Ltd.