Microphysics of Venusian Clouds in Rising Tropical Air

Abstract

As the global distribution of Venusian H2SO4?H2O clouds is strongly related to the global circulation of H2SO4 governed by wind transport and sedimentation of droplets, the circulation of H2SO4 in the Tropics was studied by simultaneously solving advection and cloud microphysics equations using a one-dimensional model that includes a weak upwelling representing the rising branch of Hadley circulation near the equator. H2SO4 vapor in the upper cloud region is supplied by photochemical production and condenses into cloud droplets that are removed from the tropical atmosphere by Hadley circulation, whereas in middle and lower cloud regions, dynamical processes supply H2SO4 vapor from below such that resultant droplets are large and fall against the upwelling. The combination of the mean upward advection of vapor and the sedimentation of droplets leads to accumulation of H2SO4 and H2O vapor near the cloud base: an observed phenomenon. A separate model run was performed to investigate the effect of transient strong winds on condensational growth, with results indicating that transient winds produce a variety of size distributions similar to those observed. Variation in cloud thickness associated with such events is thought to explain large opacity variations in near-infrared observations.