The Venusian Y-Shaped Cloud Pattern Based on an Aerosol-Transport Model

Abstract

Venusian Y-shaped clouds have been observed by ultraviolet (UV) detectors. Recently, it has been demonstrated that the Y-shaped cloud pattern is maintained by the dynamical combination of an equatorial 4-day wave and a Rossby wave. In the model of the Y-shaped cloud, however, material transport was not considered. Present numerical simulations, including not only dynamical transport, but also the essential microphysics of aerosols, are conducted together with the chemical reactions of the aerosol precursor gas, SO2. Results indicate that aerosols are accumulated in the polar regions due to poleward transport. As a result, the scattering coefficient becomes higher with increasing latitude. At the low-latitude cloud-top heights, the dark (bright) region is formed by the small (large) aerosol concentration associated with planetary-scale waves since the zonal-mean aerosol concentration is relatively small. On the other hand, the longitudinal contrast of the scattering coefficient becomes very small at the high-latitude cloud top since the zonal-mean aerosol concentration is much larger than the perturbed one. In the cloud-top regions where aerosol scattering is weak, solar radiation may penetrate the area without suffering much extinction. The UV radiation is strongly absorbed in the middle cloud layer (?55 km), while visible radiation is scattered within this layer. As a result, weakly scattering regions appear dark in UV images. On the other hand, visible radiation is scattered in the middle cloud layer, so that visible image contrast hardly occurs. The incorporation of planetary-scale waves leads to the Y-shaped cloud pattern at low latitudes. Bright polar bands are also formed in the high-latitude regions, where the total surface area of aerosols reaches a maximum.