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M. E. Koukouli, P. G. J. Irwin, F. W. Taylor (Atmospheric, Oceanic and Planetary Physics, Oxford University)
For a period of almost eighty days, from December 1978 to February 1979, the Pioneer Venus Orbiter Infrared Radiometer measured the thermal emission from the middle atmosphere of Venus. Temperature, cloud optical depth and water vapour mixing ratio were retrieved as a function of height. A remarkably high abundance of water vapour in the early afternoon, almost a hundred times the planet-wide average was reported in a localised region above the cloud-tops at equatorial latitudes, accompanied by an increase in cloud top height further downwind.
A set of microphysical and photochemical models of the composition and growth of Venus' cloud droplets and their optical properties has been developed to investigate the mechanisms responsible for the observed features. In the models, solar heating in and below the clouds causes convection that transports water vapour and sulphuric dioxide from lower levels of the atmosphere, enhancing the chemical reaction rates and increasing the optical depth of the clouds. The water vapour maximum is displaced from the subsolar point by the prevailing super-rotational zonal wind and coincides with what appears in ultraviolet images to be a convective and sulphur dioxide-rich region. The rise in cloud-top height is produced as the excess water reacts with sulphur dioxide increasing the cloud opacity and raising its effective height.
Four distinct physical processes that attempt to simulate the above characteristics have been investigated. We conclude that no one physical mechanism can account both quantitatively and qualitatively for the Pioneer Venus observations. The basis for a more sophisticated model incorporating a combination of the above processes is set for use in analysing the combined Pioneer Venus and Venera 15 data sets.
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