Venus cloud structure and radiative energy balance of the mesosphere

A dense cloud layer located at 50–75 km altitude completely covers Venus. The cloud aerosol consists mainly of sulfuric acid mixed with still unknown constituents. Due to their great opacity, the clouds play an important role in the radiative energy balance, the general circulation, and the atmospheric chemistry. The knowledge about the upper part of the clouds is especially important to understand the Venus mesosphere, because it is responsible for thermal cooling and solar heating that eventually results in the strongest retrograde zonal wind near the cloud tops in all altitudes. Remote observations have indicated significant temporal and spatial variations of the cloud structure that should affect radiative energy balance of the mesosphere. This thesis investigates the upper cloud layer of Venus. Firstly, the vertical structure of the upper clouds and its variation with latitude are retrieved from the Venus Express observations. For this purpose, we have developed a new technique which uses temperature profiles from the radio science occultation experiment VeRa and thermal emission spectra in the 4.4–5.0 μm range collected by thermal infrared spectrometer VIRTIS. We implemented a comprehensive radiative transfer code including line-by-line calculations of gaseous opacities, and taking account for multiple scattering of aerosols. This study shows significant latitudinal trends in the upper cloud structure. Secondly, we investigated the influence of clouds, and their variability on the thermal fluxes and cooling rates calculated in a broad spectral range (3.86–200 μm). We developed and used an atmospheric model based on the derived parameters of the upper cloud and the observed temperature field. The result, thermal cooling rates, is compared with the solar heating rates to derive the radiative energy balance.