Vertical structure of the Venus cloud top from the VeRa and VIRTIS observations onboard Venus Express

We investigate the Venus cloud top structure by joint analysis of the data from Visual and Thermal Infrared Imaging Spectrometer (VIRTIS) and the atmospheric temperature sounding by the Radio Science experiment (VeRa) onboard Venus Express. The cloud top altitude and aerosol scale height are derived by fitting VIRTIS spectra at 4-5 mu m with temperature profiles taken from the VeRa radio occultation. Our study shows gradual descent of the cloud top from 67.2 +/- 1.9 km in low latitudes to 62.8 +/- 4.1 km at the pole and decrease of the aerosol scale height from 3.8 +/- 1.6 km to 1.7 +/- 2.4 km. These changes correlate with the mesospheric temperature field. In the cold collar and high latitudes the cloud top position remarkably coincides with the sharp minima in temperature inversions suggesting importance of radiative cooling in their maintenance. This behaviour is consistent with the earlier observations. Spectral trend of the cloud top altitude derived from a comparison with the earlier observations in 1.6-27 mu m wavelength range is qualitatively consistent with sulphuric acid composition of the upper cloud and suggests that particle size increases from equator to pole. (C) 2011 Elsevier Inc. All rights reserved.