Cyclostrophic wind in the mesosphere of Venus from Venus Express observations

The dynamics of Venus mesosphere was investigated by using observations from the Venus Express (VEx) mission. In the frame of this work, zonal thermal winds were derived from the VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) and VeRa (Radio Science Experiment) temperature soundings by applying a special approximation of the thermal wind equation, the cyclostrophic balance. Thanks to Venus Express capabilities, the variability of zonal wind with latitude, altitude and local time was analysed in detail. The main features of the retrieved winds are: (1) the midlatitude jet with a maximum speed up to 140 ± 15 m s¹ which occurs around 50°S latitude at 70 km altitude; (2) the fast decrease of the wind speed from 60°S toward the pole; (3) the decrease of the wind speed with increasing height above the jet. Thermal winds were compared to the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images that allowed to validate the cyclostrophic assumption. From the temperature and thermal wind fields we calculated the Richardson number, the parameter characterising the stability of the atmosphere. We have found that the atmosphere is dominated by convection from ~45 km altitude up to the cloud top. High values of Richardson number in the region of midlatitude jet indicate highly stable atmosphere. Verification of the necessary condition for barotropic instability implies that it can occur on the poleward side of the midlatitude jet where planetary waves are expected to play an important role in the maintenance of the circulation.