Cyclostrophic wind in the mesosphere of Venus from Venus Express observations

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The dynamics of Venus' mesosphere were studied using data from the Venus Express mission. Zonal thermal winds were derived from VIRTIS and VeRa temperature soundings through a specialized approximation of the thermal wind equation known as cyclostrophic balance. This analysis revealed significant variability in zonal wind patterns based on latitude, altitude, and local time. Key findings include: (1) a midlatitude jet with peak speeds of 140 ± 15 m/s at 50°S latitude and 70 km altitude; (2) a rapid decrease in wind speed from 60°S toward the pole; and (3) a reduction in wind speed with increasing altitude above the jet. The thermal winds were compared to cloud-tracked winds from the Venus Monitoring Camera, validating the cyclostrophic assumption. Additionally, the Richardson number, which indicates atmospheric stability, was calculated from the temperature and thermal wind fields. Results showed that convection dominates the atmosphere from approximately 45 km altitude to the cloud top, with high Richardson numbers in the midlatitude jet region indicating a stable atmosphere. The verification of conditions for barotropic instability suggests it may occur on the poleward side of the midlatitude jet, where planetary waves are likely influential in maintaining circulation.