Zonal Wind Speeds in the Venusian Atmosphere from the Akatsuki and Venus Express Temperature Fields

Abstract

Between 50 and 100 kilometers, superrotating zonal winds characterize the Venusian atmosphere. As a result of the superrotation, cyclostrophic balance can approximate the zonal winds in the Venusian mesosphere, where a pressure gradient counteracts the equatorward centrifugal force. Direct wind measurements are limited; the current available data on winds in the Venusian atmosphere use single-altitude cloud-tracked features. The existing calculations for zonal wind velocities using the cyclostrophic approximation do not utilize temperature fields from the Akatsuki Radio Science instrument or the Venus Express Solar Occultation in the Infrared (SOIR) instrument. Using the available information about cloud-tracked winds as boundary conditions, the assumption of cyclostrophic balance is applied up to 100 kilometers to calculate zonal wind velocities through upward and downward integrations. Above 100 kilometers, cyclostrophic balance breaks down due to a subsolar-to-antisolar circulation. Results demonstrate high wind velocities that peak between -100 m/s and -170 m/s, depending on the boundary condition and the temperature field used. Across all calculations, there is no evidence of a strong mid-latitudinal jet between 40 and 50 degrees latitude in either hemisphere, but there are slight increases in wind speed at latitudes around 30 degrees. Another finding that previous works on the zonal wind speed in the Venusian atmosphere do not account for is an increase in zonal winds with altitude at certain latitudes. A significant dependence of the zonal winds on the lower boundary and temperature field used is clear; however, all calculations agree on a superrotating atmosphere with significantly reduced wind speeds at high latitudes. The characterization of the zonal wind field will aid in future studies, like the characterization of gravity waves in the Venusian mesosphere