Zonal Winds in the Middle Atmosphere of Venus from Pioneer Venus Radio Occultation Data

Abstract

Pioneer Venus radio occultation data for four seasons between December 1978 and October 1981 are used to construct meridional cross sections of temperature and zonal wind velocity for the middle atmosphere of Venus in the altitude range of ?40?80 km and in the latitude range of ?1 5?85°. Wind speeds are derived assuming cyclostrophic balance. The wind field contains as many as three jets: first, a prominent one above the clouds, situated between 50 and 55° latitude, with a maximum speed of ≈140 m s?1; a second with a speed of 95 m s?1 centered at 70° and at 60 km altitude; and a possible third jet at 15° and 65 km altitude, with a speed of ≈100 m s?1. The notable midlatitude jet is associated with the cold polar collar. Wind speeds above ?70 km generally decrease with height because of the warm pole at these levels. However, within the jet the speed is 100 m s?1 at 83 km. The jets may be barotropically unstable and inertial instability may occur above the clouds on the equatorial side of the main jet. Cyclostrophic balance breaks down in polar latitudes above the clouds where the zonal wind speed becomes small enough that eddy effects may be important. Our derived wind speeds compare favorably with Pioneer Venus probe winds below ?65 km and with ultraviolet cloud-tracked winds in equatorial latitudes.