Cloud-Tracked Winds from Pioneer Venus OCPP Images

Abstract

Analysis of ultraviolet image sequences, obtained from the Pioneer Venus Orbiter Cloud Photopolarimeter and covering five 80-day periods from 1979--1985, provides the first climatological description of the cloud top circulation on Venus. The average zonal winds can be characterized as a 5-day retrograde rotation of the whole cloud-level atmosphere with weak ``jets'' at middle to high latitudes. Both the midlatitude and equatorial zonal winds vary by about 5--8 m s−1 over time spans of 1--6 years. The average meridional circulation is poleward in both hemispheres up to at least 60° latitude, consistent with the presence of a thermally direct Hadley circulation associated with the clouds. The strength of the Hadley circulation also varies with time. Four wave modes are clearly identified: a diurnal solar tide, a semi-diurnal solar tide, a ``4-day equatorial'' wave, and a ``5-day midlatitude'' wave. The semidiurnal tide appears to have an amplitude of about 5 m s−1 and to be approximately constant with time; the diurnal tide varies in amplitude from about 10 m s−1 to less than 5 m s−1. Both tides have phases such that maximum zonal windspeeds occur near the evening terminator. The ``4-day'' wave is wavenumber 1 and has an amplitude of about 5 m s−1 that peaks at the equator and varies with time; in 1982 no wave with this period was apparent in the data. This wave mode is identified as a Kelvin mode by Del Genio and Rossow. The ``5-day'' wave is wavenumber 1 and has an amplitude of about 5 m s−1 that peaks at midlatitudes and varies in time: in 1982 no wave with this period was apparent. This wave mode is identified as an internal Rossby-Haurwitz mode.