Zonal and meridional circulation of the lower atmosphere of Venus determined by radio interferometry

Abstract

From earth‐based Doppler and interferometric radio observations we determined the paths, in three dimensions and as functions of time, taken by the Pioneer probes as they fell to the surface of Venus. From the motion of each probe below about 65‐km altitude we were able to infer the ambient wind velocity with an estimated uncertainty of about 1 m s−1 in each vector component. The magnitude of the velocity was about 1 m s−1 or less near the surface of the planet and about 100 m s−1 near 65‐km altitude at all four probe locations. Distinct strata of high wind shear were centered at altitudes of 15, 45, and 60 km, where the atmosphere is most stable against verticle motion. Except within a few kilometers of the surface, the wind velocity was always directed within a few degrees of due west. At the day and the night probe sites, which were separated by 100° in longitude and 3° in latitude, the altitude profiles of the westward velocity were virtually identical. Thus the dominant motion of the lower atmosphere seems to be a retrograde zonal rotation. Eddies appear to account for most of the instantaneous meridional velocity. However, in the radiatively heated middle cloud layer, between 50‐ and 55‐km altitude, equatorward flow of 1–7 m s−1 was observed for all four probes. These observations, coupled with other observations of ∼10 m s−1 poleward average meridional velocity for cloud top features at about 65‐km altitude, suggest that within the clouds a thermally driven mean meridional circulation is superimposed on the much more rapid zonal rotation.

Venus Atmospheric Thermal Structure and Radiative Balance

“…For entry probe data such as Pioneer Large and Small probes, the hydrostatic law is required for pressure and altitude calculations (Seiff et al 1980). Interestingly, although the Pioneer probes were tracked using DLBI (Counselman et al 1980), there appear to be no estimates of the distance of the falling probes relative to the planet, which should have been possible from the DLBI data. A comparison of the tracking derived altitudes with the Seiff et al (1980) values would have revealed a discrepancy due to the constant molecular weight assumption.…”

“…Longitudinal differences from surface to the cloud tops (∼ 70 km) are < 5 K in low latitudes (Seiff et al 1980;Kliore 1985) and can be ∼ 20 K in higher latitudes (> 60°). In the deep atmosphere, at 10 km altitude, winds are as fast as 5 m s −1 (Counselman et al 1980), suggesting at least some meridional temperature gradient must exist if the flow is cyclostrophic. Given the large thermal inertia, the temperature differences in the near surface atmosphere are expected to be small in the deep atmosphere (Stone 1975).…”

Theoretical Estimation of the Superrotation Strength in an Idealized Quasi-Axisymmetric Model of Planetary Atmospheres

“…The mechanism of such strong superrotation is a mystery of the general circulation of planetary atmospheres. The observed equatorial westward wind of the Venus atmosphere achieves a speed of -100 m s −1 at the cloud-top level of -65 km altitude (e.g., Counselman III et al 1980). This zonal wind is about 55 times faster than the planetary rotation speed at the equator.…”

Global-Scale Winds at the Venus Cloud-Top Inferred from Cloud Streak Orientations

“…or albedo of the clouds, or it may represent real changes While this is not strictly true, cloud-tracked wind velocities in the flow field at a fixed elevation. We make the assumpdo agree with in situ and spectroscopic measurements (for tion that average streak orientations within a single epoch example, spectroscopy: Traub and Carleton 1975; Pioneer can be used to infer a meaningful estimate of the average Venus probes: Counselman et al 1980; VEGA balloons: flow field. Sagdeev et al 1986).…”