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

Abstract

The orientations of the small cloud streaks that are always found at mid-latitudes in ultraviolet images of the Venus cloud-top provide a means for characterizing the global-scale circulation, including the wind field on the night side of Venus. Assuming that small clouds act as passive tracers of the cloud-top wind field, the orientation of cloud streaks is determined by the wind shear that the cloud has recently experienced. The typical deformation time for small clouds is about 50 hr, or one-half of a cloud-top rotation, so clouds viewed on the sunlit half of Venus show the influence of daytime and nighttime winds. By recording the orientations of cloud streaks and their variation with local solar time, we for the first time use cloud orientation information to place a constraint on the amplitudes of the Hadley circulation and solar thermal tides at the cloud-top level. A large solar thermal tide with a cloud-top meridional wind amplitude at 45° latitude of between 5.5–10.5 m sec−1is found to be necessary to account for the observed variation with local solar time of cloud streak orientations. Maximum poleward tidal winds occur in the early afternoon. The cloud-top Hadley circulation is estimated to have an amplitude at 45° latitude of between 3.5 and 8.5 m s−1. This means the Hadley circulation may not be the dominant contributor to cloud-top meridional velocities, and that equatorward meridional velocities may exist over a portion of the night side at the Venus cloud-top.