Winds In The Lower Cloud Level On The Nightside Of Venus From Virtis-M (Venus Express) 1.74 Mu M Images

The horizontal wind velocity vectors at the lower cloud layer were retrieved by tracking the displacement of cloud features using the 1.74 mu m images of the full Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M) dataset. This layer was found to be in a superrotation mode with a westward mean speed of 60-63 m s(-1) in the latitude range of 0-60 degrees S, with a 1-5 m s(-1) westward deceleration across the nightside. Meridional motion is significantly weaker, at 0-2 m s(-1); it is equatorward at latitudes higher than 20 degrees S, and changes its direction to poleward in the equatorial region with a simultaneous increase of wind speed. It was assumed that higher levels of the atmosphere are traced in the equatorial region and a fragment of the poleward branch of the direct lower cloud Hadley cell is observed. The fragment of the equatorward branch reveals itself in the middle latitudes. A diurnal variation of the meridional wind speed was found, as east of 21 h local time, the direction changes from equatorward to poleward in latitudes lower than 20 degrees S. Significant correlation with surface topography was not found, except for a slight decrease of zonal wind speed, which was connected to the volcanic area of Imdr Regio.