Abstract
The horizontal wind velocity vectors at the lower cloud layer were retrieved by tracking the displacement of cloud features using the 1.74 µm 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° 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° 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° 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.
Highlights
The thick cloud layer of Venus rotates around the planet in the westward direction with a peak velocity 60 times higher than that of the planet itself at approximately 100 m s−1 for the altitudes of 65–70 km above the surface [1,2]; this is a phenomenon known as retrograde superrotation
By tracking the displacement of these cloud features with time, one can derive horizontal wind speed at the altitude of remote sensing [5]
We found that the sign change of the meridional wind at 20◦ S coincides with the increase of the zonal speed (2–5 m s−1)
Summary
The thick cloud layer of Venus rotates around the planet in the westward direction with a peak velocity 60 times higher than that of the planet itself at approximately 100 m s−1 for the altitudes of 65–70 km (upper cloud level) above the surface [1,2]; this is a phenomenon known as retrograde superrotation. The velocity decreases for altitude levels farther from the upper cloud level and for latitudes closer to the poles [3]. Thermal emissions from the hot lower atmosphere and surface are observed on the nightside in the near-infrared range in spectral “windows” between CO2 bands. By tracking the displacement of these cloud features with time, one can derive horizontal wind speed at the altitude of remote sensing [5]
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