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Venus, atmospheric circulation and winds

The fast rotating layer in the atmosphere of Venus is surrounded by two regions with completely different thermal properties.

Venus thermosphere thermal system

In the thermosphere, above 90km, the characteristic cooling time by thermal conduction with transmission of the heat to the mesopause (upper boundary of the mesosphere) followed by radiative dissipation, is short compared to the length of a Venusian day. Due to the day-night symmetry, a circulation system is created from the sub-solar point to the anti-solar point. The pressure gradient at the terminator (limit day-night) is so high that the flow occurs at the speed of sound (800 km/h for CO2).

The day-night circulation at high altitude is compensated for by an inverse circulation in the neighbourhood of the mesopause. Since the total gas flux in each direction must be the same, the flow speeds night to day in the mesopause are much slower than the inverse flow at high altitudes, where the density is much lower.

Venus troposphere thermal system

On the other hand, in the troposphere, above 50km, the characteristic cooling time is large compared to the length of a day. Therefore energy transport by radiation is very difficult, due to the greenhouse effect on Venus.

Like on Earth, we believe that two Hadley cells form, one on each side of the equator. Hot air rises at from the tropics and is transported to the poles where it drops and it transported back to the equator. On Earth, the Hadley cell doesn’t reach the poles. The Coriolis force, linked to the fast rotation of the planet, creates an instability (barocline) at mid-latitude that is responsible for the system of cyclones and anticyclones and which assures the meridian transport to mid-latitudes.

The day-night thermospheric convective cell at high altitude and the equator-pole tropospheric convective cell at low altitude assure the circulation of heat from warm to cold regions. Between them there is a large atmospheric zone (from 30 to 90km altitude), centred on the cloud layer, where the thermal regime is complex, bounded by flows at its base and its summit. It is in this layer that super-rotation takes place.

False-colour image of cloud features seen on Venus by the Venus Monitoring Camera (VMC) on Venus Express. Copyright: ESA/MPS/DLR/IDA, 2011.