Structure and circulation of the Venus atmosphere

Pioneer Venus has revealed important new features of the structure and the circulation of Venus' atmosphere. The temperature decreases from nearly 750 K at the surface to about 180 K at about 100 km. Above 100 km, there is a marked contrast between the day-side and the night-side thermal structures. On the day side there is a thermosphere in which temperatures increase with height to an exospheric temperature of about 300 K. On the night side there is a `cryosphere' in which temperatures decrease with height to an exospheric temperature of about 100 K. The atmosphere is stably stratified from the highest altitudes down to about 28 km except for a layer in the clouds, between about 50 and 55 km, which is nearly adiabatic. Between about 20 and 28 km, the lapse rate is also nearly adiabatic while there is evidence for stable stratification between about 10 and 20 km. Horizontal thermal contrasts are of the order of 1-2% in the deep atmosphere and 100% in the upper atmosphere. At and below the clouds, temperatures generally decrease with latitude on constant pressure surfaces; above the clouds, between about 70 and 90 km, there is a reversed zonally averaged latitudinal temperature gradient. The dominant circulation of the atmosphere above the lowest one or two scale heights is a zonal retrograde motion with 100 m/s winds at 60 km altitude. There is also a superrotation of the atmosphere at altitudes of 150 km and above. Low latitude height profiles of the zonal wind have alternating layers of high and low shear which correlate with structure in the vertical profiles of static stability. Advection of heat by the large zonal winds helps maintain the relatively small longitudinal thermal contrasts throughout the atmosphere below the clouds. Latitudinal temperature and pressure contrasts are consistent with a zonally rotating atmosphere in approximate cyclostrophic balance. Meridional winds below 60 km vary in speed from a few to about 10 m/s; the winds are poleward at the cloud tops. A cloud level Hadley cell driven by solar heating combines with the zonal circulation to produce a cloud top polar vortex. Eddies in the form of convective cells, small-scale gravity waves, and planetary scale waves are found throughout the atmosphere. Eddies, as well as mean meridional circulations, may be important in the transport of energy and momentum. Venus' atmospheric circulation is not steady despite the planet's small obliquity and nearly circular orbit.