The Dynamics of the Atmosphere of Venus

Abstract

The wide range of radiative time scales in the Venus atmosphere, together with observations of temperature structure and winds, indicate that the atmosphere contains two distinct regimes. In the deep atmosphere, at altitudes below 40 km, diurnal effects are negligible, motions are weak, and the lapse rate is near-adiabatic. In the upper atmosphere, at altitudes above 70 km, diurnal effects are important, strong retrograde zonal motions ?100 m s? occur, and the lapse rate is sub-adiabatic. The transition region between these two regimes is complicated by the presence of two layers of small-scale turbulence at altitudes of 45 and 60 km. Analytical and numerical studies show that the Hadley cell hypothesis for the circulations in the deep atmosphere is consistent with all the observations, provided that the greenhouse effect is strong enough to explain the high surface temperatures. Under these conditions the Hadley cell circulation produces an adiabatic, nonturbulent temperature structure, with equator-to-pole temperature contrasts ?0.1 K, meridional velocities ?2 m s?1, zonal velocities ?1 m s?1, and vertical velocities ?½ cm s?1. Studies of the motions in the upper atmosphere are more ambiguous. Suggestions for explaining the strong zonal motions include the ?moving flame? mechanism, the instability of diurnal convective cells to a mean shear, tidal forcing, momentum transport by internal gravity waves, and momentum transport by a Hadley cell. The ?moving flame? mechanism has not yet been analyzed for conditions appropriate to the Venus upper atmosphere. The other mechanisms all require at least one ad hoc assumption in order to produce the desired velocities. The location and properties of the two layers of small-scale turbulence suggest the possibility that the lower one is generated by local shear instability, and the upper one by local convection.