Momentum transport by the thermal tide in the stratosphere of Venus

The response of a stratified Boussinesq fluid subjected to a moving periodic thermal forcing is analysed and the role of thermal stratification in the generation of a mean ‘zonal’ motion discussed. In the parameter range where the phase speed of this forcing is much less than the speed of the gravest gravity wave mode, the problem may be treated as one of momentum transport by damped internal gravity waves, though the analysis does not require that the damping be small. Results from a simple model show that mean flow speeds much larger than the speed of heat source motion can be generated by this mechanism. Whilst, in the parameter range appropriate (to the best of our present knowledge) to the stratosphere of Venus, some assumptions of the analysis are violated, it is argued that mean winds of the order of 100m s −1 may be generated, thus supporting the suggestion of Schubert and Whitehead that this mechanism is responsible for the observed four‐day rotation of that planet's equatorial stratosphere. It is shown that the ratio of mean wind velocity to the speed of the subsolar point is independent of the length of the solar day, and that it is largely determined by the distribution of radiative heating and cooling in the planetary atmosphere.