Deep convective clouds generate gravity waves throughout their development, maturity, and dissipation. During their vertical propagation, these waves may be dissipated in some way, and subsequently exert a drag on the mean flow via a divergence of the vertical flux of horizontal momentum. This drag is thought to shape the mean flow of the middle atmosphere, and in the tropics may be important in driving the Quasi-Biennial Oscillation. Arguably the most important mechanism contributing to this drag is the breakdown of the vertically-propagating gravity waves. However, the spatial and temporal scales of such gravity waves and their subsequent breakdown are typically smaller than the scales that can be resolved by current general circulation models; therefore their effects must be parameterized. The development of an accurate and physically-realistic convectively-generated gravity-wave drag parameterization requires an understanding of the way convective clouds generate gravity waves, and the way these waves are dissipated. As an incremental step towards this goal, this study examines the breaking of high frequency gravity waves above deep convection.

Results from a very high resolution cloud resolving model will be presented. The numerical model explicitly resolves the deep convective cloud, the gravity waves that are generated, and the subsequent breakdown of those waves. In a systematic series of model integrations, the effect of background wind shear on the wave breaking is investigated. Gravity-wave breaking is attributed to the interaction between the propagating waves and a critical level. The height of this critical level is reduced by the nonlinear cloud-induced circulations. These results also show that the background shear has a profound effect on the wave spectrum through critical level dissipation and wave trapping. Subsequently, in the lower stratosphere the highest frequency, shortest wavelength waves are removed from the spectrum. Thus, it will be shown that in cases of moderate wind shear, the net effect of such waves is to exert a drag in the lowest part of the stratosphere. Also, other issues involving resolution dependence will be briefly discussed.