Momentum Transport Due to a Squall Line System over the Tropical Oceans

Fast-moving squall lines observed in the tropical oceans are treated as a gravity wave generated by the latent heat release associated with deep cumulus convection. A simple two-dimensional diagnostic linear gravity wave model forced by a zonally propagating heat source is presented. The effect of cumulus friction, or momentum exchange due to cumulus convection, is included in the wave momentum equation. Both monochromatic and localized distributions of precipitation are considered. We will focus on the momentum transport. In the troposphere, within the context of our model, vertical transport of momentum owing to either clouds or gravity waves can be easily separated, whereas above the clouds, only gravity wave transport exists. It is discovered that, in the cloud layer, for the scales characteristic of squall lines, mean cloud transport is the dominant mean momentum flux contributor. Compared to aircraft observations, our model is able to simulate the mean momentum flux with both the correct sign and magnitude. Within the wave momentum equation, cumulus friction, however, is found not to be a major term. Above the cloud layer, impacts of these gravity waves due to thermal forcing from below are also inspected. In particular, the amount of energy gets radiated out of the cloud layer on a global scale, the levels where wave breakings occur, and the associated wave-induced acceleration. We find that thermally forced gravity waves are potentially as important as mountain waves (perhaps the most well known and extensively studied gravity waves in the atmosphere) at least in the lower stratosphere. Therefore, when studying the momentum budget of the middle atmosphere, roles played by gravity waves triggered by cumulus convection in troposphere cannot be overlooked. In addition, our results indicate that only gravity waves forced by localized precipitation are subject to breaking near the tropopause where the static stability of the environment is locally low, and we think this breaking phenomenon is likely to happen in the real atmosphere. However, in terms of the final overall residual momentum flux leaked into the stratosphere, at least for the parameters chosen in this study, both monochromatic and localized precipitation seem to be able to give rise to similar results.