The effect of large-scale convergence on the generation and maintenance of deep moist convection

Abstract

Convergence is included in the model by imposing a momentum forcing at the lowest levels of the domain. Before convection commences, the convergence lifts the atmosphere to saturation, or close to saturation, over a wide region. This means that once convection begins, the air entering the system requires little further lifting and it is demonstrated that this allows the system to maintain itself without the additional lifting that evaporative cooling produces. The average rainfall rate decreases by approximately 40% when the convergence is removed. For the first two hours of convective activity, the low-level flow has the structure of a gravity wave in that air passes through the system and remains near the surface. The amplitude and structure of this gravity wave, which is primarily forced by evaporative cooling and low-level waterloading, is compared with the predictions of linear theory. Another feature described is the formation of convective cells some 10-15km ahead of the main updraft and the low-level cold pool. -from Authors