Water Vapor Deficit-Induced Equilibrium Shift in Convective Self-Aggregation: Dry Expansion and Wet Moistening

Convective self-aggregation (CSA) involves not only the drying of dry areas and the moistening of wet areas, but also an expansion of dry areas and a significant shift in the domain's climate. This study reveals that the occurrence of CSA is a finite-amplitude problem. A dry area with low water vapor content or large size is a necessary pathway to lead to the subsequent evolution. Based on this finding, the whole CSA evolution can be roughly divided into two stages, with the first stage responsible for self-producing dry disturbances (overturning circulations), and the second stage responsible for the expansion of dry regions and the domain-wide climate change. This research delves deeper into the mechanisms behind the second stage from the perspective of water vapor budget. It is revealed that the transport of moisture by overturning circulations makes wet areas wetter, which increases precipitation rate significantly and leads to a domain-wide water vapor deficit. This deficit manifests as an expansion of the dry areas, which further increases moisture transport from the dry to the wet ones, boosting the precipitation rate and exacerbating the water vapor deficit. When the dry areas become large enough, the water vapor deficit is alleviated due to a reduction in the precipitating area and an increase in surface water vapor flux. To deepen the understanding of these processes, a simple mathematical model focusing on the second stage of CSA is proposed, which generally reproduces the gross characteristics in full-physics CSA simulations.