The Unreasonable Efficiency of Total Rain Evaporation Removal in Triggering Convective Self-Aggregation

The elimination of rain evaporation in the planetary boundary layer (PBL) has been found to lead to convective self-aggregation (CSA) even without radiative feedback, but the precise mechanisms underlying this phenomenon remain unclear. We conducted cloud-resolving simulations with two domain sizes and progressively reduced rain evaporation in the PBL. Surprisingly, CSA only occurred when rain evaporation was almost completely removed. The additional convective heating resulting from the reduction of evaporative cooling in the moist patch was found to be the trigger, thereafter a dry subsidence intrusion into the PBL in the dry patch takes over and sets CSA in motion. Temperature and moisture anomalies oppose each other in their buoyancy effects, hence explaining the need for almost total rain evaporation removal. We also found radiative cooling and not cold pools to be the leading cause for the comparative ease of CSA to take place in the larger domain. Convective clouds are not randomly scattered across the sky but tend to clump together, a phenomenon known as convective self-aggregation (CSA). The interaction between clouds and radiation is a key mechanism for CSA to occur. Curiously, CSA can still take place without this radiative feedback, provided that rain is prohibited from evaporating in the lowest layers of the atmosphere (similar to 1 km), called the planetary boundary layer (PBL). To investigate the physical processes behind this type of CSA (no-evaporation CSA, or "NE-CSA"), we ran high resolution atmospheric model simulations and reduced rain evaporation in steps in the PBL. We found that the additional heat resulting from the reduction of evaporative cooling is crucial in triggering NE-CSA, thereafter the invasion of dry air into the PBL in the dry region takes over and intensifies aggregation. Surprisingly, allowing even a minuscule amount of rain to evaporate prevents NE-CSA from taking place. This is because removing rain evaporation has two opposing effects on convection: heating and drying. The former aids convection while the latter hinders it. Only when rain evaporation is almost completely eliminated can the heating effect be powerful enough to overcome the drying effect and kick-start NE-CSA. When rain evaporation is removed in the PBL, convective self-aggregation (CSA) is triggered by convective heating of the moist regions Surprisingly, CSA only occurs when rain evaporation is almost totally removed in the PBL, due to opposing temperature and moisture effects CSA occurs more easily in a larger domain due to stronger radiatively induced subsidence, while cold pools play a less significant role