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 (frequently referred to as “moisture memory aggregation”), but the precise mechanisms underlying this phenomenon remain unclear. We conducted cloud-resolving simulations with two domain sizes (L = 128 and 256 km; Δx = 1 and 4 km) with homogenised radiation and progressively reduced rain evaporation in the PBL by multiplying it with a factor 𝛼 = [1.0, 0.8, 0.6, 0.4, 0.2, 0]. Surprisingly, self aggregation only occurred when rain evaporation was almost completely removed (𝛼 ≈ 0). Similar to conventional radiatively-driven aggregation (RDA), a shallow circulation that leads to an upgradient moist static energy transport is present, but in this case it is the additional convective heating resulting from the reduction of evaporative cooling in the moist patch that triggers this circulation, thereafter a dry subsidence intrusion into the PBL in the dry patch takes over and intensifies aggregation. Hence, this type of aggregation should be more appropriately referred to as “convectively-driven aggregation” (CDA). Contrary to RDA, in CDA temperature and moisture anomalies oppose each other in their buoyancy effects, hence explaining the need for near-zero 𝛼 values: only when rain evaporation is almost completely removed can the additional heating trigger aggregation. Lastly, we found radiative cooling and not cold pools to be the leading cause of the domain size dependence of CDA. Runs with similar amounts of cold pools aggregate in the large but not small domain due to stronger radiative cooling rates and concomitant broadening of the range of precipitable water in the larger domain.