The Onset of Convective Self-Aggregation

The atmosphere can spontaneously organize into a convecting moist patch and a sinking dry patch over uniform surface temperature in cloud-resolving simulations. This phenomenon is referred to as the convective self-aggregation and has been argued to be important for tropical weather and climate systems. Here we ask what processes are responsible for the onset of self-aggregation. We combine theory and cloud-resolving simulations to demonstrate that diabatic processes (radiative cooling, convective heating, surface fluxes, and re-evaporation of raindrops) in the boundary layer help generate available potential energy and are therefore essential for the onset of convective self-aggregation. This theory successfully predicts that enhanced virtual effect of water vapor can lead to self-aggregation. This result challenges the prevailing theory of self-aggregation, which does not account for the effect of the molecular weight of water vapor.