New Critical Length for the Onset of Self-Aggregation of Moist Convection

Convective self-aggregation (CSA) in an idealized modeling framework is key to understanding the role of clouds. To investigate the existence of characteristic length of CSA onset, we conducted systematic cloud-resolving simulations, with a scope covering the horizontal domain size and resolution. In the high-resolution simulation, CSA can occur with a square domain larger than similar to 500 km. Based on the competition between two near-surface horizontal divergent flows, we discuss the characteristic length existence. While the flow induced by radiative cooling in the subsidence region acts as positive feedback for moisture perturbation and scales with the domain size, the other flow induced by evaporative cooling of falling rain in the convective region acts as negative feedback and does not scale. The study suggests characteristic length existence for the organization of moist convection, even in real-world conditions. Plain Language Summary In the tropics, deep convective clouds play significant roles in the redistribution of heat and water. They are often organized into hierarchically clustered forms, such as tropical cyclones. We investigated the condition for the onset of spontaneous clustering by a series of simulations in an idealized situation. We discovered that clusters appear if the horizontal domain size is larger than 500 km. Thus, behind a complex tropical atmospheric phenomenon, a characteristic length exists. The relationship between two near-surface horizontal divergent flows explains the characteristic length existence; cloud clusters can occur when the flow induced by subsidence in the cloud-free area overwhelms the flow induced by the cooling of rainfall evaporation in the cloudy area.