The atmosphere can self‐organize into long‐lasting large‐scale overturning circulations over an ocean surface with uniform temperature. This phenomenon is referred to as convective self‐aggregation and has been argued to be important for tropical weather and climate systems. Here we present a boundary layer centric framework based on the available potential energy budget of convective self‐aggregation. We show that boundary layer diabatic processes dominate the available potential energy production and are, therefore, essential to convective self‐aggregation. We further show that the enhanced virtual effect of water vapor can lead to convective self‐aggregation.

Organized rainstorms and their associated overturning circulations can self-emerge over an ocean surface with uniform temperature in cloud-resolving simulations. This phenomenon is referred to as convective selfaggregation. Convective self-aggregation is argued to be an important building block for tropical weather systems and may help regulate tropical atmospheric humidity and thereby tropical climate stability. Here the author presents a boundary layer theory for the horizontal scale l of 2D (x, z) convective self-aggregation by considering both the momentum and energy constraints for steady circulations. This theory suggests that l scales with the product of the boundary layer height h and the square root of the amplitude of density variation between aggregated moist and dry regions in the boundary layer, and that this density variation mainly arises from the moisture variation due to the virtual effect of water vapor. This theory predicts the following: 1) the order of magnitude of l is ;2000 km, 2) the aspect ratio of the boundary layer l/h increases with surface warming, and 3) l decreases when the virtual effect of water vapor is disabled. These predictions are confirmed using a suite of cloud-resolving simulations spanning a wide range of climates.