Simulating Multiple Aggregates in a Small-Domain 3D Cloud-Resolving Model

Organized rainstorms and their associated overturning circulations can self-emerge in cloud-resolving models (CRMs) with uniform sea surface temperature (SST). This phenomenon is referred to as convective self-aggregation and is argued to be an important building block for multiple tropical weather systems, such as tropical cyclones and the Madden-Julian Oscillation (MJO). Here we present a boundary-layer theory for the spatial scale of convective self-aggregation in 3D atmosphere. This theory shows that the natural lengthscale of self-aggregation is about 2000 – 3000 km at 300 K SST. This result helps explain why convection mysteriously aggregated into a single cluster in previous 3D CRM simulations even with the domain of 3,000 km x 3,000 km. This theory can also help us simulate multiple convective aggregates in a small domain. Figure 1 shows snapshots of precipitable water (PW) from a 3D CRM simulation. In this simulation, we have perturbed the model physics according to our theory to make self-aggregation smaller. Two convective aggregates are then successfully simulated, and their lengthscale is about 300 km, an order of magnitude smaller than its typical value. This success of simulating multiple aggregates in 3D CRMs is a step toward understanding convective self-aggregation. Implications on understanding the scale selection mechanism of the MJO will also be discussed.