Sensitivity of Convective Self-Aggregation to Domain Size

A three-dimensional cloud-resolving model (CRM) was used to investigate the sensitivity of simulations of aggregated convection in radiative-convective equilibrium (RCE) to the domain size. The purpose of the simulations was to determine how large a domain is needed to allow multiple convective aggregations, and also how the size of the aggregated convective cluster depends on domain size. Simulations were performed with doubly-periodic square domains of widths 768 km, 1536 km, 3072 km and 6144 km, over a time period of 350 simulated days. In the 768 km, 1536 km, and 3072 km domains, the simulations produced circular convective clusters surrounded by broader regions of dry, subsiding air. In the 6144 km domain, the convection formed two bands. The radial and vertical structures of the aggregated RCE were analyzed. With increasing domain size, the moist static energy (MSE) in the convective region boundary-layer (CBL) increased as a result of an increase in the temperature difference between the CBL and the dry region boundary-layer. We hypothesize that the increase in CBL MSE contributed to more melting and freezing around the freezing level, and, in turn, an increase in mid-level stability. The increased mid-level stability made it more difficult for convection to penetrate into the upper troposphere, resulting in a weaker mesoscale convective updraft, and an increase in the convectively active fraction of the domain by mass conservation. We also show that the warmer CBL contributed to other important domain size sensitivities, such as dry region moistening, through enhanced mid-level stability.