Organization of Tropical Convection: Dependence of Self-Aggregation on SST in an Idealized Modeling Study

Convective cloud clusters are responsible for most of the rainfall and cloudiness over the tropics, therefore modulating the radiative heating and cooling rates of the surface and atmosphere and influencing the large-scale circulation and moisture distribution. Therefore, understanding how and why tropical convection organizes is important for understanding both tropical and global climate variability. In this study, the problem is approached through the context of idealized modeling of convective organization in radiative convective equilibrium using a cloud resolving model. Previous studies have investigated interactions between the environment and the convection that allow convection to self-aggregate into a single cluster, and have found this self-aggregation to be dependent on a sea surface temperature threshold. To examine the nature of this threshold, the System for Atmospheric Modeling is used to perform 3-d cloud resolving simulations with a doubly periodic horizontal domain, interactive radiation and surface fluxes, and no rotation. Simulations at different SSTs are run for a period of 100 days. Simulations in which aggregation does occur are then closely compared to those in which it does not. Sensitivity experiments are then performed to determine the relevant feedback mechanism that controls the SST dependence of self-aggregation. In addition, perturbations such as changing the atmospheric humidity or solar insolation are then applied to the non-aggregated simulations to determine what is needed to force the system to aggregate when it is below the SST threshold.