Sensitivity of Convective Self‐Aggregation to Subsidence

Abstract

AbstractSimulations using the convection‐permitting MesoNH model with imposed vertical motion document the influence of large‐scale circulation on convective self‐aggregation. Results show that subsidence favors self‐aggregation in this model and that this sensitivity includes a hysteresis: for weak subsidence, only one stationary state is found, without aggregation; for moderate subsidence, two stationary states are found, one with aggregation and one without; for large subsidence, only the aggregated stationary state is found. In that state, the area covered by deep convection decreases with subsidence. Using the variance of column moist static energy (MSE) as a metric for aggregation, we detail the role of physical processes in the development or maintenance of aggregation. Solar radiation always contributes positively to aggregation. The effect of terrestrial radiation results from a competition between the effects of the top‐of‐atmosphere and surface fluxes, and can therefore vary in amplitude and change sign both with time in transient simulations and with subsidence intensity in stationary state. The effect of turbulent surface fluxes also result from a competition between the effect of air‐sea thermodynamic contrast and the wind‐induced contribution, and it can also vary very significantly. The contribution of MSE transport within the model domain also changes from negative to positive as the intensity of subsidence is increased because the shallow circulation accelerates and causes a change of sign in the gross moist stability associated with the internal circulation.