The effects of aggregated convection in cloud-resolved radiative-convective equilibrium

Tropical convection often aggregates into a broad class of convective systems that include large cloud clusters, mesoscale convective systems, tropical cyclones, and the Madden-Julian Oscillation. There is generally poor understanding of the relationship of convection aggregation in its various forms to climate. For example, we don't know what determines the relatively constant number of tropical cyclones on our planet each year. Will it change in the future? Here we use an idealized framework of radiative-convective equilibrium (RCE) using a cloud resolving model over the domains large enough (hundreds of kilometers) to simulate mesoscale clustering of convection and long enough (hundreds of days) to obtain steady statistics. Our preliminary results indicate that clustering of convection on mesoscale is characterized by drying of the spatial- and time-mean atmosphere (in accord with the recent observational studies of Tropics), associated increase in precipitation efficiency, among others. In our previous study of dramatic self-aggregation into relatively small precipitating regions surrounded by non-precipitating dry troposphere, we have shown that the aggregation of convection occurs in our model only for the prescribed sea surface temperatures (SSTs) that exceed a definite threshold value, and for relatively small large-scale wind shear. Above that threshold, the dramatic drying due to self-aggregation and reduction in water vapor green-house effect would cause the SST to fall, which suggests that the coupled air-sea system would tend toward a state that is right on the verge of self-aggregation; in other words, a self-organized critical state. This hypothesis is tested with a version of the model that allows the sea surface temperature to adjust to the surface energy imbalance. The implication of our findings for climate and tropical cyclones is also discussed.