The lack of cold pool interactions can initiate convective self-aggregation

Abstract:

Convective self-aggregation describes the emergence of one or more persistently dry, cloud-free areas in numerical simulations. Apart from radiation feedbacks, it is important because of its link to the formation of tropical cyclones and large-scale events such as the Madden-Julian Oscillation. Some understanding of the persistence of self-aggregation exists. However, how self-aggregation initially emerges from a homogeneously convecting state remains poorly understood. Recently, cold pools (CPs) have been linked to the organization of convective events. CPs are radially expanding pockets of dense air that form under precipitating thunderstorms. In this work, we ask whether the lack of CPs could cause the emergence of cloud-free areas. By analyzing a suite of cloud-resolving numerical simulations, we link the life-cycle and the spatial organization of CPs to the emergence of cloud-free areas. First, we show that weakening rain evaporation reduces the time at which the first cloud-free areas form. Second, we find that new convective events occur near the point where two CPs collide. Third, we identify a minimum CP radius within which CPs are too negatively buoyant to initialize new convective events. Fourth, we measure a maximum CP radius beyond which CPs do not grow any further and therefore, cannot initiate new convective events. Fifth, based on these findings, we suggest a simple idealized mathematical model that approximates CPs as expanding circles. Two colliding circles initiate a new circle if both circles are within the identified CP size limitations. We show that this model can capture the emergence of cloud-free areas. In conclusion, we propose that interactions between CPs may contribute to the initial emergence of cloud-free regions in cloud-resolving numerical simulations.