Low-Level Circulation and Its Coupling with Free-Tropospheric Variability as a Mechanism of Spontaneous Aggregation of Moist Convection

Abstract

The organization of clouds has been widely studied by numerical modeling as an essential problem in climate science. Convective self-aggregation (CSA) occurs in radiative–convective equilibrium when the model domain size is sufficiently large. However, we have not yet reached a comprehensive understanding of the mechanism of CSA onset. This study argues that low-level circulation is responsible for horizontal moisture transport and that its coupling with variabilities of diabatic heating and moisture in the free troposphere is essential. We simulated scattered and aggregated convection by varying the domain size as a control parameter constraining the horizontal scale associated with the CSA onset. Based on a new analysis method quantifying the circulation spanning dry and moist regions, we found that 1) the upgradient moisture transport in the aggregated cases is associated with low-level circulation development, amplifying the horizontal moisture contrast; 2) the horizontal buoyancy gradient due to strong radiative cooling in the dry region intensifies the low-level circulation; 3) the free-tropospheric subsidence intrudes into the boundary layer in the dry region preceding the intensification of low-level circulation; and 4) the subsidence intrusion is due to a weakening of convective heating in the free troposphere associated with the moisture variability at a larger horizontal scale. This study provides new insights into the organization mechanism of clouds unifying the different mechanisms impacting CSA: the free-tropospheric moisture, radiation, convection, and low-level circulation.