The interaction of cumulus convection with soil moisture distribution: An idealized simulation

In order to investigate the interaction between cumulus convection and soil moisture distribution, two‐dimensional numerical experiments using a regional atmospheric model are performed. The model roughly resolves each convective cell and represents cloud processes by a microphysics parameterization. Two long‐term (60‐day) integrations with relatively dry and wet conditions are made with the atmosphere‐land system in a quasi‐equilibrium state. Though the initial and boundary conditions are horizontally homogeneous, horizontal contrasts in soil moisture spontaneously develop due to the spotty nature of convective precipitation. When intense soil moisture contrasts develop, they cause surface temperature contrasts through a change in evaporation. As a result, thermally induced local circulations occur in the daytime, with upward branches over the dry and hot regions and downward branches over the wet and cool regions. Most cumulus convection events are initiated by the upward motion of this local circulation over the dry region. They mostly occur in the afternoon (1300–1700 LT), while convection that forms over regions that are wet throughout may occur at any time during the day. The intense precipitation over the dry region “overdamps” the soil moisture contrast, which results in the maintenance of a heterogeneous distribution of soil moisture.