Shallow Cumulus Convection

Abstract

Shallow cumulus convection plays a crucial role in determining the vertical thermodynamic structure of the atmosphere and influences the large-scale circulation significantly in both tropics and mid-latitudes. This is most clearly demonstrated in the Hadley circulation over the tropical oceans, such as schematically illustrated in Fig. 1. In the subtropical belts, the surface evaporation from the ocean increases significantly due to the enhanced vertical transport of heat and moisture by shallow cumulus clouds, which are usually referred to as trade-wind cumuli. This enhanced moisture, collected in the trade-wind boundary layer, is transported by the trade winds towards the intertropical convergence zone (ITCZ), where it is finally released as latent heat in deep convective precipitating cumulus towers. Since this latent heat release is an important engine of the Hadley circulation, the enhanced surface evaporation upstream in the trade-wind region can be regarded as a fuel supply for this circulation. It is in this context that the presence of shallow cumuli, though small in individual size and non-precipitating, intensifies the large-scale circulation. Locally, the vertical transport of heat and moisture by the shallow cumulus clouds is also important to counteract the drying and warming effects of the large-scale subsidence induced by the Hadley circulation. As a result, a steady thermodynamic state of the vertical structure of the trade-wind region can be maintained.