Multi-scale Analysis of Surface Heterogeneity Induced Convection on Isentropic Coordinates

Abstract:

Land-atmosphere interactions and boundary layer processes play important roles in the formation of shallow cumulus clouds and their potential transition to deep convective precipitation over land. In this study, we investigate multiple scale processes associated with soil moisture-heterogeneity induced convective clouds observed during the 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) field campaign at the U.S. Southern Great Plains. Analysis on isentropic coordinates in Large-eddy Simulations enabled the tracking of circulations that transport energy upward and to capture the key convective processes induced by land heterogeneity. The isentropes associated with upward motion are found to connect the ground with high latent heat flux regions to cloud bases directly over high sensible heat flux regions, while isentropes associated with downward motion connect precipitation to the ground with high sensible heat flux regions. The analysis shows that soil moisture gradients provide the two ingredients of shallow convection: (1) initial updraft from the surface sensible heating flux over dry regions and (2) moist air from the wetter soil regions. The mixing of the air from these two regions leads to a buoyant mixture and formation of clouds, as shown in Figure 1. We also examined the energy transport at various spatial scales and found that the turbulent scale dominates vertical energy transport. At scales larger than 27 km, convection does not transport surface energy upwards in the lower boundary layer but contributes to vertical energy transport aloft. Moreover, larger soil moisture gradients lead to deeper convection at scales between 0.3 to 2.7 km and favor the upscale growth of convective clouds to scales larger than 27 km.