Structure and Evolution of Mesoscale Convective Systems: Sensitivity to Cloud Microphysics in Convection-Permitting Simulations Over the United States

Abstract

Regional climate simulations over the continental United States were conducted for the 2011 warm season using the Weather Research and Forecasting model at convection-permitting resolution (4 km) with two commonly used microphysics parameterizations (Thompson and Morrison). Sensitivities of the simulated mesoscale convective system (MCS) properties and feedbacks to large-scale environments are systematically examined against high-resolution geostationary satellite and 3-D mosaic radar observations. MCS precipitation including precipitation amount, diurnal cycle, and distribution of hourly precipitation intensity are reasonably captured by the two simulations despite significant differences in their simulated MCS properties. In general, the Thompson simulation produces better agreement with observations for MCS upper level cloud shield and precipitation area, convective feature horizontal and vertical extents, and partitioning between convective and stratiform precipitation. More importantly, Thompson simulates more stratiform rainfall, which agrees better with observations and results in top-heavier heating profiles from robust MCSs compared to Morrison. A stronger dynamical feedback to the large-scale environment is therefore seen in Thompson, wherein an enhanced mesoscale vortex behind the MCS strengthens the synoptic-scale trough and promotes advection of cool and dry air into the rear of the MCS region. The latter prolongs the MCS lifetimes in the Thompson relative to the Morrison simulations. Hence, different treatment of cloud microphysics not only alters MCS convective-scale dynamics but also has significant impacts on their macrophysical properties such as lifetime and precipitation. As long-lived MCSs produced 2–3 times the amount of rainfall compared to short-lived ones, cloud microphysics parameterizations have profound impact in simulating extreme precipitation and the hydrologic cycle.