Abstract
Low-level convergence induced by land surface heterogeneities can have substantial influence on atmospheric convection and rainfall. Analyses of heavy convective rainfall in the Baltimore–Washington metropolitan area are performed using the Weather Research and Forecasting (WRF) Model, coupled with the Princeton Urban Canopy Model (PUCM) that resolves urban subfacet heterogeneity. Analyses center on storms that produced heavy rainfall and record urban flooding in Baltimore on 1 June 2012. The control simulation using PUCM shows a better performance in reproducing the surface energy balance and rainfall than the simulation using a traditional slab model for the urban area. Sensitivity experiments are carried out to identify the role of the land surface heterogeneities, arising from land–water and urban–nonurban contrasts in the Baltimore–Washington metropolitan area, on heavy rainfall from organized thunderstorm systems. The intersection of low-level convergence zones from thunderstorm downdrafts and from the bay breeze from the Chesapeake Bay enhances the upward motion of preexisting convective storms. The larger sensible heating from the urban area modifies the low-level temperature and wind fields, which in turn modifies the bay breeze. The enhanced moisture supply in the deepened bay-breeze inflow layer due to urban heating promotes intense convection and heavy rainfall in conjunction with the enhanced upward motion at intersecting convergence zones. This study suggests that better representations of surface heat and moisture fluxes in urban areas along major water bodies are required to better capture the timing and location of severe thunderstorms and heavy rainfall.
