The METROpolitan Meteorological EXperiment (METROMEX) results show St. Louis enhances convective rainfall in and downwind of the city. Possible mechanisms considered since METROMEX include enhanced convergence from the urban heat island (UHI) and barrier effects from the buildings, which produces convergence downwind of the city. Microphysical alterations to cloud systems may also play a role. To date, the complex interactions within the urban boundary layer (UBL) prevented a complete understanding of these phenomena and how they modify thunderstorms.

The current study uses the Regional Atmospheric Modeling System with LEAF-2 (Walko et al. 2000) and the Town Energy Budget (TEB) (Masson 2000) surface parameterizations, in order to simulate ordinary thunderstorms over the St. Louis region. The TEB accounts for the complex, three-dimensional urban surfaces in each urban portion of any grid cell. The remaining land cover is parameterized with LEAF-2. Land use land cover (LULC) information is gathered from 30 meter resolution Landsat Thematic Mapper data for the finest domain. The finest grid is centered over St. Louis, with 1.5 km grid spacing and a 150 km by 150 km horizontal domain.

St. Louis provides an ideal experimental platform to study the effects of an urban surface on thunderstorms, since it is located in a midlatitude moist, temperate climate. Moreover, Saint Louis is relatively flat, essentially land-locked, and isolated from other large metropolitan areas. Our experimental setup involves a case study of isolated convective cells over St. Louis, devoid of significant synoptic scale forcing. The case study investigates 8 June 1999, a day in which the St. Louis area experienced severe wind, hail, and heavy rainfall. The control run, using LULC and topographic data, simulates not only the general synoptic environment, but also the convective situation fairly well.

Sensitivity tests are performed by altering surface characteristics over St. Louis to isolate the effects of topography, the UBL and UHI, and increased roughness of the city on the thunderstorm environment. A comparison of two urban parameterizations on the thunderstorm environment is also presented. A major conclusion derived from these experiments is that the UHI has a substantial effect on the modeled storms. The simulated UHI causes a mesoscale circulation with surface winds directed toward the city. Resulting convergence produces enhanced boundary layer convection, triggering storms in the given moist and unstable environment. Even though the urban surface is somewhat drier than the surrounding areas, the higher temperatures account for overall higher convective available potential energy over the city. Detailed analysis of the sensitivity tests are designed to investigate many of the complex interactions between forcing mechanisms in the convective environment. In addition, we assess the urban parameterizations used in this study and their role in producing model convection. Future work will focus on more case studies including nocturnal mesoscale convective systems and also aerosol influences on urban precipitation.