Radar analysis of St. Louis urban thunderstorm initiation

A number of studies have provided evidence that large urban areas tend to locally enhance precipitation during the warm season. However, the relative importance of urban-induced influences (e.g., mesoscale circulations, stability changes, and microphysical processes) are not well understood. Quantitative precipitation forecasts for major cities are more important than ever. With growing metropolitan areas comes increased urban flooding vulnerability. Outdated drainage infrastructure can be overwhelmed beyond capacity by extreme rainfall events, putting lives and property at stake. Furthermore, heavy rainfall can augment the washout of surface and airborne urban pollutants to nearby water bodies.

The overall goal of the Riverside Urban-Produced Extreme Rainfall (RUPER) project is to investigate the frequency and location of heavy precipitation events induced by several riverine Midwestern cities, including St. Louis, Missouri. It was found that the development of storms with NOAA WSR-88D radar reflectivities >35dBZ occurred nearly twice as often within a 40km circle centered on St. Louis as compared to a same-sized circle over a nearby rural area. A mid-sized urban area, Terre Haute, Indiana, experienced storm initiation frequencies between those for the St. Louis and rural areas. While storm initiation and heavy rainfall were observed at all locations around St. Louis, the occurrence of heavy precipitation was most often seen near and southeastward of the urban center.

The process which drives urban storm development is not always clear. An interesting feature observed was a recurring bowed reflectivity “fine line”. This line was first observed along the Mississippi River and then propagated south and westward across the northern portion of the city. While the origin of this convergence zone is unclear, the boundary may enhance convective activity. As seen in a case study, the boundary appeared to interact with an eastbound post dissipated-stage thunderstorm, reigniting it to >60dBZ as it passed just north of downtown St. Louis. Additional >50dbZ cells subsequently erupted over the urban area from the reinvigorated storm's outflow. Further details of this process will be discussed.