Locations and Trends in US Midwest Urban-induced Heavy Rain Events

It is well known that large urban areas (e.g., Atlanta, GA; Chicago, IL; Beijing, China) can enhance warm-season convective activity and heavy rainfall in some regions and atmospheric conditions. Recent studies have shown that such enhancements of convective rainfall can be detected downwind of the urban centers, in some instances for hundreds of km. The impacts of these rainfall events depend critically on their locations with respect to the urban population and land-use areas. For example, heavy rainfall in urban areas is much more likely to produce street flooding than in rural downwind areas due to the predominance of impervious surfaces. Heavy rainfall occurring in urban areas can be an important source of washed-out surface and airborne urban pollutants to nearby rivers and other water bodies.

This presentation will focus on the locations of heavy rain development and climatic trends in frequencies of heavy rain events for a range of urban centers located close to large rivers in the US Midwest. Heavy rain initiation frequencies were quantified by identifying the development of precipitation with at least 35dBZ radar reflectivity as observed by the NOAA WSR-88D network. Long-term trends in heavy rain frequencies were determined using NOAA surface-based daily precipitation sites and multi-sensor precipitation fields.

It was found that heavy rains initiated nearly twice as often over a five-year period (2009-2013) within a 40-km diameter circle encompassing the St. Louis urban land-use area than over an equal-sized nearby rural area. Mid-sized urban areas (Terre Haute, IN, and Davenport, IA) experienced storm initiation frequencies between those for the St. Louis and rural areas. Areas of maximum daily rainfall frequencies greater than 2.5, 5.1 and 7.6 cm typically extended from over the urban areas to within about 100 km of the urban centers.