Urban effects on weakly forced thunderstorms observed in the Southeast United States

Abstract

To comprehensively examine the impact of the urban rainfall effect (URE) on weakly forced thunderstorms (WFTs), analyses of numerous cities over extended time periods are necessary. Unfortunately, previous studies finding that urban development influences thunderstorms have typically focused on smaller spatiotemporal regions. In this paper, an Urban Convection Ratio (UCR) was developed, representing the degree to which WFT activity was attributable to urban land cover, for 32 Southeast U.S. urban environments from 2001 to 2015. The UCR was calculated at various buffer distances from each urban core by comparing the observed WFT climatology to a four-variable random forest estimate of the WFT climatology, which excluded urban influences. The largest UCR for the 0–20 km buffer was observed in New Orleans (Louisiana) while Atlanta (Georgia) ranked sixth. Thirteen urban areas exhibited statistically significant amplifications of WFT frequency in the 0-20 km buffer, with urban enhancements diminishing with distance from the urban core. A complex relationship was also observed between the UCR values and city size. These findings and the developed UCR methodology establish a foundation for furthering the understanding of the mechanisms underlying urban initiation and modification of WFTs as well as how they may contribute to the broader URE.