A Climatological Analysis of the Extent of Rainfall Produced Over the U.S. by Atlantic Basin Tropical Cyclones

Precipitation from tropical cyclones (TCs) makes in important contribution to the hydroclimatology of the eastern United States. Previous studies have focused on the magnitude or contribution of tropical cyclone precipitation (TCP) to a region’s annual precipitation totals. In the present study, we examine the spatial extent of TCP as these systems move over U.S. after making landfall along the Gulf and Atlantic coastlines. A Geographic Information System is employed to delineate rainfall swaths for 257 U.S. landfalling TCs during 1948 to 2014 from U.S. Unified Precipitation Data (UPD) gridded daily rainfall totals. Our technique separates TC rainfall from that produced by other weather systems by constructing polygons around 12.5 mm per day rainfall regions and considering the distance between their centroid and the TC track. All polygons identified as being produced by the TC are merged into a single polygon. Every 10 km along the track, a straight line was drawn perpendicular to the track segment until it intersected with the TCP polygon edge and the lengths of these lines were calculated to get the width of rainfall extent. Finally, we calculated the average width and changes of width on the right and left sides of the track when 70% of measurements along the track were available on a side. Results show that the average TC’s rainfall region extends about 200 (295) km from the left (right) side of the track. Seventy of 85 TCs with a swath length > 500 km have rain swaths that expand on the left side at some point as they move inland. Multiple Chi-square tests show that the TCs exhibiting expansion were hurricanes at landfall, re-intensifying over land, undergoing extra-tropical transition, and/or moving near the coastline. Additionally, we construct a series of maps with return intervals and frequency distributions for inland TCP that are comparable to those of tropical storm-force winds created by previous researchers. Nearly 95% of 2435 counties over the eastern U.S. are more frequently exposed to rainfall than wind from TCs. Also, many inland regions have received 5-6 TCP events in a single season, which confirms that TCP should be a concern for people living inland as well as near the coast. This study’s analysis of the spatial characteristics of TCP inland extent from a climatological perspective should benefit forecasting, hazard mitigation, and risk analysis of TC hazards.