Measuring the width of rainfall swaths produced by landfalling tropical cyclones over the northeastern U.S

Rainfall from tropical cyclones (TCs) can induce freshwater flooding that leads to loss of life. An improved understanding of the spatial distribution of rainfall associated with TCs as they move over land could help to identify which flood-prone regions should be warned in advance of the TC's arrival. Many TCs tracking northward over the northeastern U.S. have produced heavy rainfall that caused flooding due to their interaction with elevated terrain and/or their transition into an extratropical cyclone. Previous research has found that when passing through this region, the rain shields of TC become wider on the left side of their track due to the process of extratropical transition. Yet few studies have quantified how wide the area receiving TC rainfall is and how this width changes as the TC moves inland. In this study, a geographic information system (GIS) is utilized to define the spatial characteristic of landfalling TC rain shields from long term observational rainfall data. Thirty-one TCs tracking northward over the eastern U.S. are examined since 1970. Daily rainfall data are from the National Centers for Environmental Prediction (NCEP)-Climate Prediction Center Unified Precipitation Dataset (UPD), which provides daily precipitation amounts on a 0.25°× 0.25° latitude-longitude grid over land surfaces. Daily rainfall totals were analyzed beginning 24 hours before the storm's first landfall, and ending at 24 hours after the storm left the U.S or dissipated over land. Most rainfall totals accumulated over a four-day period. A contour method is utilized to generate TC rainfall regions. For each day, all polygons that are over a threshold value of one inch and within 500 km of track are selected. Next, all selected daily rainfall regions in TC period are merged to define the TC rainfall region. This method excludes rainfall produced by other weather systems. Then, the width of rain shield on the left side of the track is calculated every 100 km along the track beginning at the point of landfall. The results show that the average distance from the storm track to the left edge of the region receiving one inch of rainfall was about 200-250 km. Rainfall swaths of some TCs were over 350 km. For most TCs, the width broadens 120 -150 km as the TC moves north. Future research will utilize the same spatial analysis technique to define the widths of all TCs affecting the U.S. over the 1970-2012 period and compare right and left sides of the track when possible.