Lagrangian analyses of rainfall structure and evolution for organized thunderstorm systems in the urban corridor of the Northeastern United States

In this study, a climatology of the structure and evolution of rainfall for organized thunderstorm systems in the urban corridor of the northeastern United States is developed. These storm systems are major agents of flash flooding for urban regions of the northeastern United States and, more generally, for the United States east of the Rocky Mountains. The analyses are motivated by problems that center on characterizing flash flood hazards. The authors focus on spatial heterogeneities of rainfall associated with urbanization in a region of complex landscape including mountainous terrain and land-water boundaries along the geometrically complex coastline of the New York City-New Jersey metropolitan region. The sample of storms selected for investigation consists of the 50 days from April to September 2001-09 with the largest cloud-to-ground lightning flash density derived from National Lightning Detection Network (NLDN) observations over the study region. Storm-tracking analyses of 3D radar reflectivity fields are performed for the 50 storm days and used to develop a Lagrangian climatology of storm structure and evolution for the study region. Rainfall analyses for the 50 storm days are based on high-resolution (1 km, 15 min) bias-corrected radar rainfall fields developed from the Hydro-NEXRAD system. The analyses suggest that complex terrain and land-water boundaries have large impacts on Lagrangian storm properties. Areas of increased heavy rainfall and lightning flash density over New York City were identified. The authors found evidence for changing storm structure as thunderstorms pass over New York City, but little evidence that thunderstorms split as they approach New York City.