Flash Flooding in Arid/Semi-arid Urban Environments: Storm Structure, Evolution and Storm Environment

We examine the hydrometeorology and hydroclimatology of flash floods in the arid/semi-arid watersheds of the Phoenix, Arizona metropolitan region. The study region is affected by infrequent, but severe floods associated with thunderstorms during the North American Monsoon (July-September). The principal research questions that motivate this study are: 1) what are the characteristic modes of structure and motion for thunderstorm systems that produce flash floods in arid/semiarid, urban watersheds, and 2) what is the role of urban land use and cover in modifying the storm environment of flash flood producing storms. We develop climatological characterizations of rainfall and thunderstorm variability over the study region using observations from a dense network of rain gages, radar rainfall fields and the National Lightning Detection Network. We examine Lagrangian properties for a catalog of flash flood producing storms using 3D reflectivity fields together with the TITAN storm tracking algorithms. We further investigate the storm environment based on analyses of four-year (2012-2015) downscaling simulations using the Weather Research and Forecasting (WRF) model coupled with the Princeton Urban Canopy Model (PUCM). Numerical experiments with WRF are also implemented to identify the role of urbanization in modifying storm environment, including mountain-valley circulation, convergence/divergence, moisture availability, over the Phoenix metropolitan region.