Urban Impacts on Extreme Monsoon Rainfall and Flooding in Complex Terrain

Hydrometeorological impacts due to urbanization for cities close to complex terrain are poorly understood due to the complexities of terrain-related circulation and urban perturbations of atmospheric flow. In this study, we examine urban impacts on extreme monsoon rainfall and the resultant flooding over central Arizona based on high-resolution atmospheric and hydrological model simulations. Strong positive rainfall anomalies at the urban-rural interface downwind of the city are mainly related to dynamic effects (increased surface roughness) on convective outflow boundaries. Urban-related thermodynamic disturbances slightly increase rain rates over the downtown core of Phoenix. Contrasting rainfall anomalies for two consecutive storm episodes highlight the importance of flow regime analysis in understanding urban impacts on extreme rainfall in complex terrain. Urban-induced rainfall anomalies result in amplification of flood peak magnitudes by as much as a factor of 2 for Phoenix watersheds. Our results highlight the urban impacts on regional flood hydrology through land-atmosphere interactions.