Urban effects on the North American monsoon precipitation of 2002–2010: a study within the context of severe weather events

The Southwest U.S. experiences an increase in moisture, instability, and precipitation during the summer months associated with the North American Monsoon System (NAMS). During this time period, organized convection causes severe weather with impacts of flash flooding, strong winds, dust storms, and lightning. This study investigates the effect of a incorporating an urban canopy model and urban land cover within the Phoenix simulating severe weather events with the Weather Research and Forecasting model at a convective-permitting scale (2.5 km grid spacing) for the period (2002-2010). Approximately 75 event days for retrospective simulation were selected based on the presence of favorable thermodynamic conditions for the development of severe thunderstorms. Two experiments have been conducted considering each simulated individual severe weather event. The urban experiment incorporates the urban canopy model and 2006 land cover classifications. The desert control simulation replaces the Phoenix area with its pre-settlement, natural land cover. The mean differences in simulated precipitation for between the urban and desert control cases are relatively small and statistically insignificant. However, the urban simulations do produce significantly more intense precipitation around the periphery of the Phoenix area for the most extreme weather cases, defined as those precipitation events in Phoenix exceeding the 80-90% threshold a cumulative distribution function. Nearly all of these most extreme events occur during the evening hours. The changes in model simulated precipitation are largely tied to differences in sensible and latent heat fluxes that affect the height of the planetary boundary layer and atmospheric instability.