The Impact of Urbanization on North American Monsoon Precipitation in Phoenix, Arizona, within a Context of Modeled Severe Weather Events

This study investigates the effect of incorporating an urban canopy model (UCM) and urban land cover, in a series of sensitivity experiments for severe weather events during the North American monsoon in Arizona. One single severe weather event is first simulated with the Weather Research and Forecasting (WRF) model at a convective-permitting scale. The URBAN experiment uses the UCM and 2006 land cover classifications whereas the DESERT experiment replaces Phoenix and Tucson areas with their pre-settlement, natural desert land cover. The URBAN simulation produces less precipitation within the urban core of the city and more precipitation around the periphery of the city. Instead of being a “heat island,” like cities of comparable size in the eastern and central United States, the presence of vegetation and irrigation makes Phoenix a “heat sink” in comparison to its surrounding desert. Greater partitioning of surface energy to latent heat fluxes causes the boundary layer to be suppressed with a local increase in stability and surface pressure. Repeating the same experiment on a series of objectively identified severe weather events in the Phoenix area for the period 1991-2010 confirms that the result of the case study applies to an entire climatology of severe weather events. Results tend to confirm observed long-term changes in monsoon precipitation within the Phoenix metropolitan area.