The analysis of historic precipitation records indicates that urbanization in the Phoenix metropolitan region affects monsoon thunderstorms. Previous studies showed a statistically significant increase in mean precipitation from the pre-urban to the post-urban period for locations in northeastern suburbs of Phoenix. However, the mechanisms leading to the observed changes in precipitation patterns are not clear.

In this research the Weather Research and Forecasting Model (WRF) was applied in order to investigate the effects of Phoenix' urban land use/cover on the dynamic and thermodynamic forcing of monsoon thunderstorms. Larger scale atmospheric motions control the broad distribution of water vapor and hence the large scale stability of the atmosphere, but local topographic and land cover effects arecritical to the geographic and temporal distribution of convective activity in this region. In order to properly capture the meso-alpha and beta scale of the atmospheric environment and forcing and allow accurate simulation of precipitation location and timing, the NCAR observation-nudging based real-time four-dimensional data assimilation (RTFDDA) and forecasting system is employed. To study the urban effects on precipitation in Phoenix and nearby regions, the unique Phoenix urban properties, such as the extended areas of irrigated vegetation, are parameterized in the WRF model.

Based on NEXRAD stage IV and local sounding data a typical Phoenix monsoon thunderstorm event (August 2/3 2005) during which an outflow boundary convergence situation allowed for a storm to form over Phoenix was chosen for a case study. Impact of the Phoenix metropolitan area on the thunderstorm development and propagation are studied. It is found that the increased surface roughness due to the urban buildings and the evapotranspiration from extended areas of irrigated vegetation and anthropogenic open water surfaces in the Phoenix region can significantly affect thunderstorm outflows and the propagations.