P20  What is Responsible for the Semi-Arid Urban Area ÒOasis     EffectÓ?

Georgescu, Matei, Mohamed Moustaoui, Alex Mahalov, Arizona State University, and Jimy Dudhia, National Center for Atmospheric Research

The climatic summertime representation of the diurnal cycle of near-surface temperature is evaluated using the Weather Research and Forecasting System [WRF]) over the rapidly urbanizing and water-vulnerable Phoenix metropolitan area. A suite of monthly, high-resolution simulations are conducted during a number of July months with both a contemporary landscape and a hypothetical Pre-Settlement scenario. We demonstrate excellent agreement between WRF and observations in the representation of daily to monthly diurnal cycle of near-surface temperature. The WRF-simulated characterization of the diurnal cycle, supported by previous observational analyses, illustrates two distinct and opposing impacts on the urbanized diurnal cycle of the metro-area: evening and nighttime warming is partially offset by daytime cooling. The simulated nighttime Urban Heat Island (UHI) is explained by well-known mechanisms (e.g., slow release of heat from within the urban fabric stored during daytime). During daylight hours the limited vegetation and dry semi-desert region surrounding urban Phoenix warms at greater rates than the urban complex. We show that lower urban relative to semi-desert daytime temperatures occur as a result of PhoenixÕ surrounding environment rather than the addition of residential and agricultural irrigation (i.e., Òoasis effectÓ).