Urban modifications in a mesoscale meteorological model and the effects on near-surface variables in an arid metropolitan region

A refined land cover classification for the arid Phoenix (Arizona) metropolitan area and some simple modifications to the surface energetics were introduced in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). The single urban category in the existing 24-category U.S. Geological Survey land cover classification used in MM5 was divided into three classes to account for heterogeneity of urban land cover. Updated land cover data were derived from 1998 Landsat Thematic Mapper satellite images. The composition of the urban land use classes in terms of typical fractions of vegetation and anthropogenic surfaces was determined from ground-truth information, allowing a variety of moisture availability for evaporation by land cover class. Bulk approaches for characteristics of the urban surface energy budget, such as heat storage, the production of anthropogenic heat, and radiation trapping, were introduced in MM5's Medium Range Forecast boundary layer scheme and slab land surface model. A 72-h simulation was performed with MM5 on a 2 km X 2 km grid during June 1998. The new land cover classification had a significant impact on the turbulent heat fluxes and the evolution of the boundary layer and improved the capability of MM5 to simulate the daytime part of the diurnal temperature cycle in the urban area. The nighttime near-surface air temperatures were improved significantly by adding radiation trapping, heat storage, and anthropogenic heating to the model.