Urban modifications in a mesoscale meteorological model and the effects on surface energetics in a semi-arid metropolitan region

A refined land cover classification for the semi-arid Phoenix (Arizona, USA) metropolitan area and some simple modifications to the surface energetics were introduced in the fifth-generation PSU/NCAR mesoscale meteorological model MM5. The single urban category in the existing 24-category United States Geological Survey (USGS) land cover classification used in MM5 was divided into three classes: built-up urban, urban mesic residential and urban xeric residential. 30 meter/pixel land cover data were derived from 1998 LANDSAT Thematic Mapper satellite images. The data were upscaled to a 30-second grid and used to augment and correct the existing USGS land use/cover scheme in MM5. Ground truth information was used to determine the composition of mesic and xeric residential areas in terms of typical fractions of irrigated and total vegetation and man-made surfaces. These changes allowed us to vary the moisture availability for the urban land cover classes and hence to consider the influence of urban irrigation practices in the surface energy budget. Furthermore, 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.

Two 72-hour simulations were performed with MM5 on a 2 km x 2 km grid during the early summer of two different years. It is at this time of year that the solar forcing is largest and the model response to the land cover most critical. The updated land cover data and 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. The nighttime near surface air temperatures across the urban area were accurately simulated by adding radiation trapping, heat storage and anthropogenic heating.