Climatic Effects of 30 Years of Landscape Change Over the Greater Phoenix, AZ, Region

The climatic impact of recent landscape change for one of the nation's most rapidly expanding metropolitan complexes, the Greater Phoenix, AZ, region, is examined. The region's landscape evolution over an approximate 30-year period since the early 1970s is first documented based on analyses of satellite imagery and land-use/land-cover (LULC). We make use of Regional Atmospheric Modeling System (RAMS) simulations (2-km grid spacing) in conjunction with consistently defined LULC datasets and associated biophysical parameters for circa-1973, circa-1992, and circa-2001, and quantify the effect(s) of intensive landscape change on the July near-surface temperature and surface radiation and energy budgets for the Greater Phoenix region. The main findings are as follows: since the early 1970s the region's landscape has been altered by a significant increase in urban/suburban land area, primarily at the expense of decreasing plots of irrigated agriculture, and secondarily by the conversion of semi-natural shrubland. Mean regional temperatures for the circa-2001 landscape were 0.12°C warmer than the circa-1973 landscape, with maximum temperature differences, located over regions of greatest urbanization, in excess of 1°C. The significant reduction in irrigated agriculture, for the circa-2001 relative to the circa-1973 landscape, resulted in peak dew-point temperature reduction in excess of 1°C. In addition to monthly spatial averages, we also illustrate the impact of distinct land-use conversion themes in order to assess the climatic effect of the region's chief landscape changes, on a diurnal scale. The two urbanization themes studied (from an initial landscape of irrigated agriculture and semi-natural shrubland) have the greatest positive effect on near-surface temperature, increasing maximum values by 1°C. Overall, sensible heat flux differences between the circa-2001 and circa-1973 landscapes result in a 1 Wm-2 increase in domain-wide sensible heating with a similar order of magnitude decrease in latent heating. In addition, the role of the surface budget changes on the mesoscale dynamics/thermodynamics, in context of the large-scale environment, is addressed. Results indicate that differences in surface heating are responsible for the development of preferentially located mesoscale circulations, which are shown to form on most days, and which increase in vigor for the 2001 as compared to the 1973 landscape, due to the increased planetary boundary layer (PBL) heating via enhanced turbulent heat flux. The effect of these stronger circulations was to warm and dry the lower part of the PBL and moisten the upper part of the PBL for the 2001 relative to the 1973 landscape. The precise physical mechanism(s) whereby precipitation enhancement is initiated with evolving landscape, since the early 1970s, reveals a complicated interplay among scales (from the turbulent to the synoptic-scale) warranting future research. However, precipitation recycling, was found to be an important driver in the overall sustenance of rainfall enhancement.