Simulation of potential impacts of man-made land use changes on US Summer climate under various synoptic regimes

This study evaluates impacts of land use changes due to human settlement on regional summer climate over the central and western United States by perf orming 30-day simulations during normal, drought, and flood years. Under cu rrent land use the simulated evapotranspiration increased noticeably over t he central United States where grassland has been replaced by crops. Simula ted evapotranspiration decreased slightly in the western United States. The se changes produced wetter and cooler surface air over the central United S tates and slightly drier and warmer air over the western United States. Res ponses of surface fluxes and thus screen height variables to land use chang es were consistent from year to year, whereas rainfall showed strong intera nnual variations because of the combination of various dynamic processes in volved in precipitation. For normal year conditions, average evapotranspira tion and rainfall under current land use increased by 18% and 8%, respectiv ely, over the central United States, whereas they slightly decreased in the western United States. In both flood and drought years, current land use e xhibited a rainfall increase in the western United States and a decrease ov er the central United States. The decrease of rainfall with increased evapo transpiration in the central United States was likely associated with weake ning of the dynamic forcing needed to produce precipitation.