Dynamically Downscaled IPCC RCM Climate Projections for the Southwest U.S. and Their Applicability in Hydrologic Resource Projection

Current global scale climate projections from the Intergovermental Panel on Climate Change (IPCC) are not adequate for regional and fine scale projections, which require better resolved terrain forcing, land surface and vegetation feedback. There is an increasing demand for regional scale climate projections at a spatial scale appropriate for hydrologic modeling. The Weather Research and Forecast (WRF) model is used for the long-term regional climate simulation, driven by two sets of ‘well-performing' IPCC AR4 data (MPI-ECHAM5 and UKMO-HadCM3). Summer precipitation in Southwest is largely convective and closely related to the North American monsoon (NAM). Trends in temperature and precipitation climatology and extremes are analyzed in both climate control and climate change periods. The free-running RCM-IPCC simulations have shown ability to represent historical climate with reasonable driving physical mechanism in the Southwest U.S. warm season precipitation, demonstrating value added using dynamical downscaling with spectral nudging. However, the different GCM simulations project both increased and decreased NAM summer precipitation. Additional high resolution domains are identified in the core NAM region as well as river basins that are crucial for water supply in the Western U.S. A hydrological study for both Colorado and Rio Grande River basins indicated a decrease in projected runoff, regardless of the precipitation trend from the forcing RCMs.