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" ~;I\p. SecoocI Sympwium oa OloW au.,. ..... 71" AMS Aaaaal MMciD&. IIDUUJ 1),.1, 199.. New 0dMM. Loallll- }/ rt MODEUNG REGIONAL CONTROLS OF WATERSHED PRECIPITATION FOR CUMA TE CHANGE sruDlES David Matthews U.S. Bureau of Reclamation P. O. Box 25007, Denver CO 80225 Filippo Giorgi and Gary Bates National Center for Atmospheric Research, I Boulder CO 80307 1. INTRODUcnON The Global Oimate Change Response Program (GCCRP) of the U.S. Bureau of Reclamation (Reclamation) is concerned with the possible impacts of global climate change upon precipitation, evapotranspiration, and streamflow in the Western United States. Dennis (1991) has described in a companion paper Reclamation's strategy to develop scenarios of precipitation under changed climate conditions. This paper presents initial results from Reclamation's GCCRP climate modeling studies. It focuses on collaborative efforts to linlc large-scale general circulation models (GCMs) used in global climate simulations at the National Center for Atmospheric Research (NCAR) to a simple local-scale precipitation model described in a companion paper by Medina (1991). Current GCMs have resolutions of 300 to 600 kIn. They represent the region from the Sierra Nevada to the Rocky Mountains as one broad ridge; therefore, they cannot adequately simulate precipitation patterns over the Western United States. This problem is caused by limitations in computational resources that restrict the dimensions of the GCM grid, which does not resolve Ioc:al forcing patterns. Computational restrictions also limit the representation of physical processes that occur on different spatial and temporal scales. It. may be many years until high-resolution GCM simulations become feasible. Therefore, to improve the simulation, a system of nested models is being developed at NCAR using supercomputer technology. Recently. the climate modeling team at NCAR has succeeded in nesting the Pennsylvania State University/NCAR Mesoscale Model (MM4) into the NCAR Community Climate Model (CCM) for regional climate simulations. The first stages of developing the nested model system are described by Dickinson et al. (1989), Giorgi and Bates (1989), and Giorgi (1990). The one- way nested technique uses the GCM results from the NCAR CCM to provide the large-scale circulation response to global climate forcing. Then the regional- scale MM4 model is used to describe the effect of sub- GCM grid scale forcing over a specific region. This forcing includes the effects of complex topography. I The National Center for Atmospheric Research is sponsored by the U.s.A. National Science Foundation. large bodies of water. and surface vegetation characteristics that may significantly affect local climates. Reclamation will then use output from the regional simulations to initialize a local-scale precipitation model. This paper provides a brief report on the initial efforts to determine the feasibility of using the nested MM4 model outputs to describe regional controls of precipitation over specific basins. The regional structure of storms and its effect on regional precipitation is described for the Verde and Salt River basins of Arizona. Local-sca1e precipitation model results from MM4-derived soundings are dc:scribed separately by Medina (1991). 2 REGIONAL MODEUNG FRAMEWORK The two models used in the nested modeling system have been thoroughly tested and widely used for more than 10 years. The NCAR CCMl model (Williamson et al. 1987) uses a grid of 4S:17S (R15 spectral truncation) latitude-longitude resolution. The standard MM4 model is described in Anthes el al. (1987); however, in this study we have used results from the augmented version descn"bed by Giorgi and Bates (1989). The augmented version of the MM4 model includes an improved sophisticated surface physics-soil hydrology package, an explicit boundary layer formulation, and a more detailed treatment of radiative transfer in place of the corresponding standard MM4 formulations. The model is a. compressible and hydrostatic model with primitive equations written in terrain-varying sigma (a)-vertical coordinates, where" = (P-pJ/p., p. = P,-PI' P, is the surface pressure, and PI is the pressure at the top of the model. The improved surface physics-hydrology package included in the augmented version ()f MM4 is known as the Biosphere Atmosphere Transfc:r Scheme (BATS). It was developed by NCAR and university collaborators and is descnbed by Dickinson (1984), Dickinson et al. (1986). and Wilson et al. (1987). It includes processes of sensible heat. water vapor and momentum fluxes at the surface, evapotranspiration, and soil hydrology. The soil hydrology calculations account for precipitation, snowmelt, canopy f,~liage drip, evapotranspiration, surface runoff, infiltration below the root zone, and diffusive exchange of water between soil layers. The augmented MM4 also