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,.4 . The Nested Modeling Approach - Current Climate Simulations In phase 1, the nested modeling approach uses a regional model to provide the regional-scale forcing that largely controls the evolution of local-scale clouds and precipitation that is simulated by a local-scale model. The regional model is the MM4 (pennsylvania State University/NCAR Mesoscale Model Version 4). The standard MM4 model is described in Anthes et al. (1987); however, in this study we use results from an augmented version for regional climate studies described by Giorgi and Bates (1989). This version of the MM4 model includes a sophisticated surface physics/soil hydrology package. The compressible, hydrostatic model solves the equations of motion written in terrain-varying sigma vertical coordinates. This study focuses on the Gunnison Basin in western Colorado marked by the bold square in Figure 1a nested within the domain analyzed by MM4. The MM4 model provides the large-scale (synoptic) and regional-scale (mesoscale) forcing as shown by the summer monsoon flow over the West in Figure 1a and corresponding 24-h precipitation in Figure lb. Local-scale model simulations are examining the detailed evolution of clouds and precipitation within the 440- by 440-:-km domain (dx= 10 km) marked by the bold square in Figure 1a. Within this area, the dashed square defines the nested 5-km resolution, 260- by 260-kin domain. Small dots show each MM4 grid point and large dots indicate the CCM (Community Climate Model) R-15 (rhomboidal spherical harmonic truncation at wave number 15) 500-km resolution grid points. The CCM has been used widely by modelers. to examine the effects of doubled CO2 on global climate (Washington and Meehl, 1984). Clearly, nested regional and local-scale models are required to examine precipitation processes at the watershed level. Reclamation is using two models to simulate local orographic precipitation: Rhea and Clark models. Medina (1992) has adapted the steady-state Rhea model (Rhea, 1977) to simulate local precipitation over the Gunnison River Basin in Colorado, and has adapted the model to use initial conditions from soundings derived from MM4. This approach approximates a one-way, local-scale nesting. Comparisons between Rhea model simulations from rawinsondes and MM4-derived soundings are encouraging; however, convective precipitation is not accurately simulated in either the MM4 or Rhea models. Therefore, we are also using the more sophisticated, three-dimensional, time- dependent, local-scale Clark model to simulate the local evolution of convective clouds and precipitation over complex terrain in the central Colorado Rockies. The Clark model is used extensively throughout the research community to simulate phenomena ranging from airflow over complex terrain to deep convection (Clark, 1979). The model is ail anelastic, finite difference model with one- and two-way interactive nesting described by Clark (1977), and Clark and Hall (1991). The Clark model will be used to understand the complex three-dimensional airflow and evolution of clouds and precipitation in a limited sample of cases. It will contribute to better parameterizations of airflow structure for the Rhea model, which may be run 3 Matthews et al.