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<br />0'\ <br />~.:t~ ~ <br />,\ - <br />, <br /> <br />Presented at the 1993 ASCE Conference on Irrigation and Drainage Engineering, Park City, Utah <br /> <br />Nested Modeling of Watershed Precipitation <br /> <br />David A. Matthews1 <br /> <br />Gary Bates and Filippo Giorgi2 <br /> <br />Abstract <br /> <br />The U. S. Bureau of Reclamation's GCCRP (Global Climate Change Research <br />Program) is concerned with the possible impacts of global change upon precipitation, <br />environmental variables and streamflow in the Western United States. This paper <br />presents an assessment of regional model simulations of the unusually heavy <br />precipitation in the El Niiio of 1983, and the dry 1988-89 winter season. It outlines <br />the nested modeling approach of our GCCRP that will use synoptic-scale forcing <br />from the global NCAR (National Center for Atmospheric Research) CCM <br />(Community Climate Model) to initialize the regional Pennsylvania State <br />University/NCAR Mesoscale Model .version 4 (MM4) developed by Giorgi and <br />Bates. Two phases of Reclamation's collaborative GCCRP research with NCAR are <br />outlined. Phase 1 focuses on validation of the nested modeling approach in current <br />climate. Phase 2 will apply the nested modeling to future climate to determine the <br />long term impacts of global change. This paper focuses on results from phase 1. <br /> <br />Comparisons of the observed winter orographic precipitation and model- <br />diagnosed precipitation indicate good regional predictions by MM4; howeve.r, <br />summer convective precipitation was significantly overestimated. Daily cumulative <br />precipitation time series for seven western watersheds indicate that MM4 generally <br />describes the timing and intensity of precipitation and clearly discriminates among <br />different watersheds. Modeled precipitation patterns are consistent with observed <br />storm structure. Simulations of the 1982-83 winter precipitation in the Gunnison <br />Basin of Colorado are much better than that for 1988-89. These results suggest that <br />MM4 provides useful climatic information. However, the exact timing of modeled <br /> <br />1 u. S. Bureau of Reclamation, Denver, CO 80225-0007 <br />2 National Center for Atmospheric Research, Boulder, CO 80307 <br /> <br />I <br /> <br />Matthews et al. <br />