SNOWMELT DERIVED FROM COMPLEX MOUNTAINOUS REGIONS SUPPLIES A SIGNIFICANT PORTION OF THE WORLD S POPULATION WITH FRESHWATER. RECENT STUDIES SUGGEST THAT PEAK SNOW WATER EQUIVALENT (SWE) MELT AND STREAMFLOW OCCUR EARLIER IN THE YEAR DUE TO CLIMATE CHANGE. GENERAL CIRCULATION MODELS ARE OFTEN USED IN CLIMATE STUDIES; HOWEVER THEIR COARSE SPATIAL RESOLUTION INADEQUATELY RESOLVES THE HIGH HETEROGENEITY OF TOPOGRAPHY THAT IS IMPORTANT FOR ESTIMATING ACCUMULATION/MELT SEASON SUBGRID PROCESSES IN MONTANE SYSTEMS. REANALYSIS PRODUCTS (E.G. NASA S MODERN-ERA RETROSPECTIVE ANALYSIS FOR RESEARCH AND APPLICATIONS (MERRA)) RELY ON STATE-OF-THE-ART MODELS (E.G. GODDARD EARTH OBSERVING SYSTEM V5.2.0 ATMOSPHERIC GENERAL CIRCULATION MODEL (GEOS-5 AGCM)) TO PRODUCE ROBUST HYDROLOGIC STATE/FLUX PREDICTIONS. A PATHWAY TO BETTER QUANTIFY HYDROLOGIC STATES (SWE SOIL MOISTURE) FLUXES (PRECIPITATION LATENT HEAT) AND STORAGES (SNOWPACK WATER VAPOR) IS THROUGH IMPROVING REANALYSIS DATASETS AND MODELSSUCH AS MERRA AND GEOS-5 AGCM. THEREFORE THIS PROJECT S OVERARCHING GOAL IS THREE-FOLD: 1) EVALUATE MERRA SNOWPACK PREDICTIONS IN SNOW-DOMINATED MONTANE REGIONS OVER THE SATELLITE ERA 2) DYNAMICALLY DOWNSCALE MERRA FOR A HIGH-RESOLUTION REGIONAL CLIMATE MODEL TO GAIN INSIGHT INTO SUBGRID PROCESSES IN COMPLEX MOUNTAIN SYSTEMS AND 3) DEVELOP IMPROVED SUBGRID PARAMETERIZATIONS THAT COULD ENHANCE MERRA AND GEOS-5 AGCM SNOWPACK ESTIMATES IN MONTANE DOMAINS. VARIABILITY TRENDS AND CLIMATOLOGY OF PEAK SWE TIMING/VOLUME MELT-OUT DATE AND SNOWPACK DISTRIBUTION WILL BE INVESTIGATED IN THE CONTEXT OF EACH OBJECTIVE OVER THE HYDROLOGICALLY IMPORTANT SIERRA NEVADA. THE EXTENT TO WHICH RAINFALL/SNOWFALL AND MELT ESTIMATES DEPEND ON TOPOGRAPHIC RESOLUTION WILL BE ASSESSED. INSIGHT FROM AMULTI-DECADAL HIGH-RESOLUTION REGIONAL MODELING APPROACH WILL BE USED TO CHARACTERIZE SUBGRID PROCESSESAND DEVELOP ROBUST PARAMETERIZATIONS THAT COULD ULTIMATELY IMPROVE LARGE-SCALE MODEL PERFORMANCE IN MONTANE SYSTEMS WORLDWIDE. AN EXISTING SWE REANALYSIS DATASET WILL BE USED TO INVESTIGATE MERRA SNOWPACK ESTIMATES IN MONTANE REGIONS AND PROVIDE THE SPATIALLY-DISTRIBUTED FIELDS FOR VALIDATING THE REGIONAL CLIMATE MODEL ACCUMULATION/MELT SEASON PREDICTIONS.