Mass-conserving downscaling of climate model precipitation over mountainous terrain for water resource applications

A mass-conserving method to downscale precipitation from global climate models (GCMs) using sub-grid-scale topography and modeled 700-mb wind direction is presented. Runoff is simulated using a stand-alone hydrological model, with this and several other methods as inputs, and compared to runoff simulated using historical observations over the western contiguous United States. Results suggest the mitigation of grid-scale biases is more critical than downscaling for some regions with large wet biases (e.g., the Great Basin and Upper Colorado). In other regions (e.g., the Pacific Northwest) the new method produces more realistic sub-grid-scale variability in runoff compared to unadjusted GCM output and a simpler downscaling method. The presented method also brings the runoff centroid timing closer to that simulated with observations for all subregions examined.