Climatology of Orographic Precipitation Gradients Over High Mountain Asia Derived From Dynamical Downscaling

Abstract

Within High Mountain Asia (HMA), the annual melting of glaciers and snowpack provides vital freshwater to populations living downstream. Precipitation over HMA can directly affect the freshwater availability in this region by altering the mass balance of glaciers and snowpack. However, available reanalyses and downscaling simulations lack the resolution required to understand important glacier-scale variations in precipitation. This study aimed to determine the current characteristics of orographic precipitation gradients (OPG) by curve-fitting daily precipitation as a function of elevation from a 15-year, 4-km grid spaced Weather Research and Forecasting (WRF) model simulation focused on the Himalayan, Karakoram, and Hindu-Kush mountain ranges. To facilitate precipitation curve-fitting, the WRF model grid points were separated into regions of similar orientation, referred to as facets. Akaike Information Criterion-corrected values and an F-test p-value identified the need for a curvature term to account for a varying OPG with elevation. Regions with similar seasonal variability were found using $k$-means clustering of the monthly mean OPG coefficients. The central Himalayan slope's intra-seasonal variability of OPG depended on synoptic scale conditions, in which cyclonically-forced heavy-precipitation events produced strong sublinear increases in precipitation with elevation. Initial testing of precipitation estimates using monthly coefficients showed promising results in downscaling daily WRF precipitation; the daily mean absolute error at each grid point had a lower magnitude than the daily mean precipitation total, on average. Results provide a physically-based context for machine learning algorithms being developed to predict OPG and downscale precipitation output from global climate models over HMA.