Evaluating Topographic Effects on Kilometer-Scale Satellite Downward Shortwave Radiation Products: A Case Study in Mid-Latitude Mountains

Downward shortwave radiation (DSR) is critical to many surface processes, and many satellite-derived DSR products have been released. Few studies have validated DSR over mountains where it is highly heterogeneous, and so, the shortwave flux measured at ground stations does not match kilometer-scale DSR products. To tackle this challenge, we used a high spatial resolution (30 m) daily DSR over Sierra Nevada, Spain, for 2008-2015, and a mountainous radiative transfer model to explore how topographic effects impacted the performances of DSR products. Four widely used satellite products were selected as proxies for our evaluation: 1) MCD18A1 V6.1 (with a spatial resolution of 1 km); 2) Meteosat Second Generation (MSG) DSR ( similar to 3.3 km); 3) Global LAnd Surface Satellite (GLASS) DSR V42 (0.05 degrees); and 4) Breathing Earth System Simulator (BESS) DSR (0.05 degrees). There are three main findings under clear skies. First, the product accuracies were slope-dependent, decreasing by 59.8%-134.6% with a slope of >= 25 degrees compared with areas with a slope of <10 degrees . Second, the product accuracies were aspect-dependent, exhibiting a higher degree of overestimation (i.e., average of 27.6 W/ m2 ) on the north side and underestimation (i.e., an average of -1.3 W/ m2 ) on the south side. Third, and finally, the product accuracies were time-dependent, exhibiting seasonal variations and pronounced overestimation in summer (i.e., 8.8-18.2 W/ m2 ). Moreover, the impact of topography decreased with increasing cloud cover. Our findings can be applied to various mountainous areas due to the same mechanism of how topography influences the DSR estimation. This study corroborates the substantial uncertainties of the current DSR products in mountains and the necessity of incorporating topographic information into DSR estimations.