Comprehensive assessment of five global daily downward shortwave radiation satellite products

Abstract

The downward shortwave radiation (DSR) is a critical parameter of the surface radiation budget. Several DSR satellite products have been developed in recent years. In this study, five updated global satellite daily DSR products were evaluated using in situ measurements from 142 global sites with a special focus on high latitudes in 2004. These five products are Clouds and the Earth's Radiant Energy System Synoptic TOA and surface fluxes and clouds (CERES), Clouds, Albedo and Radiation Edition 2 data (CLARA), Global Land Surface Satellite Downward Shortwave Radiation (GLASS), Breathing Earth System Simulator (BESS) shortwave radiation product, and Moderate Resolution Imaging Spectroradiometer land surface Downward Shortwave Radiation (MCD18) with a spatial resolution of 100, 25, 5, 5, and 1 km, respectively. The CERES, BESS, and MCD18 provide full global coverage throughout the year, whereas CLARA and GLASS present different levels of seasonal data loss over high-latitude areas. The products were aggregated and compared at various spatial resolutions over different subareas. The overall accuracy increased after the products were aggregated to 100 km. However, the highest accuracy was achieved at a resolution of 25 km over high-latitude areas for GLASS and MCD18. When all products were evaluated at a resolution of 100 km, the global root-mean-square error of CERES, CLARA, GLASS, BESS, and MCD18 was 27.6, 29.1, 30.3, 29.6, and 31.6 W/m2, respectively, and the mean bias difference was 2.2, −1.5, −1.8, −3.4, and −8.0 W/m2. The accuracies of most products are ~7 W/m2 lower over high-latitude areas. A seasonal variation of the accuracies was observed for all products. It is particularly pronounced over high-latitude areas. With respect to the long term, both in situ data, BESS, and CERES show insignificant trends, while CLARA and GLASS present dimming trend. Besides, CLARA and GLASS exhibit slight annual changes of −0.250 and −0.387 W/m2 in the bias and 0.357 and 0.310 Wm−2 in the RMSE in the past two decades. GLASS and MCD18 exhibit a superior performance over coastal regions but degrade over snow-covered areas. Potential refinements of current high-resolution DSR retrieval algorithms are suggested, which will improve the retrieval accuracy. Highly accurate products with a long-term stability, especially over high-latitude areas, are required for future climate change analyses.