Assessment of Three Satellite-Estimated Land Surface Downwelling Shortwave Irradiance Data Sets

This letter assesses three satellite-estimated surface downwelling shortwave irradiance data sets: 1) GEWEX-SRB; 2) ISCCP-FD; and 3) CERES-FSW, using ground measurements collected at 36 globally distributed sites from 2000-2002. SRB and FD solar irradiance are available at three hourly intervals during daytime and FSW hourly solar irradiance is available at late morning. The data are compared to ground measurements at the temporal resolutions of the satellite measurements. Results indicate that the downwelling solar irradiances of the three products show good overall agreement with ground measurements. FSW has an overall coefficient of determination of R² = 0.69, a bias of 29.7 Wm^-2 (6.0% in relative value), and a standard deviation (STD) of 123.2 Wm^-2 (25.1% in relative value). The values are 0.83, -5.5 Wm^-2 (-1.9%), 101.3 Wm^-2 (35.0%) for SRB, and 0.83, 2.8 Wm^-2 (0.3%), 101.7 W m 2 (35.0%) for FD. However, there are substantial uncertainties in these products in some regions. For example, large biases ranging from -90.2 Wm^-2 to 45.8 Wm^-2 are found in SRB in Southeast Asia. FD has large biases in Southeast Asia (58.9 Wm^-2) and Greenland (-33.2 Wm^-2). FSW has substantial biases in Southeast Asia (72.6 Wm^-2) and Japan (66.5 Wm^-2); while R² is 0.35 in Tibetan Plateau and 0.47 in Southeast Asia.