Evaluation of MERRA-2 hourly surface solar radiation across China

Accurate global horizontal irradiance (GHI) is key to solar energy resource assessment and prediction. The GHI provided by MERRA-2 (Version 2 of the Modern-Era Retrospective Analysis for Research and Applications) still needs to be evaluated over China. In this study, GHI observed by 37 ground stations in 2018 are used to systematically assess the MERRA-2 GHI performance for all-sky, clear-sky and cloudy days. Moreover, aerosol and cloud products from MERRA-2 are compared with satellite counterparts to determine the dominant factor affecting MERRA-2 GHI. The results show that MERRA-2 overestimates the hourly and daily mean GHI by 69.35 W/m(2) and 27.54 W/m(2) on all-sky days, respectively. The mean bias error and root mean square error of hourly GHI between MERRA-2 and CERN are 4.98 W/m(2) and 124.85 W/m(2) on clear-sky days and 107.16 W/m(2) and 215.24 W/m 2 on cloudy days, respectively. These values indicate a much greater overestimation of MERRA-2 GHI on cloudy days than on clear-sky days. By comparing cloud fraction (CF) and aerosol optical thickness (AOD) between the Moderate-resolution Imaging Spectroradiometer (MODIS) and MERRA-2, we find that the overestimation of MERRA-2 GHI in most areas is induced by the substantial underestimation of MERRA-2 CF. MERRA-2 underestimates AOD in North China and further enhances the overestimation of GHI caused by the underestimation of MERRA-2 CF, whereas the overestimation of AOD in the Sichuan Basin and Southeastern China partly counterbalances the overestimation of GHI caused by the underestimation of MERRA-2 CF.

Automated summary

This study evaluates the performance of MERRA-2's global horizontal irradiance (GHI) in China by comparing it with ground station observations. The results show that MERRA-2 overestimates GHI in all-sky, clear-sky, and cloudy conditions. The underestimation of cloud fraction is identified as the dominant factor causing the overestimation of MERRA-2 GHI.