Trends in downward surface shortwave radiation from multi-source data over China during 1984-2015

The clear knowledge of decadal variability of surface solar radiation (SSR) is of vitally significant for understanding hydrological and biological processes and climate prediction. However, existing studies have shown observed SSR over China may have large discrepancies and inhomogeneity in decadal variability due to sensitivity drift, inaccurate calibrations and instrument replacement. Therefore, a new procedure of station selection was proposed to eliminate errors and to derive true SSR values in this study. Afterward, two satellite retrieves of SSR, including Clouds and the Earth's Radiant Energy System-energy balanced and filled product (CERES-EBAF) (edition 4) and Global Energy and Water Cycle Experiment-Surface Radiation Budget (GEWEX-SRB) (Version 3.0), and three reanalysis products, including National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR), national centers for environmental prediction-/department of energy (NCEP-DOE) and Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) were evaluated using true SSR values at 39 homogeneous stations from the China Meteorological Administration and it was found that although all five products overestimated SSR, two satellite retrieves showed better accuracy with an overall R of 0.95, an root mean squared error (RMSE) of 20.4 W m(-2) and mean absolute bias error (MAE) of 14.9 W m(-2) for CERES-EBAF and an overall R of 0.92, an RMSE of 27.7 W m(-2) and MAE of 21.2 W m(-2) for GEWEX-SRB across China. Meanwhile, inter-comparisons between trends of observations and trends of two satellite retrieves in this study showed that the new trends derived from two satellite retrieves (+0.78 W m(-2) decade(-1)) were good agreement with trends of observation (+0.92 W m(-2) decade(-1)) from 1994 to 2015. However, trends of SSR (+5.8 W m(-2) decade(-1)) in situ measurements were still in disagreement with the trends of SSR (-3.7 W m(-2) decade(-1)) derived from two satellite retrieves from 1984 to 1993 because of the sensitivity drift and instrument replacement in this period. The possible reasons for decadal variability of SSR in China were detected and it was found that variations in aerosol optical depth (AOD) and aerosol-cloud interaction, rather than cloud, were suggested to be likely the main influencing factor of decadal variability of SSR across China from 1984 to 2015.