Decadal variability of surface incident solar radiation over China: Observations, satellite retrievals, and reanalyses

Existing studies have shown that observed surface incident solar radiation (R-s) over China may have important inhomogeneity issues. This study provides metadata and reference data to homogenize observed R-s, from which the decadal variability of R-s over China can be accurately derived. From 1958 to 1990, diffuse solar radiation (R-sdif) and direct solar radiation (R-sdir) were measured separately, and R-s was calculated as their sum. The pyranometers used to measure R-sdif had a strong sensitivity drift problem, which introduced a spurious decreasing trend into the observed R-sdif and R-s data, whereas the observed R-sdir did not suffer from this sensitivity drift problem. From 1990 to 1993, instruments and measurement methods were replaced and measuring stations were restructured in China, which introduced an abrupt increase in the observed R-s. Intercomparisons between observation-based and model-based R-s performed in this research show that sunshine duration (SunDu)-derived R-s is of high quality and can be used as reference data to homogenize observed R-s data. The homogenized and adjusted data of observed R-s combines the advantages of observed R-s in quantifying hourly to monthly variability and SunDu-derived R-s in depicting decadal variability and trend. R-s averaged over 105 stations in China decreased at -2.9 W m(-2) per decade from 1961 to 1990 and remained stable afterward. This decadal variability is confirmed by the observed R-sdir and diurnal temperature ranges, and can be reproduced by high-quality Earth System Models. However, neither satellite retrievals nor reanalyses can accurately reproduce such decadal variability over China.