Decadal variability of surface solar radiation over China

Abstract:

Observations show that national average surface solar radiation (R_s) decreased by -8.0 W m^-2 per decade from 1960 to 1990 and sharply increased from 1990 to 1993. However, none of the state-of-the-art climate models can reproduce such decrease/increase of R_s. This study shows that R_s observations over China have significant inhomogeneity. Before 1989, R_s was calculated as a sum of direct (R_sdir) and diffuse (R_sdif) solar radiation observations measured by pyrheliometers and shaded pyranometers separately. Due to technical limitations and irregular calibration, pyranometers before 1990 had a strong sensitivity drift problem, which introduced crucial spurious decreasing trends into R_sdif and R_s data. From 1990 to 1993, instruments and measurement methods were replaced and measuring stations were restructured in China, which resulted in an abrupt increase in the observed R_s.

R_s calculated from Sunshine duration (SunDu) provide a reliable reference in assessing decadal variability of R_s. SunDu derived R_s have no sensitivity drift problem because of its daily changed recording material. SunDu-derived R_s averaged over China decreased by -2.9 W m^-2 per decade from 1961 to 1990, and had a negligible trend afterward. During the period of 1994-2012 when R_s observations were free of inhomogeneity mentioned above, the observed and SunDu-derived R_s consistently show a negligible trend, being less than 0.1 W m^-2 per decade. These trends can be reproduced by high-quality CMIP5 Earth System Models (ESM). This level of agreement is due to the incorporation of a near real emission inventory of atmospheric aerosols by CMIP5 ESMs.

R_s from ERA-Interim has a good agreement with SunDu-derived R_s. However, ERA-interim does not allow aerosol loading to change annually. ERA-Interim R_s shows an unreliable increasing trend of 1.9 W m^-2 per decade from 1990 to 2013 because it does not include the impact of recent increased atmospheric aerosols over China. GEWEX R_s calculated from ISCCP cloud products show a significant but erratic decreasing trend of -3.1 W m^-2 per decade from 1983 to 2007 over China. The ISCCP cloud products aggregate cloud observations from polar orbit and geostationary satellites directly, which introduced to inhomogeneity to ISCCP cloud and GEWEX R_s products.