Century-Long Variations in Surface Incident Solar Radiation Over Japan--Characterized by Observations and Reanalyses

Surface incident solar radiation (Rs) is of great importance in determining the energy balance in the Earth system. In this study, we use century-long homogenized observations over Japan to constrain five 20th century reanalyses to explore their performance in reproducing Rs variation on different time scales (high-frequency components [HFCs], for signals with cycles less than 10 years; low-frequency components [LFCs], for signals with cycles more than 10 years) by ensemble empirical mode decomposition (EEMD) method and to quantify the impact factors on the Rs estimations by the sum of tree (SOT) model. It is found that ERA20C, ERA20CM and CERA20C overestimated Rs by 0.48-1.49 W/m-2, while 20CRv2c and 20CRv3 underestimated Rs by -0.63 and -1.18 W/m-2, respectively for 1931-2010. Poor correlation coefficient (R) was found to be 0.10 for ERA20CM and 0.22 for 20CRv2c. 20CRv2c failed for 1931-1960 but improved considerably for 1961-2010. 20CRv3 uses an upgraded model and assimilates more observations compared with its predecessor 20CRv2c; however, only the original components and HFCs in Rs were improved, with nearly no improvement in the LFCs. In general, CERA20C Rs, with small biases and higher R of 0.73 for original signals, 0.83 for HFCs and LFCs, is superior to other reanalyses. No obvious trend in clear sky Rs demonstrated that reanalysed Rs are insensitive to the aerosol forcings. Therefore, cloud cover and water vapour maybe the main factors influenced reanalysed Rs. Most of time, Rs is more sensitive to cloud cover than vapour pressure for all reanalyses except original signals (with contribution ratio of 0.29 for cloud cover and 0.71 for vapour pressure) and LFCs (with contribution ratio of 0.41 for cloud cover and 0.59 for vapour pressure) in CERA20C, and original signals (with contribution ratio of 0.37 for cloud cover and 0.63 for vapour pressure) in ERA20C. This work pointed out that aerosol related processes such as aerosol forcings or aerosol radiative effect in reanalyses should be improved in the future, which will ultimately improve the interaction between aerosol and cloud in Rs simulations. Century-long homogenized observations of surface incident solar radiation were used to evaluate five 20th century reanalyses over Japan. CERA20C performs best among the five 20th century reanalyses. The biases of surface incident solar radiation in 20th century reanalyses can mostly be explained by cloud cover.image