Homogenized century-long surface incident solar radiation over Japan

Abstract

Surface incident solar radiation (Rs) plays a key role in climate change on Earth. Rs can be directly measured, and it shows substantial variability on decadal scales, i.e. global dimming and brightening. Rs can also be derived from the observed sunshine duration (SunDu) with reliable accuracy. The SunDu-derived Rs has been used as a reference to detect and adjust inhomogeneity in the observed Rs. However, both the observed Rs and SunDu-derived Rs may have inhomogeneity. In Japan, SunDu has been measured since 1890, and Rs has been measured since 1961 at ∼100 stations. In this study, the observed Rs and SunDu-derived Rs were first checked for inhomogeneity independently using the statistical software RHtests. If confirmed by the metadata of these observations, the detected inhomogeneity was adjusted based on the RHtests quantile-matching method. Second, the two homogenized time series were compared to detect further possible inhomogeneity. If confirmed by the independent ground-based manual observations of cloud cover fraction, the detected inhomogeneity was adjusted based on the reference dataset. As a result, a sharp decrease of more than 20 W m−2 in the observed Rs from 1961 to 1975 caused by instrument displacement was detected and adjusted. Similarly, a decline of about 20 W m−2 in SunDu-derived Rs due to steady instrument replacement from 1985 to 1990 was detected and adjusted too. After homogenization, the two estimates of Rs agree well. The homogenized SunDu-derived Rs show an increased at a rate of 0.9 W m−2 per decade (p<0.01) from 1961 to 2014, which was caused by a positive aerosol-related radiative effect (2.2 W m−2 per decade) and a negative cloud cover radiative effect (−1.4 W m−2 per decade). The brightening over Japan was the strongest in spring, likely due to a significant decline in aerosol transported from Asian dust storms. The observed raw Rs data and their homogenized time series used in this study are available at https://doi.org/10.11888/Meteoro.tpdc.271524 (Ma et al., 2021).