资源环境科技发展态势分析平台

The annual to monthly average of short-term (hourly/minutely) measured global radiation (GR), hereafter called mean global radiation (MGR), has shown large secular trends (decadal or longer term) globally that are only attributable to clouds or aerosols. In order to investigate whether changes in cloud cover (CC) contribute to the MGR trend for a given location and period of interest, many studies compare secular trends of MGR and mean cloud cover (MCC, the annual to monthly average of CC). If these trends have opposite signs, such studies assume that the changes in CC cause the changes in MGR, and vice versa. However, this approach is misleading because MCC is not an appropriate measure of the averaged immediate effect of CC on GR for the year or month due to the solar-elevation dependency and the inherent nonlinearity in the instantaneous effect. As a practical solution, we derived a more reasonable measure named MRRCC (mean radiation reduction by CC) to replace MCC. This is the annual to monthly average of the immediate reduction in GR arising from CC and is determined using a nonlinear regression model that relates immediate GR to the corresponding CC, solar elevation, and year. We carefully designed the model so that the calculation of MRRCC does not require any other variables (such as aerosol measurements). According to our tests at 42 observation stations with increasing MGR trends from 1990 to 2009, 24 stations had positive trends in MCC, indicating in the conventional approach that CC is not a cause of the MGR increase. However, 14 out of these 24 stations had negative MRRCC trends, indicating that CC is one cause of the MGR increase. This demonstrates that the use of MRRCC instead of MCC can alter our understanding of the causes of MGR trends globally.