A new measure of the effect of cloud cover on annual/monthly mean global radiation for investigating the cause of secular trends in global radiation
A new measure of the effect of cloud cover on annual/monthly mean global radiation for investigating the cause of secular trends in global radiation
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DOI：
10.1002/joc.5544
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发表时间：
2018-08
期刊：
影响因子：
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通讯作者：
A. Kanno;H. Ishida;Yoji Tanaka
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文献类型：
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作者：
A. Kanno;H. Ishida;Yoji Tanaka
短期（每小时/每分钟）测量的全球辐射（GR）的年至月平均值，以下称为平均全球辐射（MGR），在全球范围内显示出大的长期趋势（十年或更长时间），这些趋势仅可归因于云或气溶胶。为了调查云量（CC）的变化是否有助于MGR趋势，许多研究比较MGR和平均云量（MCC，CC的年至月平均值）的长期趋势。如果这些趋势具有相反的符号，则这些研究假设CC的变化引起MGR的变化，反之亦然。然而，这种方法是误导性的，因为MCC不是一个适当的措施，平均直接影响的CC对GR的一年或一个月，由于太阳高度的依赖性和固有的非线性的瞬时效应。作为一个实际的解决方案，我们推导出一种更合理的测量方法，名为MRCC（CC的平均辐射减少量）来取代MCC。这是由CC引起的GR立即减少的年至月平均值，并使用非线性回归模型确定，该模型将即时GR与相应的CC，太阳高度和年份相关联。我们精心设计了模型，因此MRRCC的计算不需要任何其他变量（如气溶胶测量）。根据我们在42个观测站的测试，从1990年到2009年，MGR趋势增加，24个站的MCC有积极的趋势，表明在传统的方法，CC是不是MGR增加的原因。然而，这24个台站中有14个台站的MRRCC趋势为负，表明CC是MGR增加的原因之一。这表明，使用MRRCC而不是MCC可以改变我们对全球MGR趋势原因的理解。
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.