Construction of the method for estimating solar radiation based on weather records _|toward the climatic reconstruction in the historical period

The purpose of this study is to develop and construct the method for estimating monthly mean solar radiation based on the combination of daily weather records. At the first step, the relationship between the daily accumulated solar radiation and the daily weather conditions recorded by the local observatory of JMA (Japan Meteorological Agency) was examined. The weather conditions were classified into several categories (3,5, and 9 categories) based on the daily weather descriptions of original data sheets. As a result, it became clear that solar radiation was highly correlated with daily weather conditions. At the next step, the method for estimating monthly mean solar radiation from the daily weather conditions was applied to evaluate the monthly mean solar radiation in Tokyo for the period 1979-1998. The results show that the estimation was done in the accuracy of 4 to 8 percent by RMS relative error, although the error was changed according to the number of weather categories. This accuracy would be reasonable considering the result (4.5 percent) of Yoshida and Shinoki (1978) which estimated the monthly mean solar radiation from the equation using sunshine-ratio, snow-cover days and solar height angle. Finally, the monthly mean solar radiation was estimated in several cities in Japan using the same method as in Tokyo with the estimation error of 3 to 6 percent. Therefore, it would be possible to estimate the monthly mean solar radiation in historical times based on the daily weather records in old diaries all over the country. However, there are many problems to be solved in future when we use the historical weather records for estimating solar radiation based on the present day weather-radiation relationships between two variables.