Increase in Global Solar Radiation with Total Cloud Amount from 33 Years Observations in Japan

In general, a change in the total cloud amount (TCA) affects the solar irradiance at the surface as a result of on-the-way reflection of solar radiation. The amount of global solar radiation (GSR) increased globally after c.a. 1990, and this increase resulted from a decreases in the TCA and an increases in atmospheric transparency. Here we present the TCA and GSR trends categorized by the TCA for 33 years in Japan (from 1974 through 2006) on the basis of simultaneous observation from 53 weather stations. These trends are calculated using data obtained on a daily basis. The results averaged across Japan show that both GSR and TCA are increasing at the rate of 2.2% and 1.5% per decade, respectively. In most of Japan (regions further north than Kyushu), a high GSR increase rate mostly appears with a large TCA. This fact cannot be accounted for by the increase in the atmospheric transparency alone. There are two possible causes for increases in GSR and TCA in Japan: a change in cloud appearance frequency against the solar zenith angle and a decrease in cloud optical thickness.