Evaluation of the estimation of shortwave solar radiation in Japan using the mountain microclimate simulation model

Solar radiation is an essential parameter for ecological, agricultural, and other models. However, the availability of large-scale radiation data is usually limited. The Mountain Microclimate Simulation Model (MTCLIM) can resolve such data insufficiency by estimating missing daily shortwave solar radiation data using simple climatological and topographic parameters. Several studies have found the performance of the MTCLIM to be sufficient for the estimation of solar radiation. However, its performance in the East Asian monsoon zone, which is generally characterized by warm wet summers and cold dry winters, has not been thoroughly evaluated. We assessed the performance of the MTCLIM in Japan located at the East Asian monsoon zone. We estimated daily shortwave solar radiation using daily maximum and minimum temperature and precipitation data recorded over 57 years and topographic parameters including slope angle and direction at 41 meteorological stations. Then, we compared the estimated solar radiation with the solar radiation observed at each meteorological station using a simple linear regression equation. The slope of the regression equation without intercept was 0.96 and the coefficient of determination (R**2) was 0.61 for all data. The estimation performance was increased for the monthly mean of daily solar radiation (slope=1.03; R**2=0.93). The degree of estimation error showed geographical and seasonal trends; stations located at higher latitudes had larger slopes than those at lower latitudes. The bias was largest (positive) in spring and smallest (negative) in summer. This study confirmed that the MTCLIM performed sufficiently well to estimate solar radiation in Japan. In particular, the model's high performance for the monthly mean of daily solar radiation suggests that it provides estimates of solar radiation that are sufficiently accurate for ecological, agricultural, and other models in which values of average solar radiation for a given period of time are more meaningful than instantaneous daily solar radiation.