Diffuse solar radiation models for different climate zones in China: Model evaluation and general model development

In this paper, nine new diffuse fraction and eight new diffuse coefficient models were developed by introducing precipitation as one of the model parameters. Concurrently, thirty-four existing diffuse fraction and eleven existing diffuse coefficient models were reviewed. The existing and proposed models were compared to assess their applicability in different climate zones in China. According to the best models and climate zones, general models were established. Overall, introducing precipitation could effectively improving the underestimation problems of daily diffuse solar radiation; for diffuse fraction models, precipitation had stronger correlation with diffuse radiation than relative humidity in humid areas and weaker correlation in arid areas. For diffuse coefficient models, the models incorporating precipitation could obtain better accuracy than temperature and relative humidity in all climate zones under similar meteorological parameters. Therefore, precipitation should be preferentially adopted, followed by temperature and relative humidity. The general models developed in this paper performed well in estimating the daily diffuse solar radiation. The average coefficient of determination, root mean square error and mean absolute bias error of diffuse fraction general models were 0.82, 1.44 MJm(-2)d(-1) and 1.04 MJm d(-1), respectively; while the corresponding values of diffuse coefficient general models were 0.69, 1.98 MJm(-2)d(-1) and 1.54 MJm(-2)d(-1), respectively. The general models could represent the daily diffuse solar radiation estimation of stations without diffuse solar radiation records.