Diurnal variation of direct and diffuse radiation and its impact on surface albedo

A method for calculating the diurnal distribution of direct and diffuse solar radiation was developed. Based on this method, we analysed the changing patterns of the two radiation components under different weather conditions. The results were then used to investigate the effect of underlying snow of a forest on the surface albedo. It shows that the albedo contribution from the underlying snow is stable on overcast days when the radiation is dominated by the diffuse component. However, on clear days when the radiation is dominated by the direct component, the diurnal change of albedo contributed from the snow is significant and it exhibits a "w" shape. It was also found that under any weather conditions, the vegetation masking effect is very significant. As a result, the albedo contribution from the underlying snow in the high latitude is much less than that obtained by the fractional average method commonly used in land surface schemes of climate models.