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Estimation of Downwelling Shortwave and Longwave Radiation in the Tibetan Plateau Under All‐Sky Conditions

Downwelling shortwave radiation (DSWR) and downwelling longwave radiation (DLWR) are two important components of the Earth's surface radiation balance. In this study, the Heliosat method and the parameterization of Crawford and Duchon (1999, https://doi.org/10.1175/1520‐0450(1999)038<0474:AIPFEE>2.0.CO;2, hereafter CD99) were calibrated to make them suitable for estimation of DSWR and DLWR over the Tibetan Plateau (TP). Based on meteorological data, forcing data, and observations from polar‐orbiting satellites, the cloud albedo was calculated, and the clear‐sky index estimation scheme of the Heliosat method was improved. These improvements were then applied to derive 10‐day DSWR under all‐sky conditions over the TP by combining the clear‐sky shortwave radiation scheme with a clear‐sky index. The coefficient of the CD99 parameterization scheme clear‐sky DLWR was also calibrated, and 10‐day all‐sky DLWR was then determined and validated using ground‐based measurements. The spatiotemporal distributions of DSWR and DLWR were analyzed in detail. The results showed that the modified methods are efficient and applicable for downward radiation retrieval under all‐sky conditions over the TP with a reasonable accuracy. The mean percentage errors for DSWR and DLWR were −4.75% and 0.11%, respectively. The variation in the monthly DSWR (DLWR) showed a convex shape, with a maximum appearing in May (July). The spatial distributions of DLWR showed a southeast‐high and northwest‐low pattern. As the subsolar point moves northward, DSWR increases gradually and is clearly influenced by the Asian summer monsoon.