Estimation of Surface Longwave Radiation over the Tibetan Plateau Region Using MODIS Data for Cloud-Free Skies

This study addressed the estimation of downward surface longwave radiation (R_L,D) for the Tibetan Plateau region using the MODerate resolution Imaging Spectroradiometer (MODIS) measurements observed at the top of the atmosphere (TOA) for clear-sky conditions. To extend, the method developed by Tang and Li with only regard to MODIS/Terra data and with surface elevation only lower than 2.5 km, two separate look-up table (LUT) coefficients corresponding to different elevations and viewing zenith angles (VZAs) were developed for MODIS/Terra and MODIS/Aqua, respectively. The upward surface longwave radiation (R_L,U) was estimated using the Stefan-Boltzmann law with MODIS surface temperature/emissivity products (MOD11_L2/MYD11_L2). Combining the R_L,D and R_L,U, the net surface longwave radiation R_L,N can then be obtained. The R_L,D and R_L,N were validated using some in situ measurements from two sites of Asian Automatic Weather Station Network Project (AAN) (Dunhuang and Amdo) and three sites of Environmental and Ecological Science Data Center for West China (Huazhaizi, Yingke, and A´rou) in the Heihe River Basin. The results showed that the root mean square errors (RMSEs) between the estimated R_L,N and in situ measurements were about 30 W/m². In addition, the schemes developed, respectively, by Bisht et al. and Brutsaert were used to estimate the R_L,D and R_L,N for comparison with the proposed method. The results showed that the RMSE of the proposed method was the lowest one among the three methods, which indicated that the proposed method can estimate the R_L,N accurately for the Tibetan Plateau region.