Estimating clear-sky land surface longwave upwelling radiation from MODIS data using a hybrid method

Surface upwelling longwave radiation (LWUP) is one of the four components for calculating the earth's surface radiation budget. Under the general framework of the hybrid method, we developed linear models for estimating the global 1-km instantaneous clear-sky LWUP from the top of the atmosphere (TOA) radiance of the Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared channels 29, 31 and 32. Extensive radiative transfer simulations were conducted to produce a large number of representative samples, from which the linear model was derived. The derived hybrid model was first evaluated using ground measurements collected at 15 sites from two networks (SURFRAD and ASRCOP). According to the validation results, the average bias and root mean square error (RMSE) of -0.55 W m-2 and 15.76 W m-2were obtained by averaging the mean bias and RMSE for the two networks. Compared to a hybrid method developed by a previous study and the temperature-emissivity method, our linear model had a superior performance.