An improved algorithm for retrieving surface downwelling longwave radiation from satellite measurements

Abstract

Zhou and Cess (2001) developed an algorithm for retrieving surface downwelling longwave radiation (SDLW) based upon detailed studies using radiative transfer model calculations and surface radiometric measurements. The algorithm links clear sky SDLW with surface upwelling longwave (LW) flux and column precipitable water vapor. For cloudy sky cases, the cloud liquid water path is used as an additional parameter to account for the effects of clouds. Despite the simplicity of the algorithm, it performs very well for most geographical regions except for those regions where the atmospheric conditions near the surface tend to be extremely cold and dry. Systematic errors are also found for scenes that are covered with ice clouds. An improved version of the algorithm prevents the large errors in the SDLW at low water vapor amounts by taking into account that, under such conditions, the SDLW and water vapor amount are nearly linear in their relationship. The new algorithm also utilizes cloud fraction and cloud liquid and ice water paths available from the Cloud and the Earth's Radiant Energy System (CERES) single-scanner footprint (SSF) product to separately compute the clear and cloudy portions of the fluxes. The new algorithm has been validated against surface measurements at 29 stations around the globe for Terra and Aqua satellites. The results show significant improvement over the original version. Preliminary tests also suggest that the new algorithm works quite well for high elevation locations such as Tibet site where current satellite products exhibit large biases. The revised Zhou-Cess algorithm is also slightly better or comparable to more sophisticated algorithms currently implemented in the CERES processing and will be incorporated as one of the CERES empirical surface radiation algorithms.