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An Improved Algorithm for Retrieving Surface Downwelling Longwave Radiation from Satellite MeasurementsRetrieving surface longwave radiation from space has been a difficult task since the surface downwelling longwave radiation (SDLW) are integrations from radiation emitted by the entire atmosphere, while those emitted from the upper atmosphere are absorbed before reaching the surface. It is particularly problematic when thick clouds are present since thick clouds will virtually block all the longwave radiation from above, while satellites observe atmosphere emissions mostly from above the clouds. Zhou and Cess developed an algorithm for retrieving SDLW based upon detailed studies using radiative transfer model calculations and surface radiometric measurements. Their algorithm linked clear sky SDLW with surface upwelling longwave flux and column precipitable water vapor. For cloudy sky cases, they used cloud liquid water path as an additional parameter to account for the effects of clouds. Despite the simplicity of their algorithm, it performed 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 were also found for areas that were covered with ice clouds. An improved version of the algorithm was developed that prevents the large errors in the SDLW at low water vapor amounts. The new algorithm also utilizes cloud fraction and cloud liquid and ice water paths measured from the Cloud and the Earth's Radiant Energy System (CERES) satellites 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 the Terra and Aqua satellites. The results show significant improvement over the original version. The revised Zhou-Cess algorithm is also slightly better or comparable to more sophisticated algorithms currently implemented in the CERES processing. It will be incorporated in the CERES project as one of the empirical surface radiation algorithms.
Document ID
20080040138
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Zhou, Yaping (Maryland Univ. Baltimore County Catonsville, MD, United States)
Kratz, David P. (NASA Langley Research Center Hampton, VA, United States)
Wilber, Anne C. (Analytical Services and Materials, Inc. Hampton, VA, United States)
Gupta, Shashi K. (Analytical Services and Materials, Inc. Hampton, VA, United States)
Cess, Robert D. (State Univ. of New York Stony Brook, NY, United States)
Date Acquired
August 24, 2013
Publication Date
January 1, 2006
Subject Category
Earth Resources And Remote Sensing
Distribution Limits
Public
Copyright
Public Use Permitted.

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