Comparison of model-calculated and ERBE-retrieved clear-sky outgoing longwave radiation

In order to validate radiative transfer models and identify sources of errors in the satellite retrieval of radiation budgets, model calculations of clear-sky outgoing longwave radiation (OLR) over oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). The NASA Goddard Earth Observing System (GEOS-1) and the National Centers for Environmental Prediction and National Center for Atmospheric Research reanalyses of temperature and humidity and the Stratospheric Aerosol and Gas Experiment retrieval of the stratospheric humidity are used in the model calculations. Averaged over time (1985-1989) and space (60 degrees S-60 degrees N), the model-calculated clear-sky OLR has a positive bias of 1.9-2.3 W-2 when compared with that of ERBE. Nearly all of the bias can be accounted for by the inclusion of the absorption due to CO2 in the 4.3-mu m band and to the weak CO2 and O-3 molecular lines distant from band centers. The use of the two different reanalyses has only a small effect on the flux calculations (similar to 0.4 W m(-2)). Consistent with suggestions by previous studies, ERBE is found to overestimate (underestimate) the clear-sky OLR over high humid (dry) regions due to incorrect identification of clear scenes in the ERBE retrievals. The importance of the upper tropospheric humidity in affecting the Earth radiation budget is also investigated. Although only similar to 15% of the atmospheric humidity is contained in the region above the 600-hPa level, the upper troposphere is as important as the lower troposphere in contributing to the clear-sky OLR.