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 calcula tions of clear-sky outgoing longwave radiation (OLR) over oceans are c ompared with data from the Earth Radiation Budget Experiment (ERBE). T he NASA Goddard Earth Observing System (GEOS-1) and the National Cente rs for Environmental Prediction and National Center for Atmospheric Re search reanalyses of temperature and humidity and the Stratospheric Ae rosol and Gas Experiment retrieval of the stratospheric humidity are u sed in the model calculations. Averaged over time (1985-1989) and spac e (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 absorpti on due to CO2 in the 4.3-mu m band and to the weak CO2 and O-3 molecul ar lines distant from band centers. The use of the two different reana lyses has only a small effect on the flux calculations (similar to 0.4 W m(-2)). Consistent with suggestions by previous studies, ERBE is fo und to overestimate (underestimate) the clear-sky OLR over high humid (dry) regions due to incorrect identification of clear scenes in the E RBE retrievals. The importance of the upper tropospheric humidity in a ffecting the Earth radiation budget is also investigated. Although onl y similar to 15% of the atmospheric humidity is contained in the regio n above the 600-hPa level, the upper troposphere is as important as th e lower troposphere in contributing to the clear-sky OLR.