Validation of longwave atmospheric radiation models using Atmospheric Radiation Measurement data

Data taken at the Atmospheric Radiation Measurement Program's central facil ity in Oklahoma and processed as part of the Clouds and the Earth's Radiant Energy System-Atmospheric Radiation Measurement-Global Energy and Water Cy cle Experiment (CAGEX) project have been used to validate the fop-of-the-at mosphere and surface longwave radiative fluxes for two widely used radiatio n models: the Column Radiation Model from the National Center for Atmospher ic Research Community Climate Model (CCM), and the Moderate Resolution Tran smittance (MODTRAN3) radiation code. The results show that for clear skies the models slightly overestimate outgoing longwave radiation at the top of the atmosphere (OLR) and underestimate the surface downwelling longwave flu x (SDLW). The accuracy of the radiation models is quite consistent with the ir respective levels of complexity. For MODTRAN3, for example, the OLR over estimate is 7.1 Wm(-2) while the SDLW underestimate is 4.2 Wm(-2). For clou dy skies it is emphasized that the cloud input parameters, as determined fr om measurements by various instruments, require careful examination and pre processing. Spatial and temporal averaging could result in the parameters r epresenting different volumes of the atmosphere. The discrepancy between mo del calculations and observations is shown to be significantly reduced thro ugh the proper choice of input parameters.