Intercomparison of Radiation Codes in Climate Models (ICRCCM): Longwave Clear-Sky Results—A Workshop Summary

Abstract

An international program of intercomparison of radiation models has been initiated because of the central role of radiative processes in many proposed climate change mechanisms. Models ranging from the most detailed (line-by-line) to the most-highly parameterized have been compared with each other and with selected aircraft observations. Although line-by-line-model fluxes tend to agree with each other to within one percent (if the water-vapor–continuum absorption is ignored), the less-detailed models show a spread of 10–20 percent. The spread is even larger (30–40 percent) for the sensitivities of the models to changes in important radiation variables, such as carbon dioxide amounts and water-vapor amounts. These spreads are disturbingly large. Lacking highly accurate flux observations from within the atmosphere, it has been customary to regard line-by-line–model results as “the truth.” However, uncertainties in the physics of line wings and in the proper treatment of the water-vapor continuum make it impossible for the line-by-line models to provide an absolute reference for evaluating less-detailed models. Therefore, a dedicated surface-based field measurement program is recommended in order to properly evaluate model performance; the goal would be to use sophisticated spectrometers to measure accurately spectral radiances rather than integrated fluxes.