Development of a revised longwave radiation scheme for an atmospheric general circulation model

This paper describes the development of a new version of the longwave part of the radiation scheme used in the Meteorological Office 11‐layer atmospheric general circulation model. The work was initiated when comparisons with other radiation schemes revealed systematic errors in the longwave fluxes. A detailed investigation of these errors was carried out by examining the sensitivity of the scheme to many of the details of its formulation. Various improvements were made, notably to the treatment of the temperature dependence of the Planck function and the water vapour continuum absorption. It is demonstrated that the fluxes are extremely sensitive to the formulation of the continuum absorption and suggested that more observations are needed to refine the values of the absorption coefficients. Special attention was paid to the treatment of carbon dioxide absorption, as the model is being used to continue research on the effect on climate of increasing CO 2 concentrations. A simple scaling of the absorber amounts is applied in order to include the temperature dependence of the CO 2 absorption. It is demonstrated that fluxes from the new version of the scheme and the sensitivity of the fluxes to changing CO 2 concentrations in realistic atmospheres are in good agreement with those from more detailed models.