Effects of Approximate Radiation Treatments Used in the Climate Models on the Clear-Sky Thermal-Radiation Flux and Its Perturbation Due to CO sub 2 Increase

摘要

An assessment is presented of the effects of approximate radiation treatments commonly adopted in the climate models on the calculated thermal radiation flux and its perturbation due to CO sub 2 increase. Specifically, an examination was made of the uncertainties in the calculated clear sky thermal flux associated with the approximations (Curtis-Godson and correlated-k methods) for inhomogeneous atmosphere, the use of exponential kernel (diffusivity factor) approximation, the treatment of model layer temperature (constant and linear Planck function), and the representation (narrow and wide band) for the correlation of spectral features between the overlapping gases and the Planck function. The results indicate tat for an inhomogeneous atmosphere the correlated-k method calculates nearly identical flux and flux perturbation as those comptued based on the Curtis-Godson method. The results also suggest that the use of a diffusivity factor (1.66) to represent the zenith angle integration is appropriate for CO sub 2 thermal radiation calculations. However, the narrow band representation which accounts for the correlation of spectral features tends to calculate a smaller atmospheric opacity than the wide band representation which neglects such a correlation. As a result, the latter computes a much larger downward flux at surface as well as its increase due to CO sub 2 increase. It is also found that the calculated thermal flux is very sensitive to the treatment of model layer temperature. (ERA citation 08:016008)