Minor trace gas radiative forcing calculations using the k distribution method with one-parameter scaling

The k distribution method with one-parameter pressure and temperature scaling, first developed for water vapor, has now been applied to the minor trace gas (N₂O, CH₄, CFCs, and two minor bands of CO₂) absorption in the infrared window region (800-1380 cm^-1). The derivation of the k-distributions is based upon an exponential sum fitting to the monochromatically calculated transmission functions at a predetermined reference pressure and temperature. For nonhomogeneous path lengths, one-parameter scaling is utilized in conjunction with the k distribution method. To determine the accuracies of the k distribution method as compared to the monochromatic calculations, fluxes and cooling rates are calculated for a wide variety of atmospheric conditions. For the entire 800-1380 cm^-1 spectral range the effect of the minor trace gases on the fluxes calculated using the k distribution method is within 2.3% of the monochromatic method. In addition to being accurate, this method is computationally very fast. When implemented into the Goddard EOS general circulation model, the computing time for the longwave flux calculations is increased by only 20% despite the inclusion of the minor trace gas absorption bands.