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 scali ng, first developed for water vapor, has now been applied to the minor trac e gas (N2O, CH4, CFCs, and two minor bands of CO2) absorption in the infrar ed window region (800-1380 cm(-1)). The derivation of the k distributions i s based upon an exponential sum fitting to the monochromatically calculated transmission functions at a predetermined reference pressure and temperatu re. For nonhomogeneous path lengths, one-parameter scaling is utilized in c onjunction with the k distribution method. To determine the accuracies of t he k distribution method as compared to the monochromatic calculations, flu xes and cooling rates are calculated for a wide variety of atmospheric cond itions. For the entire 800-1380 cm(-1) spectral range the effect of the min or trace gases on the fluxes calculated using the k distribution method is within 2.3% of the monochromatic method. In addition to being accurate, thi s method is computationally very fast. When implemented into the Goddard EO S general circulation model, the computing time for the longwave flux calcu lations is increased by only 20% despite the inclusion of the minor trace g as absorption bands.