Investigating k distribution methods for parameterizing gaseous absorption in the Hadley Centre Climate Model

A technique for generating the coefficients of the k distribution method has been developed which uses very few exponential terms to represent absorption in a spectral region. This new technique, using Ic distributions derived from HITRAN 1992 spectroscopic data, predicts vertical profiles of irradiances and heating rates due to the major radiatively active gases with a better balance between speed of computation and accuracy than previous methods. Absorption coefficients were specified in a manner similar to that of the correlated k distribution method, whereas the weights of each term were determined using a fitting procedure similar in principle to the exponential-sum fitting of transmissions method. Errors in irradiances and heating rates for various atmospheric profiles and the reference profile (with which fitting of weights was performed) were of a similar magnitude, thereby indicating the technique's suitability for use in global model studies. This parameterization is now used by the Hadley Centre climate model.