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

A technique for generating the coefficients of the k distribution method ha s been developed which uses very few exponential terms to represent absorpt ion in a spectral region. This new technique, using Ic distributions derive d from HITRAN 1992 spectroscopic data, predicts vertical profiles of irradi ances and heating rates due to the major radiatively active gases with a be tter balance between speed of computation and accuracy than previous method s. Absorption coefficients were specified in a manner similar to that of th e correlated k distribution method, whereas the weights of each term were d etermined using a fitting procedure similar in principle to the exponential -sum fitting of transmissions method. Errors in irradiances and heating rat es for various atmospheric profiles and the reference profile (with which f itting of weights was performed) were of a similar magnitude, thereby indic ating the technique's suitability for use in global model studies. This par ameterization is now used by the Hadley Centre climate model.