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

Abstract

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 k 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. Copyright 1999 by the American Geophysical Union.