INTERCOMPARISON OF HEATING RATES GENERATED BY GLOBAL CLIMATE MODEL LONGWAVE RADIATION CODES

Longwave radiative heating, which has a pronounced impact on climate p rediction, is represented in Global Climate Models (GCMs) by an algori thm which converts model input parameters to heating rates. Since each GCM has a unique longwave radiative heating parameterization, an inte rcomparison of seven frequently used algorithms designed to assess the ir variability to input data was performed. The algorithms' heating ra te calculation, which is perhaps the most important aspect of the para meterization in that it is a principal part which the GCM actually inc orporates into its climate prediction, was evaluated by subjecting eac h to identical input parameters and comparing the resulting output. It should be noted that the overall shape of a given heating rate profil e depends strongly on the depth of the model layers over which the ave rage conditions were determined. But since GCMs ultimately see the hea ting rates only at model levels, this aspect of heating rate calculati ons is transparent to the models themselves. For clear sky conditions, the algorithms were tested with a diverse range of input data taken f rom different geographic locations and seasons and with various distri butions of vertical levels. Analysis of the results from these clear s ky experiments indicated that heating rate profiles generated by the a lgorithms were similar, with maximum variations of the order of 0.5 de grees K/d. The differences in algorithm output became substantially mo re pronounced when clouds at one or more levels with varying thickness were introduced into the input conditions, particularly if the clouds were thicker than one model level. Indeed, for some cloud configurati ons the resulting profiles of heating rates appear to have no correspo ndence whatsoever to one another. How important these differences are to ultimate GCM climate predictions is currently under study.