Systematic analysis of climatic model sensitivity to parameters and processes

Abstract

A demonstration is presented of the adjoint method of sensitivity analysis on a radiative-convective climate model with a diurnal cycle. A single adjoint calculation, which requires about the same computation time as the original model, suffices to calculate sensitivities of average surface air temperature to all 312 model parameters. The sensitivities accurately predict the effect on average surface air temperature of small variations in the model parameters. Relative sensitivities rank the importance of all the parameters, the most important parameters being those that characterize solar radiation and determine transmission functions. The uncertainties in the model results are expressed formally in terms of all the sensitivities and parameter covariances. For this model, a 10% standard deviation in surface albedo causes the increase in average surface air temperature after doubling CO/sub 2/ to have a standard deviation of 0.24/sup 0/K about the expected value of 1.66/sup 0/K. For results that cannot readily be compared with observation (for example, the results of a CO/sub 2/ doubling experiment), this method of uncertainty analysis is the only systematic way to estimate the reliability of model results. This estimate of the reliability, though, will only be realistic if all important physical processes and feedback mechanisms havemore » been included, however, crudely, in the model. The radiative-convective model contains complex on-off nonlinear processes of the type found in general circulation models. Therefore, the fact that the adjoint method works successfully and efficiently for the radiative-convective model provides valuable information about subsequent application of the method to general circulation models.« less