Gates, W. Lawrence
March 1992, 17 pp. 

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The validation of atmospheric models is a key part of the modelling enterprise, but one to which increased attention needs to be given if systematic progress is to be made in the development of predictive climate models. The validation of current AGCMs in terms of the mean seasonal distribution of primary variables such as pressure, temperature, and wind shows a reasonable ability to simulate the observed large-scale features, while at the same time identifying a number of systematic errors. More recent validations have included the simulation of variability, which reveals a modest level of skill but with further systematic errors. Recent results from mesoscale models nested within AGCMs, however, have shown substantial skill in the simulation of regional climate. In addition to conventional data sources of various resolutions, current model validation is enriched by the use of satellite observations and other special data sets, as well as by the analyses from operational models.

A comprehensive atmospheric model validation program includes examination of not only the mean and variance, but of the complete frequency distribution. Moreover, in addition to the primary dynamical and physical variables, the various derived quantities associated with fluxes and processes and the occurrence of specific events should also be evaluated. A complete validation would also include evaluation of a model's ability to simulate more than just the present climate and/or its ability to simulate observed climate change (the latter aspect necessarily including the oceans). This effort will require the acquisition, calibration and processing of global observational data sets specifically for the purpose of model validation. Useful approaches toward such a program include the reanalysis of recent decades with modern AGCMs and advanced data assimilation systems, and the extension of the various atmospheric model intercomparison projects now underway to include a wider variety of diagnostics. (pdf file)

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