A standard test for AGCMs including their physical parametrizations: I: the proposal

Abstract

To assist model intercomparison and development of a set of eight numerical experiments is proposed as a test-bed for the interaction of dynamics and physical parameterizations in atmospheric GCMs. The framework for the experiments is that of an aqua-planet and the prescribed sea-surface temperatures (SSTs) are highly idealized. * c 2001 Royal Meteorological SocietyKeywords: AGCM, sea-surface temperature, aqua-planet. 1.INTRODUCTIONIn recent years the comparison of the output from different atmospheric GCMs run under controlled conditions has been identi®ed as vital in order to probe the reasons for differences in the systematic errors of each of the models. This has culminated in the ongoing Atmospheric Model Intercomparison Project (AMIP) (Gates, 1992) which aims to compare the behaviour of AGCMs with prescribed seasurface temperatures (SSTs). The project has identi®ed systematic errors which are common to many of the models, in addition to errors which are similar across models which parametrize a particular physical process in a certain way e.g. convection. Subsequent AMIP subprojects have tried to identify the reasons for such errors. This is dif®cult due to the complex nature of the many interactions that exist in present day AGCMs. Understanding these interactions is further complicated by the complexity in the spatial and temporal variability of the boundary conditions such as the land/sea distribution, orography and SST forcing. A simpli®ed approach is taken in the study of Held and Suarez (1994) who proposed an intercomparison project comparing the dynamical cores of AGCMs. The AGCM physical parametrizations are replaced with relaxation towards a prescribed temperature distribution and dissipation terms. In this way differences between the models due purely to differences in the formulation of the dynamical core can be isolated. However for any physical parametrization there is currently no test between its performance in a single column experiment and the performance of the whole model including it in an AMIP-style integration. What is required are tests of the interaction of the physical parametrizations in an AGCM with each other and with the dynamics. The context of these tests should be simple enough that classes of behaviour and obvious problems become apparent. The proposal here is for a standard set of experiments in which the full AGCM is

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