The GCM-reality intercomparison project for SPARC (GRIPS): Scientific issues and initial results

To investigate the effects of the middle atmosphere on climate, the World C limate Research Programme is supporting the project "Stratospheric Processe s and their Role in Climate" (SPARC). A central theme of SPARC, to examine model simulations of the coupled troposphere-middle atmosphere system, is b eing performed through the initiative called GRIPS (GCM-Reality Intercompar ison Project for SPARC). In this paper, an overview of the objectives of GR IPS is given. Initial activities include an assessment of the performance o f middle atmosphere climate models, and preliminary results from this evalu ation are presented here. It is shown that although all 13 models evaluated represent most major features of the mean atmospheric state, there are def iciencies in the magnitude and location of the features, which cannot easil y be traced to the formulation (resolution or the parameterizations include d) of the models. Most models show a cold bias in all locations, apart from the tropical tropopause region where they can be either too warm or too co ld. The strengths and locations of the major jets are often misrepresented in the models. Looking at three-dimensional fields reveals, for some models , more severe deficiencies in the magnitude and positioning of the dominant structures (such as the Aleutian high in the stratosphere), although under sampling might explain some of these differences from observations. All the models have shortcomings in their simulations of the present-day climate, which might limit the accuracy of predictions of the climate response to oz one change and other anomalous forcing.