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Within CDP the purpose of diagnostic analyses is twofold: diagnosis is used to test theoretical ideas concerning the mechanisms responsible for climate variations and their relative import and also test (i.e. validate) the behavior of comprehensive climate models like the NCAR CCSM against that of the observed climate system. A particularly insightful example of this type of research is a recent CDP study exemplifying these two types of diagnoses which is detailed below.

Theoretical ideas from physics, in particular statistical physics, are occasionally beneficial in the study of climate. In this vein, Branstator has extended work he has done in collaboration with Andrey Gritsun of the Russian Academy of Science concerning application of the Fluctuation-Dissipation Theorem (FDT) to climate problems. The FDT makes it possible to construct response operators that provide estimates of how a dynamical system will react to an external forcing. In general these operators are more accurate than a simple linearization of the governing equations. Branstator and Gritsun's past efforts have been devoted to producing operators for estimating the response of the mean circulation in an atmospheric general circulation model. During the past year this has been extended to the case of the response of second moments of state variables. For example they have succeeded in constructing operators that give very accurate estimates of how storm track variances and fluxes will change in reaction to any given heat or momentum forcing. These operators can be used for optimal forcing problems in which one finds the most efficient way to excite a response with prespecified attributes. For example they have considered optimal ways to excite the Atlantic storm tracks by tropical heating. Extensive tests of this methodology have been carried out with CCM0, NCAR's original community climate model; now the methodology is being carried over to NCAR's state-of-the-art AGCM, CAM3.