carbon cycle models, but as these data are consumed, meeting the challenge of model validation must not be postponed; this must be confronted.

Coupling and Initialization

Coupled ocean-atmosphere GCMs are fundamental to the study of the climate system. Models, by definition, are reduced descriptions of reality and hence incomplete and with error. Missing pieces and small errors can pose difficulties, as indicated above, when models of major subsystems such as the ocean and the atmosphere are coupled. Inconsistencies with processes or data between the submodels and/or incompleteness can lead to numerical drift when the models are coupled. The longer-term transient integrations needed in decadal to centennial global change challenges highlight these difficulties. The overriding challenge to modeling is prediction. This challenge is particularly acute when predictive capability is sought on timescales from seasonal to decadal to centennial and where one is confronted with a coupled stiff system like the ocean-atmosphere. It is a challenge that must be met in the coming decade.

The Atmospheric Physical-Chemical Subsystem

Models that incorporate atmospheric chemical processes provide the basis for much of our current understanding in such critical problem areas as acid rain, photochemical smog production in the troposphere, and depletion of the ozone layer in the stratosphere. These formidable problems require that models include chemical, dynamical, and radiative processes, which through their mutual interactions determine the circulation, thermal structure, and distribution of constituents in the atmosphere. That is, the problems require a coupling of the physics and chemistry of the atmosphere. Furthermore, the models must be applicable on a variety of spatial (regional to global) and temporal (days to decades) scales.

Further progress in modeling the interplay between the physics and chemistry of the atmosphere requires better knowledge in five key areas:

  • Surface sources and sinks of trace gases, in particular exchanges of terrestrial ecosystems with the atmosphere and exchanges between the surface ocean and the atmosphere.

  • Chemical models with a detailed set of reactions, in which transport is ignored, need to be developed.

  • Transport models coupled to GCMs, with detailed representation of physical processes, including cloud formation and boundary layer transport, are required to simulate how advection, turbulence, and convection affect the chemical composition of the atmosphere.

  • Hydrological processes and energy exchange, especially processes involving clouds, surface exchanges, and their interactions with radiation,

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