The Thermal Balance of the NCAR Community Climate Model

Byron A. Boville National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

The thermal balance of the NCAR Community Climate Model is examined using the zonally averaged temperature tendency equation of the model. The perpetual January and perpetual July control simulations are used to determine the relative importance of individual dynamical and diabatic terms as a function of latitude and pressure. Although the terms calculated compare reasonably well with observational results, the principle intent of this paper is to understand the maintenance of the model’s temperature structure. This understanding is required in order to make sense of the changes introduced by various physical parameterizations which are being tested in the model.

Long-term means are used so that the net dynamical and diabatic heating cancel very closely. It is found that the net dynamical and diabatic heating terms in the troposphere are generally much smaller than their individual components. There is a great deal of cancellation between the eddies and the mean meridional circulation as is expected from the theory of wave–mean flow interactions. There is also a great deal of cancellation between convective heating and radiative cooling. The latter cancellation will be sensitive both to the cumulus parameterization used and to the way in which cloud fractions are determined for radiative purposes.

The surface layer is found to be a region of extremely large and nearly compensating diabatic terms. The vertical diffusion, a very important term, has been linearized for computational reasons in the current model. Using a nonlinear diffusion operator is likely to make major changes in the surface layer balances, as will any refinements in boundary layer parameterizations.

Abstract

The thermal balance of the NCAR Community Climate Model is examined using the zonally averaged temperature tendency equation of the model. The perpetual January and perpetual July control simulations are used to determine the relative importance of individual dynamical and diabatic terms as a function of latitude and pressure. Although the terms calculated compare reasonably well with observational results, the principle intent of this paper is to understand the maintenance of the model’s temperature structure. This understanding is required in order to make sense of the changes introduced by various physical parameterizations which are being tested in the model.

Long-term means are used so that the net dynamical and diabatic heating cancel very closely. It is found that the net dynamical and diabatic heating terms in the troposphere are generally much smaller than their individual components. There is a great deal of cancellation between the eddies and the mean meridional circulation as is expected from the theory of wave–mean flow interactions. There is also a great deal of cancellation between convective heating and radiative cooling. The latter cancellation will be sensitive both to the cumulus parameterization used and to the way in which cloud fractions are determined for radiative purposes.

The surface layer is found to be a region of extremely large and nearly compensating diabatic terms. The vertical diffusion, a very important term, has been linearized for computational reasons in the current model. Using a nonlinear diffusion operator is likely to make major changes in the surface layer balances, as will any refinements in boundary layer parameterizations.

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