A Simple GCM Based on Dry Dynamics and Constant Forcing

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Abstract
A dry spectral primitive equation model is used to simulate the global atmospheric circulation during northern winter. The resolution is T21 in the horizontal, with five equally spaced sigma layers. The only additional terms in the equations are those describing linear damping, linear scale-selective diffusion, and time-independent forcing. The damping and diffusion act on temperature and momentum. The forcing acts on all prognostic variables. It is calculated objectively from the tendencies produced when the model is initialized with a long time series of observational analyses, and separated into components to ease comparison with time-dependent perturbation models. The simulated climate reproduces observed features of the circulation, both time-mean fields and transient-eddy covariances, with remarkable success. The accurate simulation of tropical divergent flow is a particularly useful result. The main deficiencies are an underestimation of transient-eddy kinetic energy and a lack of transient activity in the Southern Hemisphere. In an attempt to reduce the forcing of divergent flow, a modified vertical scheme and modified forcing functions based on a calculation of balanced flow are introduced. The former still has significant divergence forcing and makes little difference to the final result. The latter tends to give solutions that are unrealistic in the Tropics. The model’s sensitivity to variations in forcing functions and damping parameters is further explored. The Southern Hemisphere transient behavior can be improved by boosting the local forcing of baroclinicity by up to a factor of 2, and a simulation of the Southern Hemisphere winter is relatively successful. The applications and limitations of such a simple fast-running climate model with a relatively realistic simulated climate are discussed.

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