A Two-Level Primitive Equation Atmospheric Model Designed for Climatic Sensitivity Experiments

Abstract

A useful but as yet under-utilized tool for climatic studies is an atmospheric model in which the time evolution of large-scale eddies is resolved explicitly, but in a relatively simple dynamical framework. One such model is described in detail in this study–a two-level primitive equation model on a sphere with variable static stability, finite-differenced in the meridional direction but Fourier analyzed and then very severely truncated in the zonal direction. Two versions of the model-moist and dry–are developed, the maintenance of the model's static stability being markedly different in the two versions. Statistically steady states are obtained for a variety of spectral truncations For both versions of the model in order to determine the fewest zonal wavenumbers one can retain and still obtain a reasonable zonally averaged circulation. Including only one wave, of wavelength typical of strongly unstable waves in mid-latitudes, results in a circulation with a subpolar jet as well as a subtropical jet in the zonal wind. The addition of a longer wave (i.e., the addition of wavenumber 3 to wavenumber 6) results in the destruction of the subpolar jet.No further dramatic changes in the zonally averaged flow occur as more waves are added to the system. Features of the model's dynamics which might limit its utility are emphasized, notably the dependence of the strength of the Hadley cell on the details of the convective adjustment scheme. We find, however, that the total energy transported by the Hadley cell is insensitive to such details. Climatic sensitivity experiments with thee models will be described in forthcoming papers.