SUMMARYNumerical experiments with a simple spectral model of tropospheric long waves are described. The model is based on the 2-parameter, ,%plane, quasi-geostrophic equations and a coarse truncation is applied. Transient baroclinic eddy motion is represented parametrically using transfer coefficient expressions which are extensions of those proposed by Green in 1970 for zonal average formulations. The long waves are forced directly by externally specified diabatic and orographic functions. Zonal average diabatic forcing is also applied. When the eddy flux parametrizations act on the zonal fields only, realistic steady states are obtained if a coarse meridional truncation is applied, but explicit baroclinic instability occurs if higher meridional modes are included. Explicit baroclinic instability is suppressed if the parametrizations act also on the long wave fields; and in this case it is found that diabatic forcing of the largest scales of motion is the most important generator of stationary long waves in the model. Various reformulations are considered, and the problem of devizing better transient eddy parametrizations is discussed.
INTRODUCTIONOne of the main objectives of climate research is the construction of reliable numerical models of the global atmospheric circulation. Most of the activity in this field is at present being channelled into the development of general circulation models which attempt to resolve the transient large-scale eddies in sufficient detail (both temporally and spatially) to reproduce their transfer properties adequately.Parametrized models, in which the transfers carried out by the transient large-scale eddies are represented in terms of the mean fields, are less developed, and nearly all deal explicitly with the zonal average fields only, (For reviews, see Schneider and Dickinson (1974), and GARP (1975).) These zonal average models are extremely useful in the development of rational theories of the general circulation, and for testing parametrization schemes (see, for example, White and Green (1982)) but it is difficult to represent in such formulations the effects of the quasi-stationary zonal asymmetries which arise in the real atmosphere because of the distribution of thermal properties and the elevation of the continents. In any case, to rival the general circulation models, parametrized models should surely attempt to reproduce the three-dimensional time-averaged circulation, and not just the zonal average fields.In this paper we explore the possibility of using a three-dimensional time-dependent model to represent the largest scales of atmospheric motion, whilst applying parametric expressions to represent the effects of all smaller scales. The explicit part of the model is composed of the zonal average fields and the first three zonal wavenumbers. We imagine the zonally asymmetric part of the model to represent the slowly-evolving long wave pattern which dominates the circulation averaged over periods of a month or more (Eliasen and Machenhauer 1965;Blackmon 1976) and w...
