A non‐linear atmospheric long wave model incorporating parametrizations of transient baroclinic eddies

Numerical experiments with a simple spectral model of tropospheric long waves are described. The model is based on the 2-parameter, β-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.