The parameterization of baroclinic instability in a simple model

Baroclinic instability of zonally forced flow in a two mode quasi-geostroph ic numerical model with periodic boundary conditions is considered. Only th e largest scale of the baroclinic mode is forced and the scale dependence o f a diffusive parameterization of baroclinic layer thickness is determined. It is shown that the effect of baroclinic instability is a decreasing func tion of scale with an exponent of about half of that corresponding to the c ommonly used Laplace operator. We furthermore show that there is no linear relation between the time averaged amplitude of the large scale streamfunct ion (or quasi-geostrophic potential vorticity) and the bolus velocity.