A note on the non-geostrophic baroclinic stability problem

The baroclinic stability properties of non-geostrophic atmospheric flow in a channel are investigated. The model is formulated so as to include the effects of latitudinal variations of the perturbations and the basic state parameters, which constitute an essential aspect of the non-geostrophic baroclinic stability problem. Spectral and finite-difference techniques are applied to obtain numerical solutions and the results are found in agreement. In order to allow for high horizontal resolution the present calculations are restricted to a particular two-layer model which appears to incorporate the major non-geostrophic aspects of the baroclinic instability problem.

A purely non-geostrophic type of baroclinic instability is found which bears some resemblance to barotropic instability in spite of the absence of latitudinal shear in the zonal flow. This instability is introduced by the twisting term in the vorticity equation and the non-geostrophic advection of temperature in the thermodynamic equation. For low Richardson numbers and short cyclonic waves this instability attains the same order of magnitude as the quasi-geostrophic baroclinic instability. The major instability found in the present model however is not different from the major quasi-geostrophic instability.