BAROCLINIC ADJUSTMENT IN A 2-LEVEL MODEL WITH BAROTROPIC SHEAR

Baroclinic instability in two-level models is characterized by a criti cal vertical shear, for values above which the flow is unstable. Exist ing studies of nonlinear baroclinic equilibration in two-level models suggest that, while equilibration does occur, it does so for values of vertical shear that are supercritical. The criterion used for the cri tical shear, however, ignores nonlinear changes in barotropic (meridio nal) shear. The present note estimates, using the two-scale formalism, the effect of both jet scale and damping on the critical value of ver tical shear. The results suggest the barotropic shear in the equilibra ted states may be sufficient, in the presence of damping, to render th e equilibrated states neutral. More generally, it appears important to take account of the nature of the evolved flow when assessing the sta bility properties of the equilibrated state.