Baroclinic Adjustment in a Two-Level Model with Barotropic Shear

Abstract

Baroclinic instability in two-level models is characterized by a critical vertical shear, for values above which the flow is unstable. Existing 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 critical shear, however, ignores nonlinear changes in barotropic (meridional) shear. The present note estimates, using the two-scale formalism, the effect of both jet scale and damping on the critical value of vertical shear. The results suggest the barotropic shear in the equilibrated states may be sufficient, in the presence of damping, to render the equilibrated states neutral. More generally, it appears important to take account of the nature of the evolved flow when assessing the stability properties of the equilibrated state.