The baroclinic instability of a “2½ dimensional model atmosphere” in which the upper level is non-divergent

The baroclinic instability of a “2½ dimensional model atmosphere” is examined by the method of small wave perturbations. The model is very similar to that of ESTOQUE (1966) but somewhat less restrictive. It is shown that the assumption of an upper isobaric level of non-divergence leads to the following unusual results: (1) Instability is impossible if the Rossby beta-parameter is zero. (2) The vertical shears required for instability increase linearly with the beta-parameter. (3) Only positive vertical shears lead to instability. (4) The instability is of the linear type as opposed to the more usual exponential type.