On the structure of potential vorticity in baroclinic instability

The structure of baroclinic modes is examined in terms of their potential vorticity. Baroclinic instabilities may be viewed as mutually reinforcinginteractions between the disturbance potential vorticity at different levels in the atmosphere. A technique is introduced for diagnosing quantitatively which levels of potential vorticity are important in the growth of the instability. Both Charney and Green modes are found to depend on the interaction between the surface thermal anomalies and a thin layer of potential vorticity anomalies centered on the steering level. In the Green modes, the growth is retarded by the influence of the eddy potential vorticity at higherlevels. The inclusion of a tropopause in the basic state does not modify the fundamental dynamics of the baroclinic modes in the presence of linear shear.