Abstract
Observational studies show that polar air stream cyclones form preferentially to the north of the polar front in regions of high vorticity and low static stability, although the baroclinicity may be stronger elsewhere. This phenomenon is investigated by considering a semigeostrophic linear stability analysis of a constant potential vorticity zonal jet in the presence of parameterized cumulus heating in the cold air mass. Three types of unstable modes are found for different amounts of heating. With relatively small heating rates, the usual baroclinic instability occurs, with the disturbance centered on the jet axis. With moderate heating, the fastest growing mode is again a mainly baroclinic system, but with significant amplitude only in the region of heating on the cold-air side of the jet. Finally, for sufficiently large heating rates, a small-scale disturbance, which is driven primarily by diabatic processes, forms in the cold air mass. There is a continuous transition between the three types of instability as the heating is varied. The short wavelength and meridionally confined structure of the second type of mode are characteristic of observed comma clouds. Sensitivity tests show that while it is necessary to have the release of latent heat confined to a part of the domain in order to produce a localized instability, confined modes only appear for physically reasonable parameter values in the high-vorticity, low-stability environment of the cyclonic shear region of the jet. It appears that this is due primarily to the effects of the reduced vertical stability in enhancing the feedback between convective heating and low-level convergence.
