STABILITY OF STEADY FRONTS WITH UNIFORM POTENTIAL VORTICITY

The linear stability of steady frontal zones is considered using the p rimitive equations. The frontal zones chosen have uniform potential vo rticity and form a sequence of ''snapshots'' of the deformation-induce d front as it evolves toward frontal collapse. The stability for a giv en along front wavelength is determined by integrating a numerical mod el forward from an initial condition of the basic-state front plus whi te noise. Two classes of instabilities emerge. In one class, the modes are modified versions of familiar, synoptic-scale baroclinic waves. T he other class consists of Kelvin-Helmholtz (KH) instabilities. These modes have wavelengths of order 1 km and exist only when the cross-fro ntal scale is of order 10 km or smaller. With e-folding times of a few minutes, the growth of IM waves may limit the cross-frontal scale of active, time-dependent frontal flows, although boundary layer processe s are probably important, at least for surface fronts, before KH modes appear. No other instabilities are found, contrary to the calculation s of Moore and Peltier.