An Instability of Mature Cold Fronts

An instability of a cold front possessing a low-level warm-band precursor is examined within an f-plane, quasi- and semigeostrophic framework. The basic frontal state is taken to be two-dimensional and of uniform potential vorticity. Theoretical considerations pinpoint the source and nature of the instability to be a vortex interaction effect acting across the thermal maximum of the warm band. They also reveal that the most unstable mode associated with this flow configuration has a mixed barotropic-baroclinic character and possesses some features (e.g., a wavelength ∼1000 km and a doubling time ∼1½ days) that are akin to surface frontal waves. In addition nonlinear numerical model simulations demonstrate that the frontal wave growth equilibrates rapidly but not before the establishment of a rich mesoscale flow structure. A brief assessment is presented of the relationship of this diagnosed instability to realized atmospheric flow patterns and to other mechanisms that have been postulated for the growth of frontal waves.