Cyclogenesis in Frontal Zones

Observational evidence has demonstrated that localized baroclinic zones in the atmosphere are unstable to disturbances with wavelengths of approximately 1000 km. Examples of such disturbances include: polar-front cyclones, Baiu cyclones, polar lows and comma clouds. In an attempt to understand the dynamical processes responsible for the generation of these diverse but apparently related phenomena a two-dimensional baroclinic zone is tested for stability against small amplitude three-dimensional perturbations. The baroclinic zone employed in this analysis is the Hoskins–Bretherton semigeostrophic model of a synoptic scale front. The fronts am found to be highly unstable and the fastest growing disturbances compatible with an origin through secondary instability in the longwave are those with horizontal length-scales on the order of 1000 km. The energetics of these disturbances demonstrate that they grow by a new “cyclone-scale” mode of baroclinic instability. The structure of the most unstable cyclone-scale baroclinic disturbance has many features in common with Bjerknes” conceptual model of polar-front cyclones. These include: the existence of warm and cold sectors that are bounded by secondary fronts; strong southerly flow in the warm sector, strong northerly flow in the cold sector, and a tendency to exist as members of a cyclone family. In addition, evidence is presented that the incipient disturbance may eventually become unstable to a moist convective instability that occurs in approximately the same location in which prefrontal squall lines are found.