A recently proposed extension to the twofold extratropical cyclogenesis classification scheme of Petterssen and Smebye is discussed. A third class of extratropical cyclone (type C) is described, in which initial development is controlled by a pre-existing upper-level potential vorticity (PV) anomaly. In its early stages, such a system is indistinguishable from the classical type B cyclone of the Petterssen and Smebye scheme. However, subsequent development cannot be understood in terms of a co-operative interaction of the upper-level feature with a low-level baroclinic zone. Rather, the cyclogenetic dynamics of type C systems are dominated by the action of strong midlevel latent heating. Such heating can generate important anomalies of PV that act to suppress the formation of a low-level thermal anomaly and that interact destructively with the pre-existing upper-level feature.
Candidate type C events are identified using recently developed, height-attributable, quasi-geostrophic, vertical-motion diagnostics. The application of one such diagnostic to a climatology of instantaneous cyclonic features suggests that type C events may occur with reasonable frequency. The generic behaviour of system types in the proposed threefold classification scheme is compared with the actual dynamics of some cyclones from the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The analysis is based on piecewise PV inversions and numerical simulations. This approach is able to provide a good description of a case of standard type B development, consistent with the qualitative description of Petterssen and Smebye and with the quasi-geostrophic diagnostics. Within the same framework, two systems are discussed whose behaviours do not fit the simple A/B classification, but are consistent with the proposed type C mechanism. Copyright © 2003 Royal Meteorological Society