Role of salient potential-vorticity elements in an event of frontal-wave cyclogenesis

Extratropical cyclogenesis is usually accompanied by both an upper-level tr ough and a surface front with its cloud-diabatic accoutrements. In the pres ent study these distinctive structures are interpreted as salient elements of the atmosphere's potential vorticity (PV) distribution, and their role e xamined for one particular event of frontal-wave cyclogenesis. The strategy adopted is a form of diagnosis by PV dismemberment. It entails adopting a 'successful' forecast with a limited-area model as a surrogate for the actual event, and then performing a sequence of simulations with mo dified initial conditions that correspond in each case to the removal of on e (or more) of the salient PV elements. This approach provides an assessmen t of the relative significance of the individual elements, permits an insig htful comparison of various extant theories of cyclogenesis, and pinpoints the particular PV elements influencing the event's predictability. It is shown that the replication of the simulated cyclone requires an adequ ate representation both of specific fine-scale PV features at tropopause le vels and of the diabatically maintained low-level pre-frontal PV band. Thus , a tenable theory of cyclogenesis that incorporates this not atypical even t should both recognize and help clarify the dependence of cyclogenesis upo n the relative locations, movement, scales and structures of the aforementi oned salient PV elements.