Dynamical characterization of the FASTEX cyclogenesis cases

A series of diagnostics based on the automatic tracking of cyclones is appl ied to the 25 life cycles captured during the field phase of the Fronts and Atlantic Storm-Track EXperiment (FASTEX). The tracking enables the various cases to be set into a common frame of reference that moves with the syste m. Time-filtering is used to further separate the events into a basic-state and perturbations. Information is obtained on environmental properties suc h as strain or baroclinicity. Combining these diagnostics, it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these inst abilities by deformation may be involved in the initial genesis of a number of cases, but do not seem to characterize specific types nor to determine a systematic behaviour. On the other hand, it is shown that rapid-deepening phases leading Eastern Atlantic systems to their maximum amplitude happen suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper-level feature. For cold-air systems, development results from meeting a diffluent zone. This is unlike the American east-coast systems that are associated with jet exit/entrance complex. Another remarkable feature is the generation of a frontal wave as a result of the parent vorticity maximum being apparently stretched and split. This has occurred in 25% of cases. A number of other properties of the FASTEX cases are shown, such as several occurrences of large phase changes in sharp contrast to the idea of phase locking.