Dynamical Characterization of the FASTEX cyclogenesis cases

Abstract

A series of diagnostics based on the automatic tracking of cyclones is applied 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 system. Time-filtering is used to further separate the events into a basic-state and perturbations. Information is obtained on environmental properties such 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 instabilities 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 exidentrance 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.

On the Northward Motion of Midlatitude Cyclones in a Barotropic Meandering Jet

“…The results obtained here suggest that the jet-crossing phase often observed of midlatitude cyclones (Baehr et al 1999;Riviè re and Joly 2006a;Wallace et al 1988) can be attributed to the combined effects of nonlinearities and deformation in presence of meridional PV gradient due to the existence of the jet. When baroclinic effects are taken into account, it is not the lowerlayer PV gradient that matters for the surface cyclone northward motion, but the barotropic meridional PV gradient, as shown by Gilet et al (2009) in a two-layer quasigeostrophic model in the case of the evolution of a surface cyclone embedded in baroclinic zonal flows.…”

“…This was the case for the December 1999 ''Lothar'' storm over Europe (Wernli et al 2002;Riviè re and Joly 2006b) and for most of the storms during the Fronts and Atlantic Storm Track Experiment (FASTEX) campaign (Baehr et al 1999). Since most cases of the jet-crossing phase occur in the jet exit region, a commonly accepted mechanism for the regeneration phase is dictated by the analogy with the role of the cold left exit region of a jet streak on cyclogenesis.…”

Factors contributing to the development of extreme North Atlantic cyclones and their relationship with the NAO

“…• The upper-air divergence (250 hPa), as areas of intense divergence north of the jet exit region are well known to induce rapid cyclone growth (e.g., Uccellini and Johnson 1979;Baehr et al 1999;Ulbrich et al 2001). It is hereafter denoted as div 250 .…”

Cyclones, windstorms and the IMILAST project

“…1). Meanwhile Baehr et al (1999), Wernli et al (2002) and others focus on jet crossing. Latent heat release is a key component too, perhaps contributing between one half and one third of the full deepening of a cyclone (as in Shutts, 1990;Nielsen and Sass, 2003).…”