Upper‐level barotropic growth as a precursor to cyclogenesis during FASTEX

During the Fronts and Atlantic Storm‐Track Experiment (FASTEX) two large‐scale cyclones (in intensive observation periods 11 and 17) developed in the North Atlantic. These can be classified, in their initial phase, as Petterssen Type B cyclones. Initiation of the cyclones follows transient upper‐level barotropic growth, caused by a pre‐existing upper‐level trough approaching a jet associated with a strong baroclinic zone. The cyclones later developed further due to baroclinic energy conversion. A favourable orientation of the trough relative to the baroclinic zone ('horizontal tilt against the shear) is crucial for this transient barotropic growth. This scenario is therefore proposed as a general dynamical mechanism leading to Type B cyclogenesis.