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

During the Fronts and Atlantic Storm-Track EXperiment (FASTEX) two large-sc ale cyclones (in intensive observation periods ii and 17) developed in the North Atlantic. These can be classified, in their initial phase, as Petters sen Type B cyclones. Initiation of the cyclones follows transient upper-lev el barotropic growth, caused by a pre-existing upper-level trough approachi ng a jet associated with a strong baroclinic zone. The cyclones later devel oped 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 cyclo genesis.