The influence of transient upper-level barotropic growth on the development of baroclinic waves

In a previous study of two cyclones within the Fronts and Atlantic Storm Tr ack EXperiment (IOP11 and IOP17), it was shown that upper-level barotropic growth dominated the initial phase of the development. The cyclones later d eveloped due to baroclinic energy conversion. These cyclones resemble Pette rssen's Type B paradigm. This contribution presents the dynamics of archety pical Type B cyclogenesis by addressing the consequences of upper-level bar otropic growth before baroclinic cyclone development within the framework o f idealized primitive-equation simulations. Localized and finite-amplitude upper-level pressure perturbations have been designed to represent a variet y of amplitudes and horizontal tilts of a localized barotropic trough struc ture. These are added to a zonal jet basic state, and it is shown that favo urable initial situations, similar to the observed cyclone, lead to much st ronger and faster cyclonic development compared with unfavourable ones. The most effective barotropic initial situation (without a vertical tilt and c onsidering growth over a time-scale of 48 hours) is one with a horizontal ' tilt against the shear' of an upper-level trough on the cyclonic side of a jet associated with a baroclinic zone.