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

Abstract

AbstractIn a previous study of two cyclones within the Fronts and Atlantic Storm Track EXperiment (IOP11 and IOP17), it was shown that upper‐level barotropic growth dominated the initial phase of the development. The cyclones later developed due to baroclinic energy conversion. These cyclones resemble Petterssen's Type B paradigm. This contribution presents the dynamics of archetypical Type B cyclogenesis by addressing the consequences of upper‐level barotropic growth before baroclinic cyclone development within the framework of idealized primitive‐equation simulations. Localized and finite‐amplitude upper‐level pressure perturbations have been designed to represent a variety of amplitudes and horizontal tilts of a localized barotropic trough structure. These are added to a zonal jet basic state, and it is shown that favourable initial situations, similar to the observed cyclones, lead to much stronger and faster cyclonic development compared with unfavourable ones. The most effective barotropic initial situation (without a vertical tilt and considering 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.