A LAGRANGIAN-BASED ANALYSIS OF EXTRATROPICAL CYCLONES .2. A DETAILED CASE-STUDY

A detailed case-study analysis is undertaken of the occurrence of sign ificant coherent ensembles of trajectories (CETs) that accompany Atlan tic cyclogenesis. The approach is based upon the Lagrangian-based anal ysis scheme introduced in Part I. The analysis reveals the existence o f several distinct moist ascending (and dry descending) CETs. Two type s of moist ascending CETs associated with strong condensational proces ses ascend from the boundary layer of the warm sector, either in the c lose vicinity of the cyclone centre, or in the trailing cold-frontal r egion. The latter is the main contributor to the precipitation along t he cold front during the early phase of the development. Another type of moist CETs-distinguished by maximum water vapour flux-travels at lo w levels through the warm sector and ascends to mid-tropospheric level s in the warm-frontal region, where it produces strong precipitation a round the time of maximum storm intensity. CETs with an upper-level or igin are linked to the classical dry intrusion. Different CETs corresp ond to: strong descent of upper-level tropospheric air that penetrated to the lower troposphere, the movement of stratospheric air toward th e centre of the maturing cyclone, and an event of stratosphere-to-trop osphere exchange upstream of the upper-level trough. Two different vis ualizations of time sequences of the moist ascending CETs indicate tha t they represent well-defined 'flexible tubes' or airstreams that exis t over a time period comparable with the duration of the main cyclone intensification. They provide a translating, flexible but coherent 'ri ver bed' for the quasi-continuous ascent of moist boundary-layer air t o the upper troposphere. These 'tubes' are akin to the concept of warm conveyor belts, and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential-vorticit y pattern. Some consideration is also given to the relationship of CET s and 'tropospheric rivers', and to possible shortcomings of the appro ach.