A Lagrangian‐based analysis of extratropical cyclones. II: A detailed case‐study

Abstract A detailed case‐study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories (CETs) that accompany Atlantic cyclogenesis. The approach is based upon the Lagrangian‐based analysis scheme introduced in Part I. The analysis reveals the existence of several distinct moist ascending (and dry descending) CETs. Two types of moist ascending CETs associated with strong condensational processes ascend from the boundary layer of the warm sector, either in the close vicinity of the cyclone centre, or in the trailing cold‐frontal region. The latter is the main contributor to the precipitation along the cold front during the early phase of the development. Another type of moist CETs—distinguished by maximum water vapour flux—travels at low levels through the warm sector and ascends to mid‐tropospheric levels in the warm‐frontal region, where it produces strong precipitation around the time of maximum storm intensity. CETs with an upper‐level origin are linked to the classical dry intrusion. Different CETs correspond to: strong descent of upper‐level tropospheric air that penetrated to the lower troposphere, the movement of stratospheric air toward the centre of the maturing cyclone, and an event of stratosphere‐to‐troposphere exchange upstream of the upper‐level trough. Two different visualizations of time sequences of the moist ascending CETs indicate that they represent well‐defined ‘flexible tubes’ or airstreams that exist over a time period comparable with the duration of the main cyclone intensification. They provide a translating, flexible but coherent ‘river bed’ for the quasi‐continuous ascent of moist boundary‐layer air to 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‐vorticity pattern. Some consideration is also given to the relationship of CETs and ‘tropospheric rivers’, and to possible shortcomings of the approach.