A LAGRANGIAN-BASED ANALYSIS OF EXTRATROPICAL CYCLONES .1. THE METHOD AND SOME APPLICATIONS

A Lagrangian-based method of analysis is introduced to examine the spa ce-time structure and dynamics of extratropical cyclogenesis. The thre e-step method involves the calculation of extensive ensembles of traje ctories, the evaluation of the Lagrangian time-trace of physical varia bles along these paths, and the application of objective selection cri teria to identify significant Lagrangian structures. The approach can provide a qualitative depiction of, quantitative information about, an d dynamical insight into, key features of mid-latitude cyclones. In th is, the first of a two-part study, examples are given of the applicati on of the method. A case-study approach has been adopted and used to i dentify coherent ensembles of trajectories (CETs) whose dynamics are s eminal to an event of Atlantic cyclogenesis, to trace the origin of su bsynoptic potential vorticity anomalies, and to analyse stratosphere-t roposphere exchange accompanying extratropical cyclogenesis. In additi on, a statistical investigation has been made of the climatology of CE Ts in the northern hemisphere for one particular winter month. Some co nsideration has been given to the shortcomings and potential of the me thod, and its complementarity with other approaches.