Mesoscale analysis of the activation of a cold front during cyclogenesis

The archetypal mesoscale structure of an ana-cold front comprises a narrow cold-frontal rainband (NCFR), associated with line convection, embedded within a wide cold-frontal rainband (WCFR) due to the slantwise ascent associated with a strong transverse-front mesoscale circulation. Whereas most mesoscale observational studies have concentrated on this mature stage, the present paper explores by means of a detailed case-study how such a mature frontal system develops from pre-existing features. The study is based upon the synthesis of data spanning many scales, in the form of satellite imagery, observations from conventional and multi-parameter Doppler radar, and surface reports, together with diagnostics from an operational mesoscale forecast model. The cold-frontal zone in this case is characterized by multiple cold fronts. The dominant second cold front is studied in detail from the point of view of its evolution and its relationship to neighbouring fronts, especially the leading front associated with the initial polar-front cloud band. The dominant cold front started as a shallow ‘rope cloud in the cold sector, and at a fairly early stage it produced a NCFR due to line convection. Then, over a period of only a few hours, a dry intrusion and an associated strong direct transverse-front circulation led to the formation of a canopy of mid-level cloud and a WCFR. The WCFR was attributable not only to the rearward-ascending outflow from the line convection but also to the re-invigoration of ascent within the initial polar-front cloud band that the line convection was undercutting. Although the transverse circulation was probably intensified by precipitation processes, it is inferred that it was mainly a manifestation of the larger-scale environmental flow that existed independently of the major precipitation features.