Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.58
Mesoscale analysis of the activation of a cold front during cyclogenesis
by K A Browning, N M Roberts and A J Illingworth
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 case study how such a mature frontal system developed from pre-existing features. The study is based upon the interpretation of satellite imagery, observations from conventional and multiparameter Doppler radar and surface reports, together with diagnostics from an operational mesoscale forecast model. The cold frontal zone in this case was characterised by multiple cold fronts. One of these was studied in detail from the point of view of its evolution and its relationship to neighbouring frontal bands. It started as a shallow 'rope cloud'. 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 accompanying 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 reinvigoration of ascent at an earlier cold-frontal zone that it was undercutting. Although this 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.