A SATELLITE-DERIVED CLASSIFICATION SCHEME FOR RAPID MARITIME CYCLOGENESIS

Guided by the conjecture that there exist characteristic synoptic-scal e flow patterns conducive to extratropical cyclogenesis, a satellite-b ased classification scheme is proposed to differentiate between variou s types of rapid maritime cyclogenesis in the western North Atlantic r egion. The scheme is derived from signatures in visible and infrared s atellite imagery observed prior to and during rapid deepening. Conside ration of the western North Atlantic region is motivated by the presen ce of a relatively dense upstream observational network and by the abs ence of the direct influence of orography in a maritime environment, t he focus on rapid development is predicated upon the assumption that t he cloud signatures will be more clearly defined than in cases of slow er, ''ordinary'' development. Examination of satellite imagery for an ensemble of 50 cyclogenesis events that occurred during the 1970s and 1980s, 46 of which satisfied the Experiment on Rapidly Intensifying Cy clones over the Atlantic criterion for rapid deepening [a pressure dro p of at least 10 mb (6 h)-1], yields four categories of cyclone evolut ion. The first category, referred to as the ''emerging cloud head,'' i s characterized by the formation of a cloud head poleward of an S-shap ed cirriform band associated with a troposphere-spanning baroclinic zo ne (i.e., the polar front). The second category, called the ''comma cl oud,'' involves development independent of a polar-front cloud band an d is distinguished by the transformation of an elongated cloud feature referred to as a ''baroclinic leaf '' into an increasingly well-defin ed comma shape. The third category, referred to as the ''left exit,'' applies to cyclones that develop beneath the left-exit region of a jet streak embedded within diffluent flow downstream of the axis of an up per-level trough and that deepen in conjunction with the merger of a b aroclinic leaf and a polar-front cloud band. The fourth category, refe rred to as the ''instant occlusion,'' involves the merger of a cold-ai r cloud cluster and a polar-front cloud band within a confluent upper- level flow environment. Diagnostic calculations relating cyclone devel opment and the evolution of characteristic cloud configurations to the synoptic-scale flow pattern for a representative storm from each of t he foregoing categories confirm the qualitative distinctions between t he various categories of the classification scheme. The proposed class ification scheme is found to be consistent with previously published s atellite-derived conceptualizations of extratropical cyclogenesis, sug gesting the possible wider applicability of the categories of developm ent proposed here to other geographical regions and to cases of less e xtreme development.