A Satellite-Derived Classification Scheme for Rapid Maritime Cyclogenesis

Abstract

Guided by the conjecture that there exist characteristic synoptic-scale flow patterns conducive to extratropical cyclogenesis, a satellite-based classification scheme is proposed to differentiate between various types of rapid maritime cyclogenesis in the western North Atlantic region. The scheme is derived from signatures in visible and infrared satellite imagery observed prior to and during rapid deepening. Consideration of the western North Atlantic region is motivated by the presence of a relatively dense upstream observational network and by the absence of the direct influence of orography in a maritime environment; the focus on rapid development is predicated upon the assumption that the cloud signatures will be more clearly defined than in cases of slower, ?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 Cyclones over the Atlantic criterion for rapid deepening [a pressure drop of at least 10 mb (6 h)?1], yields four categories of cyclone evolution. The first category, referred to as the ?emerging cloud head,? is characterized by the formation of a cloud head poleward of an S-shaped cirriform band associated with a troposphere-spanning baroclinic zone (i.e., the polar front). The second category, called the ?comma cloud,? involves development independent of a polar-front cloud band and is distinguished by the transformation of an elongated cloud feature referred to as a ?baroclinic leaf? into an increasingly well-defined 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 upper-level trough and that deepen in conjunction with the merger of a baroclinic leaf and a polar-front cloud band. The fourth category, referred to as the ?instant occlusion,? involves the merger of a cold-air cloud cluster and a polar-front cloud band within a confluent upper-level now environment. Diagnostic calculations relating cyclone development and the evolution of characteristic cloud configurations to the synoptic-scale flow pattern for a representative storm from each of the foregoing categories confirm the qualitative distinctions between the various categories of the classification scheme. The proposed classification scheme is found to be consistent with previously published satellite-derived conceptualizations of extratropical cyclogenesis, suggesting the possible wider applicability of the categories of development proposed here to other geographical regions and to cases of less extreme development.