A Fresh Look at the Type A/Type B Cyclogenesis Paradigm

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A Fresh Look at the Type A/Type B Cyclogenesis Paradigm Jace J. Bauer and Jonathan E. Martin Department of Atmospheric and Oceanic Sciences University of Wisconsin-Madison Nearly 40 years ago Petterssen and Smebye (1971) published a seminal paper that effectively summarized, with few details, work that had been ongoing for nearly a decade regarding the possibility of two broad cyclogenesis categories, labeled Type A and Type B. One of the distinctions between these two varieties was based upon which term in the traditional quasi-geostrophic (QG) omega equation appeared to control cyclone development at sea-level. Type A cyclones appeared to respond mainly to thermal advection in the lower half of the troposphere and were thought to develop in the absence of any significant upper-level disturbance (e.g. upper tropospheric shortwave, jet streak, etc.). Type B cyclogenesis occurred when a pre-existing upper trough, characterized by positive vorticity advection at its leading edge, encroached upon a low-level baroclinic zone. In such a case, the influence of vorticity advection overwhelmed that of thermal advection in the initial cyclogenetic stage. The fact that the primary distinction between these two cyclogenesis types refers to the supposedly separate forcing terms in the traditional QG omega equation has always been problematic since these terms contain considerable internal cancellation as pointed out by Trenberth (1978) and Hoskins et al. (1978). More recently, Keyser et al. (1994) considered the along- and across-isentrope components of the Q-vector and their associated vertical motion forcings, shearwise and transverse to the geostrophic vertical shear. Here we propose that a more satisfying dynamical categorization of cyclogenesis types stems from this partition – namely, that some cyclones develop in response to transverse circulations and some develop downstream of discernible upper tropospheric waves. In this proposed paradigm, all cyclones may have an upper level precursor of one kind or another, consistent with recent work on the Type A/Type B cyclogenesis (e.g. Deveson et al. 2002). The shearwise/transverse partition is a convenient diagnostic for assessing the viability of each candidate cyclogenesis type. In this talk it is demonstrated that both shearwise and transverse cyclogenesis types exist and aspects of the cyclogenesis periods of two storms, one of each variety, are examined. .

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