A Reappraisal of Petterssen's Type A and Type B Cyclogenesis - A Dynamical Process Study

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Cyclogenesis is overarching term for the development or strengthening of a closed, low pressure, circular motion of the atmosphere driven by inward spiraling winds rotating in the same direction as the planet. Extratropical cyclones form along weather fronts (baroclinic zones) and mature into cold core cyclones. This is distinct from tropical cylcones which are driven by the latent heat associated with intense convection and are warm, central core systems. The notion that extratropical cyclones evolve through a predictable sequence as they pass along the frontal boundary has led to the development of a variety of classification and diagnostic schemes. One such scheme, due to Petterssen and Smebye (1950's), adopts two broad developmental paths, Type A (directed by low level processes) and Type B (directed by upper level processes). These differ as to the forcing terms responsible for upward vertical motion and development of the kinetic energy in the systems. Despite the widespread use of this classification, details of the dynamical processes in extratropical cyclones remains unclear. This project, analyzing a number of cyclogenesis events observed during FASTEX (the Fronts and Atlantic Storms Experiment, 1997) and along with other selected data sets, seeks to employ two different methodologies to better diagnose and interpret the two category types. These methodologies are respectively conventional quasigesotrophic analyses based on Q vector analysis, and an adjoint model derived sensitivity analysis, both identifying atmospheric parameters (e.g. shearwise and transverse vertical motions) that relate to mid latitude cyclone development. Broader impacts of these studies include the furtherance of undergraduate and graduate education of University of Wisconsin students, development of community resources and the investigation of new methodologies in weather predictability research on the topic of extratropical cyclone development. The economic impact of extratropical cyclones such as the 2009-2010 US winter storms were significant examples thereof.