Theory of Rapid Extratropical Cyclogenesis

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Extratropical cyclogenesis occurs preferentially in certain geographical regions including the lee of major mountain ranges and the "storm tracks" roughly coincident with the eastward extension of western boundary currents: the Gulf Stream in the North Atlantic and the Kuroshio in the North Pacific. However, explosive cyclogenesis, characterized by a large rate of decrease in central pressure and severe winds, is almost exclusively a marine phenomenon. Synoptic meteorologists classify certain cyclones as "bombs"; a "bomb" is an extratropical cyclone with its central pressure decreasing at a rate -1 of at least 1 mb hr for 24 hours. Recent observational studies have identified a number of crucial elements common to "bombs". However, a theoretical synthesis has yet to emerge. Under this project the explosive cyclogenesis will be treated as an initial value problem (IVP) from which a theory will be derived. This approach to instability contrasts with the more traditional and common approach of normal modes. In the IVP theory, cyclogenesis results from the growth of favorably configured perturbations in a baroclinic flow; intensification proceeds whether or not the flow supports exponential instabilities and gives growth rates which are greater than the normal mode growth rates in cases where normal modes exist. As part of the project, synoptic predictors for explosive cyclogenesis will also be identified and put in the context of the new theory.