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Keywords:

  • Attribution;
  • Extratropical cyclones;
  • Quasi-geostrophic vertical motion

Abstract

A systematic and objective procedure is developed for applying the simple cyclone classification scheme of Petterssen and Smebye. This method uses a height-attributable solution of the quasi-geostrophic ! equation to identify and quantify the relative importance of upper- and lower-tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels. By applying this classification method to a sample of cyclogenesis events, during their maximum intensification stage, from the Fronts and Atlantic Storm-Track EXperiment field experiment, the Type A and Type B scheme of Petterssen and Smebye is reproduced and extended to include a Type C. Type C consists of upper-level dominated cyclones that form at high latitudes and in their initial stages resemble comma-cloud-type polar lows. Detailed examples of each of the three types are presented. Some cyclones can undergo more than one period of development and it is found that each development period can be classified as a different cyclogenesis category, A or B: these cyclones are classified as hybrid Type A=B. There is some evidence that cyclone forecast accuracy depends on cyclone type, thereby suggesting the potential for this method to be used to assign confidence levels for forecasts of cyclogenesis produced by numerical weather-prediction models. Type B cyclones appear more difficult to predict, because their development depends, initially, on the interaction between significant features in the upper and lower troposphere. Copyright © 2002 Royal Meteorological Society. T. D. Hewson's contribution is Crown copyright.