Classifying dynamical forcing mechanisms using a climatology of extratropical cyclones

A climatology of almost 700 extratropical cyclones is compiled by applying an automated feature‐tracking algorithm to a database of objectively identified cyclonic features. Cyclones are classified according to the relative contributions to the midlevel vertical motion of the forcing from upper and lower levels averaged over the cyclone intensification period (average U/L ratio) and also by the horizontal separation between their upper‐level trough and low‐level cyclone (tilt). The frequency distribution of the average U/L ratio of the cyclones contains two significant peaks and a long tail at high U/L ratio. Although discrete categories of cyclones have not been identified, the cyclones comprising the peaks and tail have characteristics that have been shown to be consistent with the type A, B and C cyclones of the threefold classification scheme. Using the thresholds in average U/L ratio determined from the frequency distribution, type A, B and C cyclones account for 30%, 38% and 32% of the total number of cyclones respectively. Cyclones with small average U/L ratio are more likely to be developing cyclones (attain a relative vorticity ≥1.2×10−4 s−1) whereas cyclones with large average U/L ratio are more likely to be non‐developing cyclones (60% of type A cyclones develop whereas 31% of type C cyclones develop). Type A cyclogenesis dominates in the development region east of the Rockies and over the Gulf Stream, type B cyclogenesis dominates in the region off the east coast of the USA, and type C cyclogenesis is more common over the oceans in regions of weaker low‐level baroclinicity. Copyright © Royal Meteorological Society, 2006.