Effects of a low-level precursor and frontal stability on cyclogenesis during FASTEX IOP17

The Fronts and Atlantic Storm-Track EXperiment (FASTEX) has provided compre hensive data to document the life-cycle of secondary frontal cyclones over the North Atlantic, and to improve the understanding of cyclogenesis mechan isms. This study analyses the processes leading to the triggering of a part icularly well-sampled frontal cyclone that developed on the trailing cold f ront of a mature primary cyclone during FASTEX Intensive Observation Period 17 (IOP17) between 16 and 20 February 1997. The case features both a classical low-level vorticity strip along the prim ary front where the frontal cyclone develops and, more unexpectedly, a pre- existing continental surface low further west, the importance of which is r evealed in the pilot study of Arbogast and Joly. In order to study possible cyclogenetic contributions along the primary front, the domain-independent vorticity-divergence attribution technique of Bishop is used to partition the flow into contributions from the continental low and from the large-sca le environment. Results indicate that the frontal cyclone is triggered wher e and when along-front stretching decreases below the theoretical critical threshold of Bishop and Thorpe. II is shown that the cyclone is developing under normalized strain less than 1, in agreement with other case-studies. Finally, the role of the continental low is investigated using a series of numerical forecasts made from different initial conditions manipulated with a potential-vorticity inversion tool. The wind attributable to the contine ntal low accounts for roughly 30% of the along-front stretching decrease. T he continental low also has a frontolytic effect on the western part of the primary front where its thermal advection pattern favours cyclogenesis. In general terms, the frontal-cyclone triggering seems to support the Bisho p and Thorpe theoretical framework of frontal instability in low-strain are as. However, a more complex picture arises when considering the role of the continental surface low, which also acts as a low-level precursor.