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

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Authors: Mallet, Isabelle, Arbogast, Philippe, Baehr, Christophe, Cammas, Jean‐Pierre, Mascart, Patrick
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1999
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1002/qj.49712556115; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fqj.49712556115; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.49712556115

Abstract The Fronts and Atlantic Storm‐Track EXperiment (FASTEX) has provided comprehensive data to document the life‐cycle of secondary frontal cyclones over the North Atlantic, and to improve the understanding of cyclogenesis mechanisms. This study analyses the processes leading to the triggering of a particularly well‐sampled frontal cyclone that developed on the trailing cold front 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 primary front where the frontal cyclone develops and, more unexpectedly, a pre‐existing continental surface low further west, the importance of which is revealed 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‐scale environment. Results indicate that the frontal cyclone is triggered where and when along‐front stretching decreases below the theoretical critical threshold of Bishop and Thorpe. It 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 continental low accounts for roughly 30% of the along‐front stretching decrease. the 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 Bishop and Thorpe theoretical framework of frontal instability in low‐strain areas. However, a more complex picture arises when considering the role of the continental surface low, which also acts as ...