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What is the 'Shapiro-Keyser' cyclone model?

During cases of rapid cyclogenesis (see the Glossary), the long accepted 'Norwegian' frontal/cyclone development model is not appropriate. M.A. Shapiro and D. Keyser, in a paper published in 1990, proposed an alternative which has gained widespread acceptance. I am grateful to Dr. David Schultz (NSSL) for permission to quote the following from an article written (with co-author H. Wernli) for the 'Mariners Weather Log', which to my mind is an excellent summary of the differences between the 'classical' frontal depression model and that proposed for rapid cyclogenesis events ...

"The Norwegian cyclone model, so named to honor the Norwegian meteorologists (e.g. Bjerknes, Bergeron and Solberg) who first conceptualised the typical life-cycle of midlatitude cyclones in the 1910's and 1920's, presents the evolution of a cyclone from an incipient frontal wave with cold and warm fronts, to a deepening cyclone with a narrowing warm sector as the cold front rotates around the cyclone faster than the warm front, and finally to a mature cyclone with an occluded front. Typically, a Norwegian cyclone is oblong, orientated roughly north-south with the cold front more intense and longer than the weak and "stubby" warm front.

The Shapiro-Keyser cyclone model is named after the authors of the study that first presented this conceptual model of the frontal structure in some marine cyclones. As with the Norwegian cyclone model, an incipient cyclone develops cold and warm fronts, but in this case, the cold front moves roughly perpendicular to the warm front such that the fronts never meet, the so-called 'T-bone'. Also, a weakness appears along the poleward portion of the cold front near the low center, the so-called 'frontal fracture' and a back-bent front forms behind the low center. (In the final stage), colder air encircles warmer air near the low center, forming a warm seclusion. Typically, the Shapiro-Keyser cyclone is oblong, elongated east-west along the strong warm front".

Schultz & Wernli then go on to state (I paraphrase) ... an important factor in determining which evolution will be preferred ... is the nature of the large-scale (i.e. mid/upper tropospheric) flow. NWP experiments have indicated significant sensitivity to the profile of the wind speed across the jet flow and other studies have indicated that the along-jet variations of wind speed can be important. Cyclones embedded within diffluent flow (e.g. jet-exit regions) tend to evolve like the Norwegian cyclone model, whereas cyclones embedded within confluent flow (e.g. jet-entrance regions) tend to evolve like the Shapiro-Keyser cyclone model.

(for background to jets, entrance and exit regions etc., see here and various entries in this FAQ and its associated Glossary.)