Role of Convective-Scale Circulations in the Explosive Development of Extratropical Cyclones

Throughout his illustrious career, Lance Bosart has made significant contributions to the understanding of the morphology and prediction of cyclones. A hallmark of Lance’s studies on cyclones is in his ability to “see” the complete picture, and being able to analyze and interpret the rich complexities apparent in each case. A prime example of such a study is his multiscale analysis and diagnosis of the Presidents’ Day (PD) snowstorm of 18–19 February 1979 (Bosart 1981). Using the available surface and upper-air observations, Lance shows that the explosive development of the PD storm is likely linked to the development of intense convection near the cyclone center. Demonstrating his ability to see the big picture, Lance makes the linkage between the convective processes driving the development of the PD storm with those that drive the development of tropical cyclones and polar lows. Additionally, he points out that untangling the relative role of convection versus jet streak dynamics (e.g., Uccellini et al. 1984) in explosive cyclone development needs further research.

As a tribute to Lance’s seminal paper on the PD storm, the aim of this presentation is to examine the role of convection in the explosive development of extratropical cyclones using high-resolution (3-km grid spacing) WRF-ARW simulations of two historical cases: the PD storm and the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) intensive operations period (IOP) 4 extratropical cyclone of 4–5 January 1989. Emerging results from a quasi-Lagrangian vorticity budget show that explosive intensification of both cyclones occurred in conjunction with system-scale convergence and axisymmetrization of intense small-scale cyclonic vorticity features that develop along the warm front. These small-scale vorticity features appear to develop in conjunction with supercell thunderstorms that form where the warm conveyor belt rapid ascends over the warm front, highlighting the role of convective-scale circulation features in the life cycle of explosively deepening extratropical cyclones.