The Role of Convective-Scale Circulations in the Development of Intense Warm Seclusion Cyclones

The aim of this presentation is to examine the role of lower-tropospheric meso- and convective-scale circulation features in the development of warm seclusion cyclones. The development of extratropical warm core seclusion in cyclones was first observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) in the winter of 1988–89. Building on the results from earlier numerical model experiments, these new observations led to the development of the Shapiro-Keyser extratropical cyclone model. Observations collected during ERICA also revealed intense meso-β to meso-γ scale cyclonic circulations that formed along the warm and bent-back frontal zones. These circulation features were coupled with intense convection and moved rearward in cyclone-relative coordinates toward the warm seclusion circulation.

Recent numerical modeling studies have also suggested that these intense smaller scale circulations are important for the development of convectively driven meso-α scale cyclones.  One example of such a development was documented in the “Super Derecho” of 8 May 2009 over the southern Plains and Mississippi Valley. During the “Super Derecho”, an intense line-end vortex developed on the north side of the parent mesoscale convective system. This line-end vortex evolved to a structure that resembled a warm seclusion, complete with a bent-back warm front and T-bone structure, except on a much smaller scale compared to that of an extratropical cyclone. Small-scale cyclonic vorticity features along the bent back warm front appeared to move rearward in system-relative coordinates and merge with the main line-end circulation, contributing to its intensification.

In this presentation, we will utilize high-resolution (3-km grid spacing) WRF-ARW simulations and a quasi-Lagrangian vorticity budget to examine the role of these mesoscale circulation features in warm seclusion development and intensification of the ERICA intensive operations period (IOP) 4 extratropical cyclone of 4–5 January 1989 and the “Super Derecho” warm seclusion cyclone. Emerging results suggest that both cyclones exhibit similar structure and intensification mechanisms in the lower troposphere. The vorticity budgets of these two warm seclusion cyclones show that intensification of the low-level warm seclusion circulation is driven by system-scale convergence and axisymmetrization of smaller-scale vorticity features that form along the warm and bent-back fronts in IOP4 and the leading convective line in the “Super Derecho”. Interestingly, the small-scale cyclonic vorticity maxima along the warm and bent-back fronts in IOP4 appear to develop in conjunction with supercell thunderstorms. We hypothesize that the development mechanisms of these two warm seclusion cyclones are consistent for the situation where an intensifying robust rotational feature is positioned along a frontal zone.