A New Theory of Supercell Propagation

From a perspective aligned with the deep-layer shear vector, a supercell moves perpendicular to the mean wind of the troposphere, with most supercells moving to the “right” in the Northern Hemisphere. This unique motion is known as “propagation”, and is one of the fundamental characteristics of a supercell. Since the 1980s, the conventional understanding of supercell propagation has been that low pressure associated with vertical vorticity on the right flank of the midlevel updraft leads to an upward acceleration that forces updraft development in the direction of propagation, thus moving the supercell to the right. In the mid-2000s, vigorous debate about the primary mechanism of supercell propagation tapered off with an uneasy resolution, with some parties arguing that “linear dynamics” play a larger role than vertical vorticity-driven nonlinear dynamics (i.e., the method previously described) in certain cases.

Our recent research ignited renewed interest in this topic, as we showed that regions of storm-generated horizontal vorticity, not vertical vorticity nor linear effects, were most strongly associated with low pressure at the updraft flank in several numerical simulations of supercells. This led us to hypothesize that these regions of storm-generated horizontal vorticity, which we call “horizontal rotors”, may play an important role in supercell propagation. To evaluate this hypothesis, we ran twelve high-resolution simulations of supercell thunderstorms across a range of background environments.

Our results show that horizontal rotors drive supercell propagation in all cases. Horizontal rotors play a larger role than both vertical vorticity-driven nonlinear dynamics and linear dynamics in initiating and maintaining the ascent of low-level, conditionally unstable air that participates in updraft growth in the direction of propagation. We believe that our findings reconcile the longstanding differences between the “vertical wind shear” and “streamwise vorticity” hypotheses. Furthermore, our findings suggest that significant changes to the conceptual model and understanding of supercells are necessary. Finally, this result has significant implications for operational forecasting of supercells. Each of these implications will be discussed in addition to our analysis, which includes a breakdown of dynamic perturbation pressure components and vertical velocity budgets.