The Formation of Small-Scale Atmospheric Vortices via Horizontal Shearing Instability

Abstract

Motivated by high-resolution observations of small-scale atmospheric vortices along near-surface boundaries, this study presents a series of idealized simulations that attempt to replicate shear zones typical of drylines and other near-surface boundaries. The series of dry, constant potential temperature simulations are initialized with a north–south-oriented constant-vorticity shear zone and north–south periodic boundary conditions. In all simulations, the shear zones develop wavelike perturbations that eventually roll up into discrete vortices. These vortices have features resembling those observed in many laboratory and numerical studies (i.e., instabilities developed into elliptical cores connected by vorticity braids that precess and contain pressure minima in their centers). To assess the instability mechanism, the results are compared to linear theory. Excellent agreement is found between predictions from linear theory for the wavenumber of maximum growth as a function of shear zone width and growth rate as a function of shear zone vorticity, suggesting to a very good first approximation, horizontal shearing instability (HSI) is responsible for the growth of initial small perturbations. It is also found that predictions of linear theory tend to extend well into the nonlinear regime. Finally, preferred regions of cumulus formation are assessed by including moisture in four simulations. Maximum updrafts and simulated cumuli tend to form along the periphery of cores and/or along the braided regions adjacent to the cores. Because of the important modulating effect of misocyclone development via HSI and subsequent moisture transport, cumulus spacing and size/depth are also dependent on the shear zone width and vorticity.

Rapid Volumetric Growth of Misocyclone and Vault-Like Structure in Horizontal Shear Observed by Phased Array Weather Radar

“…A wind shear region with small perturbation grows to an observable wave-like pattern in 5−10 minutes, resulting in the rapid formation of elliptically-shaped vortices. We also find a rapid morphological change of vortex from circular to elliptical shapes, in good agreement with numerical simulations by Buban and Ziegler (2016).…”

“…Recent theoretical calculations by Buban and Ziegler (2016) have examined a temporal change of horizontal shear zone. A wind shear region with small perturbation grows to an observable wave-like pattern in 5−10 minutes, resulting in the rapid formation of elliptically-shaped vortices.…”

“…They are vertically-oriented vortices with diameters of 1−4 km that happen along surface-based boundaries (Buban and Ziegler 2016). In the horizontal shear zone of boundaries such as sea breezes (e.g., Kingsmill 1995;Atkins et al 1995), cold fronts (e.g., Carbone 1982;Arnott et al 2006), gust fronts (e.g., Mahoney 1988), drylines (e.g., Pietrycha and Rasmussen 2004), and cold air outbreaks (e.g., Inoue et al 2011;Kato et al 2015), misocyclones often play a crucial role in initiating, intensifying, and modulating deep moist convections (e.g., Murphey et al 2006;Marquis et al 2007).…”

“…Recent theoretical studies, on the other hand, have clarified that misocyclones rapidly develop in only several minutes as a consequence of horizontal shearing instability (Buban and Ziegler 2016). This fact suggests that the crucial physical processes that govern the dynamics of misocyclones happen on a timescale much shorter than that a conventional radar resolves.…”

A warm-front triggered nocturnal tornado outbreak near Kiama, NSW, Australia

“…Trapp and Weisman (2003) argue that the mesovortices are produced by the tilting of crosswise baroclinic vorticity, which may be produced by the presence of a rear inflow jet (RIJ) in mature QLCS systems. Buban and Ziegler (2016) performed a series of numerical simulations that demonstrated that horizontal shearing instability (HSI) can produce miso-vortices (1 to 4 km) along baroclinic shear lines, with growth rates dependent on shear zone width and horizontal shear strength. Miso-vortices produced via HSI can grow into the low-level mesovortices that precede tornadogenesis.…”