Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: Shear‐zone vortex‐genesis in a developing frontal wave

M R Clark,D J Parker,K E Hanley

doi:10.1002/qj.4164

M R Clark, D J Parker + Show 1 more

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https://doi.org/10.1002/qj.4164

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Abstract

AbstractHigh‐resolution model simulations and radar observations are used to investigate the onset of vortex‐genesis in a tornadic narrow cold‐frontal rain band (NCFR). The timing and location of vortex‐genesis was strongly constrained by a developing frontal wave, which tracked northeast across the United Kingdom and Ireland on 17 October 2011. In the simulations, vortices occurred preferentially during the early stages of wave development and just down‐front of the wave centre, where large increases in vertical vorticity occurred in concert with decreases in the cross‐frontal confluence. Vortex‐genesis ceased as the frontal wave matured, due to the onset of frontal fracture. Two distinct scales of vortex‐genesis are documented: primary vortex‐genesis on the meso‐γ‐scale, and secondary vortex‐genesis on the miso‐scale. We show that horizontal shearing instability is the most likely vortex‐genesis mechanism, consistent with previous theoretical work on the stability of vertical vortex strips in the presence of horizontal stretching deformation. Secondary vortices occurred along the braid regions between primary vortices where the shear zone became particularly narrow and intense. In the model, these vortices developed extremely rapidly (from small perturbations to maximum vertical vorticity in 5–15 min) and the strongest exhibited near‐surface vertical vorticity maxima approaching 10−1 s−1. Vortices of both scales were associated with characteristic local perturbations in the NCFR and we show, by comparison with radar reflectivity data, that primary and secondary vortices were likely present in the real NCFR. Tornado reports were associated with small NCFR perturbations like those associated with the secondary vortices in the model simulations. Analysis of the sub‐structure of individual simulated vortices suggests that tornado‐genesis is most likely within a region of intense near‐surface vertical vorticity stretching at the north or northwest flank of the secondary vortices.

Highlights

Recent climatologies suggest that the United Kingdom experiences an average of approximately 30 tornadoes per annum (Reynolds, 1999; Kirk, 2007; Mulder and Schultz, 2015), of which 40–50% are associated with precipitation systems exhibiting quasi-linear morphologies in radar rainfall imagery (Mulder and Schultz, 2015; Clark and Smart, 2016)
The intense vorticity stretching maximum on the north or northwest flank of the secondary vortices, which we suggest to be a preferred region for tornadogenesis, is shallow, with stretching >2 × 10−4 s−2 extending to only ∼250 m above ground level (AGL) (Figure 15c)
It is noticeable that the two confirmed tornadoes occurred within ∼20 km of a windward coast (e.g. Figure 12). In light of these results we suggest that, whilst the frontal wave provides the requisite conditions for vortex-genesis along the shear zone, and determines the region at risk of tornadoes on the mesoscale, horizontal variability in pre-frontal convective available potential energy (CAPE) may act to modulate the tornado risk on smaller scales

Summary

Introduction

Recent climatologies suggest that the United Kingdom experiences an average of approximately 30 tornadoes per annum (Reynolds, 1999; Kirk, 2007; Mulder and Schultz, 2015), of which 40–50% are associated with precipitation systems exhibiting quasi-linear morphologies in radar rainfall imagery (Mulder and Schultz, 2015; Clark and Smart, 2016). An important type of quasi-linear precipitation system is the narrow cold-frontal rainband (NCFR: Houze et al, 1976). These systems, which are responsible for approximately one-third of the United Kingdom’s tornadoes, are characterised by strong but relatively shallow updraughts forced by horizontal convergence at the frontal boundary. In NCFR-bearing fronts the near-surface frontal boundary is marked by a narrow zone of strong cyclonic relative vertical vorticity (i.e. a vertical vortex sheet, or “vortex strip”) and large horizontal temperature gradient. Observational and modelling studies have shown that NCFR tornadoes are associated with meso-γ- to miso-scale vortices that develop along this vortex sheet. Vortex-genesis has generally been attributed to horizontal shearing instability (HSI) (e.g. Matejka et al, 1980; Carbone, 1982; 1983; Hobbs and Persson, 1982; Lee and Wilhelmson, 1997; Smart and Browning, 2009), though the mechanism of formation is not universally agreed upon and it may differ from case to case

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