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

High-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.