Two Archetypes of Tornadic Quasilinear Convective Systems in the United Kingdom: Vortex Genesis and Maintenance

Nine tornado outbreaks (days with three or more tornadoes) have occurred in the United Kingdom from quasi-linear convective systems in the 16 years between 2004 and 2019. Of the nine outbreaks, eight can be classified into two archetypes: type 1 and type 2. Simulations of each archetype were performed to determine vortex genesis mechanisms. In a type 1 event, a vortex sheet broke down into near-equally spaced misovortices, whereas in a type 2 event, disorganized, elongated cyclonic–anticyclonic couplets evolved into a small number of misovortices. The assessment of Rayleigh's and Fjörtoft's instability criteria, along with misovortices having a wavelength of about 7.5 times the width of the shear zone, implied horizontal shearing instability (HSI) was the initial mechanism for the amplification of perturbations along the vortex sheet in the type 1 event. Vorticity-tendency analysis also revealed near-equally spaced localized horizontal advection maxima prior to the growth of perturbations in the type 1 event, further reinforcing HSI as a plausible mechanism. Deformation was not strong enough to suppress vortex growth. In contrast, the type 2 event did not satisfy either criteria. Parcels acquired vertical vorticity differently dependent on the location and the height that they entered the misovortex. In the type 1 event, parcels entering the misovortex at lower heights experienced varying magnitudes of stretching and tilting, whereas tilting was considerably stronger at greater heights. In the type 2 event, parcels entering the misovortex from the leading edge of the misovortex acquired the majority of their vertical vorticity via both tilting and stretching, and parcels entering the misovortex from the trailing edge acquired the majority of their vertical vorticity via stretching. These results suggest two possible mechanisms for misovortex genesis in UK tornado outbreaks: type 1 where HSI initiates the vortices and type 2 that form from vorticity couplets along the front not formed from HSI.