Two archetypes of tornadic quasi-linear convective systems in the United Kingdom: Relevance of horizontal shearing instability to vortexgenesis and maintenance

High-resolution mesoscale model simulations of two archetypal quasi-linear convective systems producing outbreaks of three or more tornadoes in the United Kingdom are performed to determine vortexgenesis mechanisms. Type 1 events are associated with north–south-oriented cold fronts with regularly spaced misocyclones along them. In one type 1 event, a near-surface vortex sheet broke down into near-equally spaced misovortices having a wavelength of about 7.5 times the width of the shear zone. Rayleigh's and Fjørtoft's instability criteria were met preceding the development of the vortices, suggesting the presence of horizontal shearing instability (HSI). Lagrangian calculations of vorticity tendency showed that parcels entered the misovortices at lower heights, acquired their vorticity via tilting, before being further enhanced by stretching as the parcel ascended. These results implied HSI was the initial mechanism for the amplification of perturbations along the vortex sheet in the type 1 event. In contrast, type 2 events are associated with west–east-oriented cold fronts with disorganized, elongated cyclonic–anticyclonic vorticity couplets evolving into a small number of cyclonic and anticyclonic misovortices with irregular misovortices. In one type 2 event, Fjørtoft's instability criterion was not met. Lagrangian vorticity-tendency calculations showed that parcels acquired vorticity similar to type 1 events, where parcels entered the misovortices at lower heights, acquired their vorticity via tilting, before being further enhanced by stretching as the parcel ascended. However, the magnitude of tilting was typically larger in the type 2 event. Comparing these two events showed two possible mechanisms for misovortexgenesis in UK tornado outbreaks: misovortices in type 1 events form and grow via HSI along the front, whereas misovortices in type 2 events are not due to HSI.