Investigating Tornadogenesis from Quasi-Linear Convective Systems in the U.K.

The United Kingdom (UK) experiences more tornado reports per unit area than any other country. However, most tornadoes are weak and short lived, making understanding these events difficult. Tornado outbreaks in the UK (classified as days with 3 or more tornadoes) are commonly associated with quasi-linear convective systems (QLCSs). QLCSs are a line of convective storms that often occur along an atmospheric boundary, such as a cold front, and are defined as having a length of greater than 100 km, at least 10 times longer than wide and a minimum duration of 2 hours.

Tornado reports are taken from the Tornado and Storm Research Organisation (TORRO) database, where 9 tornado outbreaks from QLCSs between 2005–2015 were associated with 63 tornado reports. A common feature from the tornado outbreaks was the existence of regular-spaced precipitation cores and gaps along the QLCS. This structure on radar images has been associated with tornadoes in the previous literature, with the precipitation cores being associated with localised regions of vorticity maxima (misovortices) responsible for the tornadoes. These misovortices form in an environment of high horizontal wind shear and low convective available potential energy (CAPE).

To assess the environmental conditions of the tornado outbreaks, a mesoscale convection-permitting model (WRF) is used to simulate each event. Simulations are tested against reality using precipitation data from the UK Met Office archive. Initial analysis focuses on 500-m absolute vorticity, CAPE, surface pressure, 10-m cross-frontal wind shifts and the 2-m temperature gradient. Preliminary results show 500-m absolute vorticity coinciding with narrow frontal boundaries and a sharp cross-frontal wind shift is structured into equally spaced misovortices, each connected by a strand of weaker vorticity. However, broader frontal bands with weaker cross-frontal wind shifts have a less distinctive vorticity structure, with sporadic isolated misovortices within the frontal band. To investigate this further, the vertical structure of the misocyclones is assessed to determine the differences in structures between events and how current theories of tornadogenesis can be applied.

Keywords: Tornadogenesis, tornadoes, quasi-linear convective systems, misovortices, WRF modelling, UK.