Synoptic-scale and mesoscale controls for tornadogenesis on cold fronts

This thesis addresses the origins, dynamics and forecasting of tornadoes in narrow cold-frontal rainbands (NCFRs). A review of current understanding is undertaken, which is set in context by comparison with the situation for tornadoes in supercell thunderstorms.

Environments of 114 tornadic and non-tornadic NCFRs in the UK and Ireland are explored using ECMWF reanalysis data. A generalised measure of tornado probability is obtained using the distribution of points within the parameter space defined by the two environmental parameters exhibiting the best discrimination between event classes.

Synoptic situations commonly associated with tornadic NCFRs are identified and conceptual models are developed for each. Two distinct types are defined. Firstly, those associated with developing secondary cyclones (i.e., frontal waves), which account for over half of all tornadic cases. Secondly, those associated with upper-level jet streaks cutting across the front within amplifying large-scale flow patterns, at the rear flank of a longwave trough (so-called ‘north-westerly flow’ events), which account for just over one-quarter of all tornadic cases.

For one example of each type, further analysis is undertaken using convection-permitting models and observations. Physical links are found across a wide range of spatiotemporal scales in both events. In the frontal wave case, changes in the relative magnitudes of vertical vorticity and horizontal strain at the front favoured meso-ɣ- to miso-scale vortex-genesis, likely by release of horizontal shearing instability, just down-front of the wave centre in the early stages of secondary cyclogenesis. In the north-westerly flow case, although the front was generally weaker, interaction of an upper-level jet streak and tropopause fold with the surface front’s transverse circulation initiated a sequence of events that culminated in locally tornado-favourable conditions at the surface front, near the left exit of the cross-cutting jet.

Finally, results are reviewed and compared with existing cyclone paradigms and conceptual models of cold fronts, and directions for further study are outlined.