Synoptic-scale and mesoscale controls for tornadogenesis on cold fronts: A generalised measure of tornado risk and identification of synoptic types

Abstract

Environments of tornadic and non-tornadic narrow cold-frontal rain bands (NCFRs) are investigated using ERA-Interim reanalyses for a sample of 114 events over the United Kingdom and Ireland (44 tornadic). The results offer a practical tool for prediction of the likelihood of tornadoes in these potentially high-impact events. Of 22 analysed parameters, a bulk measure of shear vorticity, and the front-normal wind component on the cold side of the front, yield the best discrimination between event classes, showing significantly larger values in tornadic events. A generalised measure of tornado probability, p[TN], is obtained using the distribution of points within the two-dimensional parameter space defined by these parameters. Synoptic situations commonly associated with tornadic NCFRs are identified and conceptual models describing the large-scale evolution are developed. Most events are associated with developing secondary cyclones (i.e., frontal waves) along trailing cold fronts (≥54.5%), generally within west to southwesterly large-scale flow. Another significant class of event corresponds to situations where a strong mid- to upper-level jet streak cuts across the front within an amplifying large-scale flow pattern (upstream ridge building and downstream trough extension), generally within northwesterly flow (27.3%). In frontal waves, tornadoes occurred relatively early in the wave's development and just down-front of the wave centre, where rapid increases in p[TN] occurred as the wave amplified. In northwesterly flow cases, tornadoes occurred along a well-defined NCFR bulge close to where the mid- to upper-level jet streak and an associated positive potential-vorticity anomaly intersected the front. Analysis of a high-tornadic subset of tornadic events (NCFRs producing ≥7 tornadoes) revealed an even stronger association with frontal waves (72.2% of cases), suggesting that the highest-impact events are usually associated with secondary cyclogenesis. The possible relevance of identified environmental parameters to candidate vortex-genesis and tornadogenesis mechanisms within NCFRs and quasi-linear convective systems is discussed.