Synoptic-Scale Environments and Precipitation Morphologies of Tornado Outbreaks from Quasi-Linear Convective Systems in the United Kingdom

Abstract

Nine tornado outbreaks (days with three or more tornadoes) have occurred in the United Kingdom from quasi-linear convective systems (QLCSs) in the 16 years between 2004 and 2019. Of the nine outbreaks, eight can be classified into two synoptic categories: type 1 and type 2. Synoptic categories are derived from the location of the parent extratropical cyclone and the orientation of the surface front associated with the QLCS. Environmental differences between the categories are assessed using ERA5 reanalysis data. Type 1 events are characterized by a confluent 500-hPa trough from the west, meridional cold front, strong cross-frontal wind veer (about 90°), cross-frontal temperature decrease of 2°–4°C, prefrontal 2-m dewpoint temperatures of 12°–14°C, a prefrontal low-level jet, and prefrontal 0–1- and 0–3-km bulk shears of 15 and 25 m s−1, respectively. In contrast, type 2 events are characterized by a diffluent 500-hPa trough from the northwest, zonal front, weaker cross-frontal wind veer (≤45°), much smaller cross-frontal temperature decrease, lower prefrontal 2-m dewpoint temperatures of 6°–10°C, and weaker prefrontal 0–1- and 0–3-km bulk shears of 10 and 15 m s−1, respectively. Analysis of the Met Office radar reflectivity mosaics revealed that narrow cold-frontal rainbands developed in all type 1 events and subsequently displayed precipitation core-and-gap structures. Conversely, type 2 events did not develop narrow cold-frontal rainbands, although precipitation cores developed sporadically within the wide cold-frontal rainband. Type 1 events produced tornadoes 2–4 h after core-and-gap development, whereas type 2 events produced tornadoes within 1 h of forming cores and gaps. All events produced tornadoes during a relatively short time period (1–3 h).A total of 22% of tornado outbreaks (days with 3 or more tornadoes) in the United Kingdom occur with convective storms where heavy precipitation occurs in a line over 100 km long. Such outbreaks fall into two types. The first is a south–north-oriented cold front with a strong wind shift, producing tornadoes within 2–4 h after a characteristic precipitation structure called cores and gaps develops. The second is a west–east-oriented front with a weaker wind shift, producing tornadoes within 1 h after core-and-gap formation. These results point the way toward better understanding of the conditions under which tornadoes form in the United Kingdom, and they suggest that future studies may show that the mechanisms producing tornadoes differ between these two types of storms.