Doppler radar observations of mesovortices within a cool-season tornadic squall line over the UK

On 7 December 2006 a tornado of T5 (F2) intensity struck northwest London injuring six people and damaging over 200 houses. In this paper, the squall line responsible for the tornado is analysed using reflectivity and radial wind data obtained from Met Office Doppler radars. The environment of the squall line is also analysed using available observations. The squall line developed in a weakly unstable polar maritime airmass to the rear of an active cold front and south of a deep depression moving eastwards over northern Scotland. The tornado was associated with a strong vortex of approximately 2–4 km diameter which formed along the leading edge of a bowing segment embedded within the squall line. The tornado occurred close to the time of maximum vortex strength, which followed a short period of rapid vortex intensification.This vortex was one of the strongest, and the longest-lived, of a number of mesovortices traceable in the Doppler radial velocity field for periods of up to 75 min. Mature vortices were often associated with characteristic ‘broken-S’ signatures in the reflectivity field, with inflow notches to their north and localised line-bowing to the south. Velocity couplets were also observed in the radial velocity field, especially when vortices were located within 50 km range of the radar. The presence of cyclonic and anti-cyclonic vortex pairs early in the evolution of at least one cyclonic vortex, straddling a small bulging section in the leading edge of the line, suggests that localised tilting of the ambient horizontal vorticity was the most likely vortex-genesis mechanism.The analysis reveals that individual vortices developed and evolved rapidly, suggesting that in similar cases, the issuance of tornado warnings based on the detection of individual vortices may result in limited success. However, a tendency for stronger and longer-lived vortices to form along and north of the bowing line segment was observed. In similar cases this could prove useful for identification of sections within a given squall line most prone to stronger mesovortex development, with their attendant risk of localised damaging winds.

Research Highlights
► Mesovortices observed by Doppler radar within a squall line over southeast England.
► Longest-lived vortices tended to occur along a bowing segment within the line.
► One vortex produced a tornado and other significant wind damage over London.
► A counter-rotating vortex pair was observed in the early stages of this vortex.
► Suggests tilting of horizontal vorticity was the likely vortex-genesis mechanism.