Doppler radar observations of non‐occluding, cyclic vortex genesis within a long‐lived tornadic storm over southern England

Abstract

AbstractOn 3 November 2009 a long‐lived tornadic storm developed close to the apex of a small frontal wave moving eastwards over southern England. Ten tornadoes and several instances of non‐tornadic wind damage occurred along a narrow path extending for over 120 km. The storm passed within 5 km of two of the radars in the Met Office's network, providing an opportunity for detailed observation. Initial development was in the form of an intense segment of line convection at the northern flank of the frontal wave and associated deepening surface mesolow (diameter 60–70 km). The line evolved rapidly; the southern portion bowed out to form a transient ‘S’‐shaped echo before dissipating. The northern section persisted, leaving a hook‐shaped storm which became quasi‐steady in overall echo shape.Radars revealed the cyclic development of five discrete low‐level mesovortices (diameter 1–4 km) between 1130 and 1330 UTC, along a zone of intense cyclonic vertical vorticity at the southeastern edge of the storm. These vortices intensified rapidly (typically from initial detection to maximum observed differential velocity in ∼10 min), some attaining low‐level differential velocities exceeding 40 m s−1. However, significant differential velocities (>20 m s−1) were not observed above 2.5 km above ground level, and no persistent mid‐level mesocyclone was present. The cyclic behaviour was unusual in that vortices were ‘non‐occluding’, moving southeastwards relative to the storm (to the right of the storm motion vector), away from the core. This is in contrast to the more commonly observed evolution of vortex occlusion (by storm downdraughts) and subsequent leftward and rearward storm‐relative movement during decay. Copyright © 2011 British Crown copyright, the Met Office. Published by JohnWiley & Sons Ltd.