A diagnostic study of a severe wind event associated with a cold front is presented based on output from an operational numerical weather prediction model together with radar and satellite imagery and detailed surface observations. An intrusion of stratospheric air, initially with a potential vorticity of 2 PV units, is shown, as part of a backward-tilted filament, to have plunged down to the lower troposphere where it was associated with a narrow cold-frontal rainband at its leading edge and produced severe wind gusts (up to 70 knots). A close correspondence in time and space is revealed between these surface events and the appearance in the model of a pocket of diluted stratospheric air (with PV = 1) at the 2 km level. The stratospheric intrusion produced two distinguishable wind surges: one behind the narrow cold-frontal rainband and another, mainly dry, about 200 km farther behind. The strength of the winds was attributable to the downward transport of momentum as part of the large-scale dynamics; local moist processes played only a minor role. At coarse resolution the radar echo from the narrow cold-frontal rainband resembled line convection but it is shown that there were significant differences between this event and the archetypal form of line convection often seen in the British Isles.
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