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Attribution of strong winds to a cold conveyor belt and sting jet

A high-resolution version of the WRF (Weather Research and Forecasting) model has been used to study the fine structure of a cloud head and its associated cold conveyor belt jet (CJ) and sting jet (SJ) in an intense extratropical cyclone that produced damaging surface winds in northern Ireland and central Scotland on 3 January 2012. The model was run with many different initialisation times and physical parametrisations, and a run was selected that verified well against a variety of observations. New methods have been devised to visualise the 3D structure of the CJ and SJ and to attribute strong surface winds to one or other of them, and the validity of regarding the SJ as a semi-Lagrangian feature has been assessed. The model suggests that, whereas the CJ remained mainly below the 850 hPa level as it circulated around the bent-back front, the SJ consisted of a stream or streams of air within the bent-back frontal zone that first ascended from close to the surface into the middle and upper-level parts of the cloud head before descending from evaporating cloud filaments at the tip of the cloud head and reaching the top of the boundary layer slightly ahead of the CJ. The simulations did not support the idea that either the evaporation or conditional symmetric instability (CSI) played a major role in the development of these jets. The strong gusts (up to 47 m s−1) which were recorded on the north coast of Ireland appear to have been due mainly to the CJ, which by then was undercutting the SJ. The SJ was responsible for stronger surface winds than the CJ several hours earlier during the initial stage of frontal fracture, but only for a limited period.