The Dynamics of the Cold Conveyor Belt and Sting Jet in an Extratropical Cyclone and the Influence of Diabatic Processes

This thesis examines the influences on the development of high winds in a case of a damaging extratropical cyclone. Extratropical cyclones are a regular occurrence over the United Kingdom (UK) and much of western Europe: strong cyclones can cause major damage and disruption through high winds, flooding and wave damage. Cyclone Friedhelm occurred on the 7th-8th December 2011, and was observed as part of the "DIAbatic Influence on Mesoscale features in ExTratropical Storms" (DIAMET) field campaign. A case of explosive cyclogenesis, Cyclone Friedhelm exhibited a deepening of 44 hPa in 24 hours, almost double that necessary to be considered a meteorological "bomb". As the storm passed over the UK, strong winds caused widespread damage and disruption, particularly across Scotland. These high winds and their influences are the focus of this thesis, in particular the high-wind region to the south-west of the cyclone centre, in which two high wind phenomena can occur, the cold-conveyor belt (CCB) and sting jets (SJ). The Weather Research and Forecasting (WRF) model is used with a high-resolution setup to investigate the mesoscale wind features, while a lower-resolution configuration is employed to examine the large-scale storm development. The high-resolution simulation is compared to routine observations, and those made specifically as part of the DIAMET project. The WRF model is shown to recreate the storm with a good degree of accuracy. The dynamics of the high wind regions are investigated in order to identify the features and the processes that contribute to their evolution. Trajectories are used to identify parcel ensembles resembling both the SJ and CCB. The effects of forces and balanced flows are discussed and a novel way to calculate the gradient flow is presented. The results of this analysis show a systematic region of super-gradient flow to the south and south-west of the cyclone centre and a region of sub-gradient flow to the north. The SJ parcels are analysed in relation to the past literature including a number of different theories on the cause of the SJ’s descent. Unlike some theories on SJs, latent cooling is shown to have little effect on the wind field, although it does change trajectory evolution. The lower-resolution setup shows that latent heating is key to the deepening of the cyclone, but little change in the track is observed when it is removed. The low-level PV anomaly is shown to be key in driving the deepening of the storm. Varying the time during the simulation at which latent heating is switched off shows the importance of the cyclone’s deepening rate on the wind speed development. Other novel analyses are conducted in order to examine the relationships between circulation and the high winds, showing a strong relationship between the circulation energy and cyclone pressure.