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The UK Wind Regime - Observational Trends and Extreme Event Analysis and Modelling

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The UK wind regime - Observational trends and extreme event analysis and modelling Nick Earl This thesis is submitted in fulfilment of the requirements for the degree of Doctor of Philosophy at the University of East Anglia. School of Environmental Sciences June 2013 © This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that use of any information derived there from must be in accordance with current UK Copyright Law. In addition, any quotation or extract must include full attribution. 2 The UK wind regime - Observational trends and extreme event analysis and modelling Abstract The UK has one of the most variable wind climates; NW Europe as a whole is a challenging region for forecast- and climate-modelling alike. In Europe, strong winds within extra-tropical cyclones (ETCs) remain on average the most economically significant weather peril when averaged over multiple years, so an understanding how ETCs cause extreme surface winds and how these extremes vary over time is crucial. An assessment of the 1980-2010 UK wind regime is presented based on a unique 40- station network of 10m hourly mean windspeed and daily maximum gustspeed (DMGS) surface station measurements. The regime is assessed, in the context of longer- and larger-scale wind variability, in terms of temporal trends, seasonality, spatial variation, distribution and extremes. Annual mean windspeed ranged from 4.4 to 5.4 ms-1 (a 22% difference) with 2010 recording the lowest annual network mean windspeed over the period, attracting the attention of the insurance and wind energy sectors, both highly exposed to windspeed variations. A short subjective climatology (2008-2010) is developed of the ETCs and their sub- storm features which are associated with the strongest DMGSs. The little studied UK Quasi-linear convective systems (QLCSs) and pseudo-QLCSs are found to account for 22% of the top 1% of DMGSs, with the better known Sting Jet accounting for at most 5%. This same climatology of 2008-10 ETCs then forms the basis of performance assessments of global forecast ensemble systems. At T+48, an ensemble consisting of just the ECMWF and Canadian EPS members (total-70) is found to capture the same set of extreme events as an ensemble consisting of nine global centres (157-239) highlighting the value of using model physics perturbations at this range. A prominent ETC, Emma, then forms the basis of a high-resolution model sensitivity analysis using the Weather Research and Forecasting model. Surface wind simulations display greater sensitivity to different cloud microphysics schemes and to horizontal resolution than to vertical resolution, the former highlighting the importance of diabatic processes within extreme European ETCs. Acknowledgements 3 Acknowledgements This project was carried out under funding from the Worshipful Company of Insurers, at the University of East Anglia, under the supervision of Drs Stephen Dorling and Roland von Glasow. For the provision of observational wind data thanks are due to the UK Met Office, British Atmospheric Data Centre and Richard Hewston. Richard Hewston also for helping me get going with it and continued support throughout my PhD. Thanks must go to Chris Collins and Julie Harold for useful and continuous IT support, instrumental in getting the WRF model running at UEA, along with their consistent and excellent support of the UEA cluster. Thanks to Rob Neal at the UK Met Office for kindly supplying me with synoptic surface pressure charts and informing me of any interesting extra-tropical cyclone examples of interest in the operational forecasting department. Thanks goes to Chris Steele for helping me out with an awkward figure on Christmas Eve, also consistently being available to discuss anything meteorological. Andy Elvidge must be credited, always able to put a smile on my face and whose morale boosting ridiculous antics will live long in the memory. My housemates over the years deserve a mention, Tara, Amy, Webber and Julia, able to put up with me and my muddy shoes, along with Walker-Brown, Ninja and the witches for the tea breaks. I thank my running buddies Josh, Rich, Dazzer, Beaney, Glenbo and Siberia for getting me out of the office for lovely countryside runs on would be stressful days, and the rest of UEA Athletics club members for going on amazing trips with me all over the country. My fellow PhD students and office mates must be credited for their constant support, help and morale boosting games of golf and table tennis. Special thanks goes to Steve, my primary supervisor, for his constant support throughout the project. I greatly appreciate his advice and his ability to understand when I needed time to get to grips with things. Thanks must also be given for numerous free lunches and Weatherquest Christmas dinners over the years! 4 The UK wind regime - Observational trends and extreme event analysis and modelling Contents Abstract .................................................................................................................... 2 Acknowledgements .................................................................................................. 3 Contents ................................................................................................................... 4 List of Figures ........................................................................................................... 6 List of Tables .......................................................................................................... 12 Chapter 1 ........................................................................................................................ 13 Introduction ............................................................................................................... 13 1.1 Wind in the UK and Europe ............................................................................. 14 1.2 Wind climate fluctuations in the NE Atlantic region ....................................... 20 1.3 Surface observations of mesoscale ETC features ............................................ 23 1.4 Global forecasting models ............................................................................... 28 1.5 Atmospheric modelling .................................................................................... 32 1.6 Summary and research aims ............................................................................ 38 Chapter 2 ........................................................................................................................ 42 Data, methods and tools ............................................................................................ 42 2.1 Updating the observed Wind Data, data quality and applications ................. 42 2.2 The TIGGE Database......................................................................................... 52 2.3 The WRF modeling system ............................................................................... 60 2.4 Summary .......................................................................................................... 67 Chapter 3 ........................................................................................................................ 68 1980-2010 variability in UK surface wind climate ..................................................... 68 3.1 Inter-annual variability ..................................................................................... 68 3.2 NAO – driver of temporal wind climate variations .......................................... 80 3.3 Intra-annual variability ..................................................................................... 83 3.4 Spatial variability .............................................................................................. 87 3.5 Application of the Weibull function to describe windspeed distributions ...... 92 Contents 5 3.6 Wind energy implications ................................................................................. 95 3.7 Summary .......................................................................................................... 98 Chapter 4 ...................................................................................................................... 100 Sub-storm features associated with extreme surface gusts in UK ETC events ........ 100 4.1 Extreme windstorm events ............................................................................ 101 4.2 Sub-storm mechanisms .................................................................................. 112 4.3 2008-2010 provisional-climatology for sub-storm mechanisms ................... 124 4.4 Top 0.1% DMGSs ............................................................................................ 131 4.5 Summary ........................................................................................................ 135 Chapter 5 ...................................................................................................................... 137 ETC global forecast model comparison and high resolution modelling .................. 137 5.1 ECMWF analysis vs surface observations and TIGGE forecast quality .......... 137 5.2 Extreme DMGSs represented by TIGGE ......................................................... 151 5.3 Added value using the TIGGE? ....................................................................... 154 5.4 Sub-storm mechanism forecasts ...................................................................
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