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The impact of climate change on weather systems, extreme rainfall and flood episodes in the UK

Dr Cathryn Birch (SEE), Dr Mark Trigg (Civil Engineering), Dr Alan Gadian (SEE), Dr Ralph Burton (NCAS)

Project partner(s): Thomas Mackay Ltd. (Environmental Consultant)

Contact email: M.Trigg@leeds.ac.uk

Recent research suggests that global anthropogenic greenhouse gas emissions have already increased the frequency and severity of UK flooding (Pall et al. 2011; Schaller et al. 2014). Future climate change is expected to increase extreme rainfall in the UK and a recent parliamentary report has determined flooding to be the most significant climate-related risk to the UK (ConCC, 2015). This project will understand changes in weather systems associated with extreme rainfall over the United Kingdom under future climate change and assess the impact on future flood episodes.

Flooding is often the direct response to high-intensity and/or long-lived rainfall episodes (Figure 1). Traditional climate models struggle to simulate this type of rainfall because they cannot represent the processes controlling the triggering, organisation and maintenance of storms, particularly those associated with summertime convection (i.e. thunderstorms). Understanding these weather systems and predicting how they may change in the future remains a grand challenge in atmospheric science (Fowler and Ekstrom, 2009; Flato et al. 2013).

Figure 1 Wintertime flooding in central Leeds on December 26th 2016 (left) and summertime flooding in the Calder Valley of the northern Pennines in July 2012 (right). Source: ITV News.

High-resolution regional climate models, where some of the key atmospheric processes such as convection are permitted to develop explicitly, without the need for parameterisation, offer a new solution to this long-standing problem (Prein et al. 2015). This type of model is currently used routinely for numerical weather prediction for the UK and recently we have gained the computational capacity to run these models for time periods useful for climate projections. A recent study by Kendon et al. (2014) demonstrated that these state-of-the-art models provide a step-change in simulating realistic rainfall extremes, particularly in the summer months when the rainfall is driven more by convective weather (Figure 2).

Figure 2: Heavy rainfall on hourly timescales (mm/hr) in summer, for standard resolution (12km) and state-of-the-art high resolution (1.5km) climate model simulations. Top panels show the difference between model and observed radar heavy rainfall and bottom panels shown the difference between 2100 and present-day heavy rainfall. The traditional (12km) climate model fails to predict future increases in summertime heavy rainfall, whereas the high-resolution model (1.5km) predicts significant increases. Regions with statistical significance are indicated by the coloured regions. Taken from Kendon et al. (2014).

The Pennine hills of northern England are particularly prone to flood events due to the relatively wet climate and steep sided valleys, which channel water very rapidly into the valley bottom, causing river levels to rise sharply. The floods in the northern UK in December 2015 associated with Storm Desmond, Eva and Frank were devastating, with early estimates putting damage costs at £5 billion. On 26th December 2015, rainfall rates of more than 60mm in 24 hours caused the River Calder to reach its highest ever levels, causing severe flooding when the river burst its banks (fluvial flooding). Summertime flooding is also common in the United Kingdom; in 2012 heavy, often convective rainfall, caused flood damage to 4500 properties and £500 million of damage nationwide. The Calder Valley was affected by both fluvial and surface water (pluvial) flooding) during this very wet summer, where more than 900 homes and businesses were flooded. This project will use the Pennine region of northern England as a focus study area and will engage with local stakeholders such as Environmental consultants, Councils, Yorkshire Water and the Environment Agency.     

Objectives

  • Utilise state-of-the-art climate model simulations (Figure 3) to assess how extreme rainfall events may change in the future under warming due to greenhouse gases

  • Assess the mechanisms behind the changes in rainfall events. What type of weather causes severe rainfall events and how may the severity and frequency of these weather patterns change in the future?  

  • Evaluate the ability of the climate model to reproduce realistic flood-producing severe weather (intensity, duration, frequency and seasonality) using ground-based radar, surface meteorological and rain gauge observations

  • Quantify the benefits of using high-resolution climate model simulations compared to traditional climate models

  • Use existing hydrology and hydraulic models to assess how the extent, severity and probability of flood episodes in the Northern Pennines will change in response to future changes in extreme rainfall

  • Utilise newly acquired river gauge measurements from the Calderdale river catchment to evaluate the hydrological and hydraulic models

  • Work with industrial partners to ensure the catchment and river processes are represented correctly in the hydrology models and identify how project outcomes could translate into the flood risk dialogue that is ongoing within the catchments.

Figure 3: The white box illustrates the inner model domain of the state-of-the-art high resolution Weather Research and Forecasting (WRF) simulations created by the WISER research project. Data from this domain will be used in this PhD studentship.

Training

The student will receive a high-level of training in (1) observational data sets to understand atmospheric processes, (2) a computer programming language (e.g. Python) to perform complex analysis techniques, (3) running atmospheric and hydrology numerical models on supercomputers, (4) effective written and oral communication skills. There is potential for travel to the United States to visit the National Centre for Atmospheric Research (NCAR), who are conducting parallel work on the changing patterns of rainfall in the US. The student will have the opportunity to be involved in the water@leeds research centre within the University.

The student will work under the main supervision of Dr. Cathryn Birch, who is an Academic Research Fellow specialising in meteorology and climate modelling within the Atmospheric and Cloud Dynamics group in the Institute for Climate and Atmospheric Science (ICAS) at the University of Leeds. Dr Mark Trigg, an expert in hydrology, flood hazard and water related risk in the School of Civil Engineering, will co-supervise the project and provide links to industry flood models and local partners within the catchments. Dr Alan Gadian and Dr Ralph Burton work within the National Centre for Atmospheric Science, based at the University of Leeds and will co-supervise the project by contributing their knowledge of atmospheric science and the model experiments provided for the project.

Student profile

The student should have a strong interest in environmental problems related to climate change, a strong background in a quantitative science (math, physics, engineering, environmental sciences) and a flair for, or a good familiarity with, programming and scientific computing.

Links with industry

Through the Environment Agency and Consultancy Partners the student will have access to industry certified flood hazard models for the river systems that can be used to downscale the outputs from the high resolution weather model and assess flood change implications. The School of Civil Engineering have the ability to modify and run these models if bespoke scenarios are required for this research. Industry contacts will also be able to provide expert input to the project in terms of catchment/system understanding and also the translation of the findings into practice. In particular, Thomas Mackay Ltd have offered to provide in-kind time commitment to support this project as part of their continued interest in addressing wider flood management challenges within the region.

References and further reading

  • Chan, S. C. et al. (2015) Downturn in scaling of UK extreme rainfall with temperature for future hottest days, Nature Geoscience, doi:10.1038/NGEO2596.

  • ConCC (2015) Progress in preparing for climate change, Report to Parliament, Committee on Climate Change, June 2015.

  • Flato, G. et al. (2013) Evaluation of Climate Models. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

  • Fowler, H. J. and Ekstrom, M. (2009) Multi-model ensemble estimate of climate change impacts on UK seasonal precipitation extremes. Int. J. Clim., 29, 385–416, doi:10.1002/joc.1827.

  • Kay, A. L. et al. (2015) Use of very high resolution climate model data for hydrological modelling: baseline performance and future flood changes, Climatic Change, doi:10.1007/s10584-015-1455-6.

  • Kendon, E. J. et al. (2014) Heavier summer downpours with climate change revealed by weather forecast resolution model. Nature Climate Change, 4, 570–576, doi:10.1038/nclimate2258.

  • Pall, P. et al. (2011) Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000, Nature, 470, 382-385, doi:10.1038/nature09762.

  • Prein, A. F. et al. (2015) A review on regional convection-permitting climate modeling: Demonstrations, prospects, and challenges, Rev. Geophys., 53, 323–361. doi:10.1002/2014RG000475.

  • Schaller, N. et al. (2014) Human influence on climate in the 2014 southern England winter floods and their impacts, Nature Climate Change, 6, doi:10.1038/NCLIMATE2927.

Related undergraduate subjects:

  • Engineering
  • Environmental science
  • Geoscience
  • Hydrology
  • Meteorology
  • Physics