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The role of the land surface in future climate projections of extreme precipitation and heatwaves

Dr Cathryn Birch (SEE), Prof Doug Parker (SEE), Dr Giorgia Fosser (Met Office), and Dr Luis Garcia-Carreras (University of Manchester)

Project partner(s): Met Office (CASE)

Contact email: c.e.birch@leeds.ac.uk

This project capitalises on state-of-the-art climate model projections to assess the role of the land-atmosphere interaction in UK high-impact precipitation and heatwave events and to understand how this interaction may change and impact the future climate.

The magnitude and impact of UK climate extremes such as heavy rainfall, drought and heatwaves are likely to increase under climate change. Flooding associated with summertime convective rainfall (thunderstorms) is common; in 2012 heavy convective rainfall caused flood damage to 4500 properties and £500 million of damage nationwide. More than 900 homes and businesses were flooded in the Calder Valley in West Yorkshire (Figure 1).

Figure 1 Rainfall is enhanced over the UK upland areas (left, Photograph: Getty), which was a cause of summertime flooding in the Calder Valley of the northern Pennines in July 2012 (right, Photograph: ITV News).

The land surface and its interaction with the lower atmosphere have a strong influence on our climate:

  • Soil moisture and vegetation modify the surface energy budget, which controls the amount of heat and moisture entering the lowest levels in the atmosphere. Heat and moisture are key ingredients for atmospheric turbulence and convection, which ultimately control rainfall and near-surface temperatures.
  • Evaporation from the surface is a source of moisture for rainfall. Feedbacks between reduced soil moisture and minimal rainfall can occur, exacerbating drought conditions (Figure 2). In addition, rain that falls on already saturated soil runs quickly and directly into low-lying areas and river systems, increasing the likelihood of flooding.
  • Mountains force air masses to rise, cool and condense, forming clouds and precipitation. The devastating 2015 Boxing Day floods in Northern England were enhanced by the region’s upland areas.

Figure 2 Water levels were still alarmingly low in the Scammonden Reservoir, West Yorkshire, in autumn 2003 following a drought in the summer (left, Photograph: Guardian) and dry weather in Spring 2017 lead to dry reservoirs and agricultural land (right, Photograph: Netweather).

 Traditional climate models are too coarse to represent the complex coastal, mountainous and urban areas of the UK. They also struggle to simulate the processes controlling the triggering, organisation and maintenance of storms, particularly those associated with summertime convection. We have recently gained the computational capacity to run a new generation of high-resolution, kilometre-scale regional climate models that are better able to resolve these processes.  A recent study by Kendon et al. (2014) demonstrated that these models, called convection-permitting 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). The Met Office UK Climate Projection 2018 (UKCP18) project will provide the first ever ensemble of convection-permitting climate simulations, which this PhD project will have the possibility to exploit.

Figure 3: 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 UKCP18 simulations will be used by the student to understand how the surface energy budget evolves under climate change, and the impact on convective processes and temperature and rainfall extremes. The convection-permitting model’s ability to represent feedbacks between the surface and the atmosphere will be evaluated against observations and compared with coarser resolution climate models to investigate the added value of kilometre-scale models. The analysis will allow an assessment of the uncertainties related to future projections as well as the sensitivity of the land-atmosphere interaction to climate change.

The PhD project will capitalise on the ability of convection-permitting models to better resolve surface features such as cities and mountains. The climate in cities can differ considerably from that in rural areas only a short distance away and it is, therefore, important to provide reliable tailored climate projections for cities. The UK’s upland areas are known to enhance extreme precipitation events. The ability of the climate model simulations to capture land-surface interaction over cities and/or mountains will be assessed and the simulations will be used to understand how temperature, convective storms and rainfall rates may change in these regions in the future.

Objectives

There are a number of ways the project could evolve, according to the particular research interests of the student. These could include:

  • Investigate how land-atmosphere interactions impact the characteristics of the lower atmosphere in which we live, particularly in relation to cloud and storm formation, rainfall and near-surface temperature.
  • Analyse how land-atmosphere interactions may change under climate change and how this will impact UK heatwave, drought and/or extreme rainfall events.
  • Understand how and why extreme rainfall associated with the UK’s mountainous terrain may change in future climate, capitalising on the more detailed representation of land elevation in the UKCP18 climate model.
  • Evaluation of land-atmosphere interactions in the state-of-the-art UKCP18 climate model simulations and comparison with coarser climate models.
  • Comparison of model simulations with observational data from surface flux stations, satellite remote sensing and research aircraft flights.
  • Conduct additional climate model simulations to test certain physical hypotheses, for example to explore the relative importance of enhanced resolution and improved model physics.

Potential for high impact outcome

The UKCP18 project will provide the next set of UK climate projections to inform the UK’s 3rd national Climate Change Risk Assessment (CCRA). Land-atmosphere interaction is an important driver for extreme precipitation and is believed to have a crucial impact on the climate change signal. The student’s work will help gain a deeper understanding of the climate change signal and the underlying causes, and hence uncertainties in the UK projections used for CCRA. The Met Office Hadley Centre provides advice on the impacts of climate change to policymakers, providing a direct link to end users of climate data.

The social science community working on climate impacts will also benefit from the project. Suraje Dessai (Professor of Climate Change Adaptation at the University of Leeds) leads a project using UKCP18 projections to develop climate change adaptation strategies. Collaboration with Prof. Dessai’s group and project partners will provide opportunities for multidisciplinary knowledge exchange.

Student profile

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

Training

The student will work under the supervision of Dr Cathryn Birch and Prof Doug Parker within the Atmospheric Dynamics and Cloud group in the School of Earth and Environment at the University of Leeds. This project will build on the group’s expertise in land-atmosphere interaction, extreme weather and climate modelling research. The student will have access to pre-existing state-of-the-art simulations, which makes the project relatively low risk.

The student will receive a high-level of training in:

(1)   dynamical meteorology and climate science

(2)   the use of state-of-the-art climate model and observational data sets to understand atmospheric processes

(3) research skills in an inter-disciplinary environment

(4) a computer programming language (e.g. Python)

(5) effective written and oral communication skills.

The Atmospheric Dynamics and Cloud group is involved in a number of related research projects such as the Yorkshire Integrated Catchment Solutions Programme (iCASP) and Vegetation Effects on Rainfall in West Africa (VERA) projects. There is potential for the student to be involved in these projects and to engage with university, industrial and community groups. There will be the opportunity for international travel to conferences and for research visits to other universities, such as Monash University in Melbourne, Australia.

Met Office Partnership

The student will be jointly supervised by Dr Giorgia Fosser, a senior scientist at the Met Office, who researches UK heavy precipitation and its drivers. She is responsible for producing and analysing the climate model simulations that will be used in this study.

The project has a CASE award, which provides funding in addition to the NERC research funding. The student will spend periods of time (perhaps 4-6 weeks per year) working at the Met Office in Exeter, where the Met Office will provide model data, model expertise and guidance on analysing the simulations.

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.
  • 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.
  • 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.
  • Thompson, V. et al. (2017) High risk of unprecedented UK rainfall in the current climate, Nature Comms, 8, DOI:10.1038/s41467-017-00275-3.
  • Taylor, C. M. et al. (2017) Frequency of extreme Sahelian storms tripled since 1982 in satellite observations, Nature, 544, 475-478, doi:10.1038/nature22069.

Related undergraduate subjects:

  • Applied mathematics
  • Atmospheric science
  • Computer science
  • Environmental science
  • Geography
  • Geophysical science
  • Hydrology
  • Mathematics
  • Meteorology
  • Physical geography
  • Physics
  • Statistics