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Intensification of short-duration rainfall extremes and implications for flash flood risks

Discussion meeting

Starts:

February
032020

09:00

Ends:

February
042020

17:00

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Scientific discussion meeting organised by Professor Hayley Fowler.

Pedestrians on Dean Street, City Centre, Newcastle upon Tyne. Photo from the 28 June 2012 "Toon Monsoon" event in Newcastle upon Tyne, UK, where there was almost 50mm rainfall in 90 mins. Credit: ncjMedia Ltd, Paul Norris.

Extreme heavy precipitation is increasing in frequency and intensity, and results from new global sub-daily datasets and high-resolution modelling suggest that precipitation extremes will intensify more than anticipated based upon theoretical considerations. Understanding this process is vital in advancing the theoretical knowledge necessary for a step change in regional prediction from understanding the nature and drivers of precipitation extremes.

Speaker biographies and abstracts are available below. Recorded audio of the presentations is also available below. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

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Event organisers

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Schedule of talks

03 February

09:00-12:30

Session 1

6 talks Show detail Hide detail

Chairs

Professor Gabriele Hegerl FRS, University of Edinburgh, UK

09:05-09:30 UK climate change adaptation and the climate change risk assessment process

The Baroness Brown of Cambridge DBE FREng FRS, Committee on Climate Change, UK

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09:30-10:00 The INTENSE project: using observations and models to understand the past, present and future of sub-daily rainfall extremes

Professor Hayley Fowler, Newcastle University, UK

Abstract

Extremes of precipitation cause flooding and droughts which can lead to substantial damages to infrastructure and ecosystems and can result in loss of life. Historical in situ sub-daily rainfall observations are essential for the understanding of short-duration rainfall extremes but records are typically not readily-accessible and data are often subject to errors and inhomogeneities. Furthermore, these events are poorly quantified in projections of future climate change, making adaptation to the risk of flash flooding problematic. Consequently, knowledge of the processes contributing to intense, short-duration rainfall is less complete compared with those on daily timescales. The INTENSE project, part of the World Climate Research Programme (WCRP)'s Grand Challenge on 'Understanding and Predicting Weather and Climate Extremes', has used a novel and fully-integrated data-modelling approach to provide a step-change in our understanding of the nature and drivers of global sub-daily rainfall extremes and change on societally relevant timescales. This talk will present findings from the project on the intensification of sub-daily extremes and the role of local-scale thermodynamics and large-scale atmospheric conditions in driving these changes.

 

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10:00-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 The physical basis for changes in precipitation extremes at different durations

Professor Paul O'Gorman, Massachusetts Institute of Technology, USA

Abstract

Extreme precipitation is predicted to increase in intensity with global warming, but the projected rate of increase varies across different regions of the world and across seasons. In this talk, Professor O'Gorman will discuss the physical factors that contribute to changes in extremes precipitation and whether they differ between subdaily and daily extremes. Professor O'Gorman will also discuss results from idealized high-resolution models and the use of machine learning to improve simulation of extreme precipitation in global climate models.

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11:30-12:00 Linking increases in hourly precipitation extremes to atmospheric temperature and moisture changes

Dr Geert Lenderink , KNMI and Technical University Delft, The Netherlands

Abstract

While summer rain storms are very intermittent, chaotic and influenced by multiple atmospheric drivers, some statistics of short duration precipitation actually display surprisingly simple, regular behaviour. As an example, 10-min rainfall extremes derived from Dutch observations show a very regular dependency of 13% per degree over an almost 20-degree dew point temperature range. Similar behaviour has also been found in hourly precipitation observations. For dew point temperature each degree of warming reflects 6-7% more moisture in the air, following from the well-known Clausius-Clapeyron (CC) relation which is the cornerstone to understand and quantify the influence of the climate change on precipitation extremes. According to the above finding, however, precipitation intensities may be increasing with temperature at a rate twice the commonly expected CC rate. But are they, and if so, what are the impacts of these much stronger increases in precipitation extremes? These questions are at the heart of a "vivid" scientific debate. While it has become clear that these so-called scaling rates cannot be used as a straightforward predictor of the influence of warming on precipitation extremes, it will be illustrated that they are useful to understand how the most extreme rainfall events could respond to global warming.

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12:00-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 2

6 talks Show detail Hide detail

Chairs

Dr Michael Wehner, Lawrence Berkeley National Laboratory, USA

13:30-14:00 What can storm-scale models tell us about climate change impacts on extreme rainfall?

Dr Andreas Prein, National Center for Atmospheric Research, USA

Abstract

Simulating storms that cause extreme rainfall accumulations has been a long-standing challenge in weather and climate modelling. Tremendous improvements have been made over recent decades due to advances in models and computational resources that nowadays allow us to explicitly simulate storm-scale dynamics within models. This results in a step-improvement in simulating extreme rainfall and allows unprecedented insights into climate change impacts on rainfall extremes. This talk will exemplify the added value of simulating extreme rainfall producing storms in storm-scale models compared to coarser-resolution models. It will be shown how the most important storm characteristics that are associated with flooding (eg, rainfall rates, storm movement speed, and storm spatial extent) are simulated in storm-scale models and how these characteristics are projected to change in future climates. Storm-scale model-based changes in future rainfall extremes will be summarized and areas of uncertainty and opportunities for research will be highlighted.

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14:00-14:30 Using a convection permitting model ensemble for projecting future change in precipitation extremes

Dr Elizabeth Kendon, Met Office Hadley Centre, UK

Abstract

For the first time internationally a model at a resolution on par with operational weather forecast models has been used for national climate scenarios. As part of the UK Climate Projections (UKCP) project, an ensemble of 12 projections at 2.2km resolution have been carried out over the UK. These were launched in September 2019, with the aim of providing an improved simulation of extreme precipitation and also other high-impact events at local scales for the coming decades. At such high (2.2km) resolution, convection can be represented explicitly (‘permitted’) without the need for a parameterisation scheme, leading to a much more realistic representation of hourly precipitation characteristics, including extremes. In this talk initial results from the UKCP local (2.2km) projections, including changes in hourly precipitation extremes, will be presented.

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14:30-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 The role of stratification changes for the future European precipitation climate

Professor Christoph Schär, ETH Zurich, Switzerland

Abstract

There is tremendous interest into future changes of the precipitation climate, as precipitation touches upon a large number of human activities. The scientific argumentation often invokes changes in large-scale circulation and absolute humidity. Here another important aspect is considered, namely the role of stratification (or lapse-rate) changes. These changes are closely linked to climate change, as global warming will imply substantial shifts in the moist-adiabatic lapse rate. Lapse-rate changes have already been identified to instigate important impacts in the tropics, but they are also important for European climate change. Here an overview on lapse-rate changes is provided, together with examples from two recent publications. It is demonstrated that lapse-rate changes are particularly important for the European summer climate and strongly influence the projected drying in the Mediterranean, as well as short-duration precipitation events and the associated Clausius-Clapeyron scaling. It is argued that lapse-rate changes can be considered more reliable than circulation changes, and thus may help adding more confidence to some specific aspects of European climate change projections.

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16:00-16:30 Using high resolution modelling to understand the urban influence

Professor Jason Evans, University of New South Wales, Australia

Abstract

With the ever growing population living in cities worldwide, it is increasingly important to understand how the expanding cities influence their own climate. The urban environments' effect on temperature, referred to as the urban heat island (UHI), generally means urban environments are several degrees warmer than the surrounding region. While there are many factors that influence the UHI intensity, these are reasonably well understood. The influence of urban environments on precipitation is more complicated and involves the various ways in which the city (including the UHI) can alter the local circulation of the atmosphere. Here Jason Evans discusses several mechanisms to alter the local circulation and impact precipitation over urban areas. He suggests that large cities within the tropics (on coasts) are ideal places to observe these effects and show some results of studies looking at Kuala Lumpur, Malaysia, and Jakarta, Indonesia. In order for climate models to capture the UHI and changes in local circulation, resolutions at the kilometre scale are required.

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16:30-17:00 Discussion

17:00-18:00

Poster session

04 February

09:00-12:30

Session 3

4 talks Show detail Hide detail

Chairs

Professor Hayley Fowler, Newcastle University, UK

09:00-09:30 Decoupling increase in precipitation extremes from changes in urban stormwater flooding

Professor Ashish Sharma, University of New South Wales, Australia

Abstract

It is well accepted that warmer temperatures lead to greater moisture holding capacity for the atmosphere, resulting in bigger downpours, creating larger design precipitation intensities and possibly less secure flood infrastructure especially in urban areas. What is less appreciated is the importance pre-storm conditions have on urban stormwater infrastructure, especially in locations that contain significant water bodies. This presentation uses four urban case studies from the United States, Australia and Thailand to illustrate the differing impacts of warming on urban flood security. Each of these locations pose unique modelling challenges, given the near-continuous change urban stormwater networks undergo. It is shown that in spite of this change, there is clear evidence that urban flood security is compromised as a result of rise in temperature, a trend that will only worsen as time goes on.

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09:30-10:00 Challenges and opportunities in estimating impacts of climate change on future floods

Professor Seth Westra, University of Adelaide, Australia

Abstract

There is currently a significant debate in the scientific community regarding both the direction and magnitude of possible changes to flooding as a result of climate change. On the one hand, the global intensification of extreme rainfall combined with increases in sea level point to potentially dramatic increases in flood risk; on the other hand, instrumental evidence of historical flow data generally shows little change, or even a decline in the magnitude of large runoff events. This presentation will highlight the complex causal processes involved in translating large-scale atmospheric trends into measures of flood hazard, and identify some key limitations in the current generation of observational networks and modelling capabilities. To accelerate progress, possible directions forward are proposed, suggesting that an internationally coordinated effort is needed if we are to answer critical questions regarding the implications of climate change on flood risk. 

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10:00-10:30 Discussion

10:30-11:00 Coffee

11:00-12:30 Panel discussion

Murray Dale, JBA Consulting, UK
Dr Harriet Orr, Environment Agency, UK
Nalan Senol Cabi, Willis Towers Watson, UK

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12:30-13:30 Lunch

13:30-17:00

Session 4

6 talks Show detail Hide detail

Chairs

Dr Gabriele Villarini, University of Iowa, USA

13:30-14:00 Understanding from hurricanes, climate change and extreme forecast attribution

Dr Michael Wehner, Lawrence Berkeley National Laboratory, USA

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14:00-14:30 Operational event attribution in Europe

Professor Peter Stott, Met Office and University of Exeter, UK

Abstract

These are exciting times for the science of event attribution, research that aims to elucidate the links between extreme weather events and climate variability and change. With a rising toll of extreme weather, including damaging heavy rainfall and flooding, has come a rising demand for authoritative information about the risks of such events particularly from the public, the media, and from policy makers. To meet this demand, operational event attribution services are starting to be developed. At the Met Office, we are part of a consortium that is developing operational event attribution for Europe under the Copernicus Climate Change Service funded by the European Commission, initially as a prototype service. Professor Stott will describe these new developments with an emphasis on the attribution of extreme rainfall events. This includes new research demonstrating the value of high-resolution modelling for detecting changes in the probability of occurrence of the high daily rainfall totals seen during the flooding in Yorkshire in Autumn, 2019.

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14:30-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Using observational constraints on projections of change in mean and intense precipitation

Professor Gabriele Hegerl FRS, University of Edinburgh, UK

Abstract

Observed changes in mean and intense precipitation provide potentially powerful constraints on future changes when using detection and attribution methods. Some of the constraints arising suggest that CMIP class climate models underestimate some of the changes expected. Monthly precipitation changes in the tropics and subtropics are at the upper edge of simulated changes with CMIP6, with increased rainfall in the wettest third of the tropics and flat or decreased rainfall in the dry part. This diagnostic relies on detecting wet and dry regions for each month and diagnosing total rainfall in these regions. Splitting satellite/in situ blended data into driest to wettest climatological regions shows that climate models follow consistent physics between them and with data, even if features are misplaced, and even with high variability in the location of the features. In mid-high latitudes, increased precipitation is detectible, but changes are impacted by modes of variability such as the North Atlantic Oscillation. Intense (1-day) precipitation has shown a detectible and attributable change, with significant uncertainty associated with observational data. Changes recorded in the satellite and in situ record contain increasingly powerful information on changing precipitation. 

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16:00-16:15 Discussion

16:15-17:00 Panel discussion

Dr Michael Wehner, Lawrence Berkeley National Laboratory, USA
Professor Hayley Fowler, Newcastle University, UK
Professor Paul O'Gorman, Massachusetts Institute of Technology, USA
Dr Andreas Prein, National Center for Atmospheric Research, USA
Professor Seth Westra, University of Adelaide, Australia

Show speakers
Intensification of short-duration rainfall extremes and implications for flash flood risks

3 - 4 February 2020

The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK
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