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The eye of the storm for flood risk models

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By Nalan Senol Cabi and Prof. Hayley Fowler | March 17, 2020

The march is on to reflect more frequent, high intensity rainstorms in the flood risk models that insurers and infrastructure planners use.

As we were writing this article, several U.K. towns and villages in Wales, Yorkshire and the West Midlands in particular remained inundated with flood waters.

The combined effects of three back-to-back, named storms – Ciara, Dennis and Jorge – made last month the wettest February in the U.K. since records began in 1862. The Met Office has reported that the U.K. as a whole received an average of 209mm of rainfall, 237% above the average for the month over the last 30 years.

What has made these storms, and the resulting flooding, particularly damaging in so many areas is the intensity of the rainfall in short periods of time. This of course is not only a U.K. phenomenon, as the rising propensity for severe floods in many parts of the world amply demonstrates. The principal culprit is fairly clear.

The link to global warming

A Royal Society conference in which we participated in London recently (the snappily titled: ‘Intensification of short-duration rainfall extremes and implications for flash flood risks’) highlighted the growing body of evidence from around the world on the link between global warming and greater rainfall extremes.

As climate scientists’ and academia’s efforts intensify to identify and isolate the underlying causes of extreme rainfall events linked to climate change, a logical next step will be to help transfer that knowledge into useful insights needed by government, regulatory bodies, businesses and insurers in particular. More specifically, this means building in more certainty into the flood risk models that underpin many infrastructure and insurance decisions.

And therein lies the challenge for all of us. While the evidence of the links between specific extreme rainfall events, an increase in their numbers, and the human influence on climate is compelling, it is a complex connection and there is still much to understand about the variety of types of extreme rainfall events, and exactly how they link changing climate conditions. For example, while there may be strong theoretical reasoning for warmer atmospheric conditions to hold greater amounts of moisture and potentially produce more extreme rainfall events, the influence of climate change on the larger scale synoptic drivers of the weather patterns that lead to the conditions conducive to severe convection and associated extreme rainfall, are much more uncertain. On the scale of an individual storm, the natural climate variability is much greater than the climate change signal, so detecting a climate change influence is a delicate process, . Furthermore, another layer on uncertainty remains about how the hydrological extremes will unfold.

Three snippets from the conference illustrate the point:


  • Rainfall increase is not uniform across the world: Professor Paul O’Gorman from the Massachusetts Institute of Technology discussed how the projected rate of increase is not uniform across different regions of the world. His research has shown, for example particularly large differences across land regions in the Northern summer (June-July-August). Since precipitation rate in extreme events in global climate models can be accurately related to upward velocity of the air throughout the troposphere, one aspect his research team is concentrating on is to analyse the fluid dynamics that influence this upward movement.
  • Urban design matters: Professor Jason Evans of the University of New South Wales described the urban influence. Among other things, his team’s research is revealing that city size and shape matters. The bigger, more compact and more circular a city is, the stronger the urban heat island effect and potentially locally induced storm circulations that affect rainfall. As he pointed out, this urban influence is also significant in that more people are projected to live in bigger cities in the future.
  • Expected frequency of extreme rainfall to rise in the U.K. : Dr Elizabeth Kendon of the U.K. Meteorological Office updated delegates on its efforts to better model rainfall variability. Among its findings are that climate change will extend the convective season into autumn, with resultant significant increases in hourly rainfall extremes. Allied to that, it estimates that the frequency of days with hourly rainfall exceeding 30mm/hour will almost double by the 2070s – increasing from a UK-average of once every 10 years now to almost once every five years.

Marching to the same tune

These examples represent just a small sample of the concerted global effort to better understand what’s causing extreme rain events and how climate change is impacting and could impact their scale and frequency.

The Willis Research Network (WRN) continues to work closely with its academic partners to identify the drivers and variables of high intensity rainstorms and improve the flood risk quantification.

Among the initiatives taking place in this area, INTENSE, led by WRN partner institution, Newcastle University has been looking at how to comprehensively analyse the response of precipitation extremes to global warming by constructing a new global sub-daily precipitation dataset, and using this together with other datasets and high-resolution climate modelling to quantify the nature and drivers of global precipitation. Other ongoing projects include FUTURE-STORMS and FUTURE-DRAINAGE, which respectively, aim to leverage high resolution climate models to develop climate adaptation tools for stakeholders, and to update guidance for U.K. urban drainage design and recommend updates to the Environment Agency’s Risk of Flooding from Surface Water maps.

The results of these and other projects can’t unfortunately provide any consolation to those, like many families currently in the UK, who have been directly affected by flooding. But they are further steps in enhancing our readiness and future mitigation measures for dealing with more sustained and serious deluges.

Authors

Senior Lead Flood Specialist

Nalan Senol Cabi is the Senior Lead Flood Specialist in the Model Research and Evaluation team at Willis Re.


Professor of Climate Change Impacts

Prof. Hayley Fowler is Professor of Climate Change Impacts in the School of Engineering at Newcastle University.


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