Winter Floods in Britain Connected to Atmospheric Rivers

27 February 2012

The AR that contributed to serious flooding in Northwest England on 19th November 2009. The image uses total column water vapour (in cm) from the ERA-Interim reanalysis.
The AR that contributed to serious flooding in Northwest England on 19th November 2009. The image uses total column water vapour (in cm) from the ERA-Interim reanalysis.
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A journal article entitled Winter Floods in Britain are Connected to Atmospheric Rivers by David Lavers, Richard Allan, Eric Wood, Gabriele Villarini, David Brayshaw, and Andrew Wade appeared in Geophysical Research Letters in December 2011.

Winter floods have recently been prevalent in the United Kingdom (UK) and Western Europe. With climate change expected to lead to an enhancement of extreme rainfall / flooding, this research was undertaken to improve hydrometeorological process understanding of flooding in UK river basins, as this is an important step in assessing the future risks of flooding in the UK and other mid-latitude regions. This work builds on results that have emerged from NOAA's HMT-West program, which had identified ARs as crucial to west coast extreme precipitation and flooding. Combined with recent publications on the role of AR conditions in Andean precipitation in Chile (Viale and Nunez, 2011, J. Hydrometeor.), it is clear that the AR phenomena are important in at least two other continents than solely North America. The findings reinforce the important HMT research and prototyping direction focused on improving physical understanding, observations, modelling, predictions, and climate projections of ARs on the U.S. West Coast.

The paper shows that winter flooding in Britain is connected to ARs. By combining river flow and rainfall records, satellite data and climate model reanalyses, winter flood events in a range of British river basins and the atmospheric circulation that occurred concurrently with the floods were analysed. A severe flood in Northwest England in November 2009 was used as a case study to highlight the link between ARs and extreme flooding (see Figure). It was both the persistent nature of this particular AR (the AR was located over the basin for at least one day) and orographic enhancement of rainfall that resulted in high rainfall totals and flooding. Furthermore, ARs were shown to occur simultaneously with the 10 largest winter floods in the study basins indicating that ARs are critical in explaining winter flooding in Britain. The article also discusses the role of basin geology in flood generation highlighting how the AR-flood connection is strongest in basins with a rapid-response to rainfall (i.e. basins with impermeable bedrock and soils).

Two factors that could affect ARs in a warming climate were discussed. Firstly, a change in AR frequency is likely to affect the frequency of extreme winter flood events, and secondly higher atmospheric water vapour content is expected to lead to ARs delivering more rainfall and a risk of larger floods. Further research is recommended to determine how ARs may alter under current projections of climate change.

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