atmospheric rivers and british winter floods david lavers department of meteorology / walker...
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![Page 1: Atmospheric Rivers and British winter floods David Lavers Department of Meteorology / Walker Institute, University of Reading, UK 19 th June 2012 Gabriele](https://reader035.vdocument.in/reader035/viewer/2022081512/551543375503465e608b6161/html5/thumbnails/1.jpg)
Atmospheric Rivers and British
winter floods David Lavers
Department of Meteorology / Walker Institute, University of Reading, UK
19th June 2012
Gabriele Villarini, Richard P. Allan, Eric F. Wood, David J. Brayshaw, Andrew J. Wade
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Contents
• Introduction
• Hydrologic study of AR-flood links
• AR-flood links from an atmospheric standpoint
• Conclusions
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Introduction• Winter Floods have been widespread in Britain (e.g.,
November 2009). • High economic losses. In Europe economic losses due to
floods have exceeded those from any other natural disaster (over last 20 years).
• Atmospheric Rivers (ARs) are regions of enhanced moisture transport across the mid-latitudes in warm sector of extra-tropical cyclones.
• Most AR-flood research undertaken in western North America.
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Basins for AR-flood link assessment
Ewe at Poolewe; BFI=0.64, 441.1km2
Ayr at Mainholm; BFI=0.29, 574km2
Eden at Temple Sowerby; BFI=0.37, 616.4km2
Dyfi at Dyfi Bridge; BFI=0.39, 471.3km2
Teifi at Glan Teifi; BFI=0.54, 893.6km2
Taw at Umberleigh; BFI=0.43, 826.2km2
Lavers et al., submitted JGR
Western Britain – impermeable geology/ mountainous; higher rainfall totals; quick response.
South-eastern Britain – permeable geology (e.g., chalk) / lowland; lower rainfall totals; lagged response.
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Seasonality of floods (POT-5)
The largest and the highest number of floods occur in winter.
Peaks-over-Threshold (POT) with on average 5 floods per year (1979-2010).
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November 2009 Cumbrian flood• Peak flow at Eden at Temple Sowerby (267m3/s) on 19th Nov
2009; 3 day rainfall total of 164.5mm near gauging station.• Other areas in Cumbria received >300mm in 24 hours.
Cum
ecs (m
3s-
1)
Data source:UK National River Flow Archive / UKMO MIDAS.
Lavers et al., 2011 GRL
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Fields at 0600 UTC 19th Nov 2009
units: ms-1
Data source:ECMWF ERA-Interim reanalysis.
SSMI F16 retrieval of column IWV
Lavers et al., 2011 GRL
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IVT at 0600 UTC 19th Nov 2009
units: kg m-1 s-1
Data source:ECMWF ERA-Interim reanalysis.
Integrated Vapour Transport from 1000 hPa to 300 hPa
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IVT at 0600 UTC 19th Nov 2009
units: kg m-1 s-1
Data source:ECMWF ERA-Interim reanalysis.
Integrated Vapour Transport in different layers
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Top 10 winter floods (in WMS) in Eden
• Top 10 floods in Winter Maximum Series (WMS) over 1970–2010. • Persistent ARs located over basin in these floods.• ARs have consistent location and orientation.
Data source: 20th Century / ECMWF ERA-Interim reanalyses.
Lavers et al., 2011 GRL
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Atmospheric Circulation (Top 10 floods)
Data source: 20th Century / ECMWF ERA-Interim reanalyses.
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Conclusions 1
• Damaging British flood (November 2009) linked to persistent AR event.
• Ten largest winter floods in a range of British river basins connected to ARs.
• ARs were particularly recognisable during winter floods in fast-responding basins; permeable basins require a series of storms to produce floods.
• Findings broadly applicable to NW Europe.
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AR screening in atmospheric reanalyses
• At 900 hPa search between 50°N and 60°N (at 4°W) for q > 5 g/kg and uv > 12.5 m/s; these criteria must exist across North Atlantic (for 20° longitude).
• If these conditions exist for 3 time steps over a specific region then a persistent AR is identified (only 4.5° latitude movement).
• Applied to the winter half-year (Oct-Mar) over 1979–2009 in five reanalyses.
60°N
50°N
4°W
Lavers et al., submitted JGR
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AR totals in each winter half-year
Twentieth Century 404CFSR 386ERA-Interim 369MERRA 319NCEP-NCAR 396
Positive NAO (hence stronger westerly winds over the N. Atlantic) tends to be associated with more ARs.
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ARs and AMS / WMS floods
NCEP-NCARMERRAERA-InterimCFSRTwentieth Century
1). Extract peak mean daily river flow in winter half-year (WMS) and water year (AMS). AMS and WMS are generally equivalent.
2). Persistent AR must start 2 days before or on day of flood.
e.g., Dyfi has 30/31 years when an AR is linked to a flood.
Lavers et al., submitted JGR
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AR-flood “hit rate”
• Strong connection between ARs and the largest winter floods.
• However, how many identified ARs in the reanalyses are not related to flood events?
• Using the POT-5 floods in the six basins, we calculate what percentage of ARs were related to these floods.
Lavers et al., submitted JGR
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Conclusions 2
• Algorithm detects persistent ARs.
• Good AR agreement between reanalyses.
• Winter floods are the largest.
• Strong connection between identified ARs and winter floods in six river basins; in one basin 30/31 years have AR-flood link.
• Up to 56 % of ARs related to POT-5 floods.