Transcript Atmospheric Rivers and British winter floods David Lavers Department of Meteorology / Walker Institute, University of Reading, UK 19th June 2012 Gabriele Villarini, Richard P.

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
Contents
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Introduction
Hydrologic study of AR-flood links
AR-flood links from an atmospheric standpoint
Conclusions
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 midlatitudes in warm sector of extra-tropical
cyclones.
• Most AR-flood research undertaken in western
North America.
Basins for AR-flood link assessment
Western Britain – impermeable
geology/ mountainous; higher 2
Ewe at Poolewe; BFI=0.64, 441.1km
rainfall
totals; quick response.
Ayr at Mainholm; BFI=0.29, 574km2
South-eastern Britain –
Eden at Temple Sowerby;
permeable
geology
(e.g.,2 chalk) /
BFI=0.37,
616.4km
lowland; lower rainfall totals;
lagged response.
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
Seasonality of floods (POT-5)
Peaks-overThreshold
(POT) with on
average 5 floods
per year (19792010).
The largest and
the highest
number of
floods occur in
winter.
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.
Cumecs (m3s-1)
Lavers et al., 2011 GRL
Data source:
UK National
River Flow
Archive /
UKMO
MIDAS.
Fields at 0600 UTC 19th Nov 2009
Data source:
ECMWF ERAInterim
reanalysis.
SSMI F16
retrieval of
column
IWV
units: ms-1
Lavers et al., 2011 GRL
IVT at 0600 UTC 19th Nov 2009
Integrated Vapour Transport from 1000 hPa to 300 hPa
Data source:
ECMWF ERAInterim
reanalysis.
units: kg m-1 s-1
IVT at 0600 UTC 19th Nov 2009
Integrated Vapour Transport in different layers
Data source:
ECMWF ERAInterim
reanalysis.
units: kg m-1 s-1
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
Atmospheric Circulation (Top 10 floods)
Data source: 20th Century / ECMWF ERA-Interim reanalyses.
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.
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.
4°W
60°N
50°N
Lavers et al., submitted JGR
AR totals in each winter half-year
Twentieth Century 404
CFSR 386
ERA-Interim 369
MERRA 319
NCEP-NCAR 396
Positive NAO
(hence stronger
westerly winds over
the N. Atlantic)
tends to be
associated with
more ARs.
ARs and AMS / WMS floods
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.
Lavers et al., submitted JGR
NCEP-NCAR
MERRA
ERA-Interim
CFSR
Twentieth Century
e.g., Dyfi has
30/31 years
when an AR
is linked to a
flood.
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
Conclusions 2
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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.
Thank you for listening
David Lavers ([email protected])
Visit www.walker-institute.ac.uk