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a windy winter season
EDITORS NOTE: Dr. Gerhard Zuba, AIR Principal Scientist, reviews
the 2011–2012 windstorm season in Europe and discusses the
climatological influences that lead to storm clustering.
By Dr. Gerhard Zuba
Edited by Nan Ma
02.2012
Season Review
Although quiet and bitterly cold in later weeks, the winter
of 2011–2012 has seen several damaging windstorms pass
in quick succession across the European continent. These
low pressure extratropical systems (also called winter storms)
form when warm moist air from the subtropical region
converges with cold air from the subarctic region. They track
across the Atlantic and arrive in Western Europe, bringing
high winds, heavy precipitation, and coastal flooding.
Because windstorms are relatively fast-moving and do not
lose their energy when they exit the ocean, they can cover
vast expanses of land and affect several countries over the
course of a few days.
The first significant storm of the season was Xaver, which
formed in the late November and brought strong winds
to Scotland. High waves caused by the storm forced
some oil platforms off the coast of Norway to shut down.
Storm activity picked up in early December with the
arrival of Friedhelm in Scotland after a process of rapid
strengthening called explosive cyclogenesis. Considered
Scotland’s worst storm in more than a decade, Friedhelm
brought violent winds to central and southern portions
of the country, prompting a rare Red Alert from the U.K.
Met Office. Sustained winds of over 170 km/h (105 mph,
akin to a Category 2 hurricane on the Saffir-Simpson
scale) were reported, with gusts exceeding 260 km/h (160
mph). Friedhelm left more than 100,000 homes without
power and caused massive closures and travel disruptions.
Fortunately, Scotland’s wind design provisions are stringent
and well enforced, and damage to the predominantly
masonry building stock was not very significant.
One week later, another powerful storm struck Europe, this
time affecting France, Switzerland, and Germany. Off the
coast of Brittany, the winds from Joachim were so severe
that a cargo ship ran aground, spilling more than 200
tons of fuel. The storm caused widespread power outages
in northern and western France, with some 300,000
households losing electricity. Flooding was reported in
several coastal departments. While wind speeds from
Joachim were not as high as those from other major storms
to affect France in recent years (like Klaus in 2009 and
Xynthia in 2010, both of which caused insured losses in
excess of 1 billion Euros), strong winds were recorded over
much of the country and into Germany and Switzerland.
This led to widespread—but mostly minor—damage, mainly
to nonstructural building elements. PERILS issued an initial
insured loss estimate of 300 million Euros.
AIRCurrents
02.12|a windy winter season
By Dr. Gerhard Zuba
Edited by Nan Ma
A Note on Naming
In Europe, the Institute for Meteorology at Freie University
of Berlin is a well-accepted authority on identifying and
tracking winter storms and has been naming all high and
low pressure systems to affect Central Europe since the
1950s. In 2002, the Institute introduced the “Adopt a
Vortex” program, allowing public patrons to submit storm
names (on average, 50–60 highs are named each year and
150 lows).
These Freie University names are widely used by the media
when covering potentially damaging events and by many
weather agencies throughout Europe. Some countries,
however, will use a different name to refer to a storm
that enters their domain. The Norwegian Meteorological
Institute, for example, assigns its own name to significant
storms that affect Scandinavia. Thus, storms can have two
names, including this season’s Patrick (named Dagmar by
the Norwegian Meteorological Institute) and Ulli (also called
Emil). The names used in this article are those assigned by
the Freie University.
A week and a half later on Christmas Day, Patrick swept
through northern Scandinavia, causing tree damage,
landslides, travel disruptions, and massive power outages
that disrupted gas processing operations. Described as
Norway’s third-worst storm in the past 50 years by the
Norway Meteorological Institute, wind gusts reached
hurricane strength at every recording station in the country
(a maximum of 233 km/h, or 145 mph, was recorded). In
Finland, Patrick affected the entire south portion of the
country and was the worst storm since Janika in 2001.
A few days later, windstorm Ulli formed off the coast of
Newfoundland, Canada. Ulli steadily strengthened as it sped
across the Atlantic Ocean, reaching Scotland with a central
pressure of just 952 mb. The storm generated a sting jet—a
localized strong downdraft that brings cold, dry air from the
mid-troposphere down to the surface—that caused intense
localized wind gusts in Scotland. Strong winds toppled trees
and power lines, overturned tall trucks, and caused some
damage to buildings. Ulli also brought heavy precipitation,
and coastal and river flooding. The region affected by Ulli
was similar to that of Windstorm Friedhelm, although Ulli’s
wind speeds were generally lower. Disruption to travel was
widespread as roads, bridges and rail lines were forced to
close.
Fast on the heels of Ulli, yet another storm—Andrea—
formed southwest of Iceland on January 3 and passed to
the north of the U.K., bringing strong winds to central
England. As it moved southeast, Andrea intensified and
struck Germany, bringing wind, rain, sleet, and snow. As is
typical of storms approaching Germany from the North Sea,
the storm pushed water against the coast, causing flooding
in low-lying areas in the north, including Hamburg. While
power outages, traffic accidents, and travel disruption were
reported, structural damage from winds was limited. To
date, Andrea was the last damaging storm of the season.
Xaver, November 22
Patrick, December 24
Friedhelm, December 7
Ulli, December 31
Joachim, December 14
Andrea, January 3
Figure 1. Tracks and maximum recorded wind gusts of major storms from the
2011–2012 winter storm season (Source: AIR)
Storm Tracks and the NAO
This season’s storm frequency and tracks are unlike those
of the previous two European winter storm seasons, which
saw only one major windstorm, Xynthia, impact central
Europe in the winter of 2009/10. Compared to recent
years, this season saw a significant northward shift in storm
tracks, resulting in an unusually high number of damaging
events in northern Europe (extratropical systems typically
exhibit the highest wind speeds on the right-hand side
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AIRCurrents
02.12|a windy winter season
By Dr. Gerhard Zuba
Edited by Nan Ma
of the track, so regions south of the storm track tend to
experience the most damage). This shift can be attributed
to a change in the North Atlantic Oscillation (NAO) from a
negative to positive phase in late 2011.
Under NAO+ conditions like the 2011–2012 season,
extratropical cyclones tend to track toward Europe in a
northeastward direction, bringing wind and precipitation
to northern Europe. This has resulted in a disproportionate
number of storms affecting the U.K. and Scandinavia. On
the other hand, during NAO- conditions, storms tend to
track due east, which takes them into the Mediterranean
region. For example in 2009, which had a strongly negative
NAO during the winter months, Klaus made landfall
on France’s Aquitaine coast, causing billions of Euros of
damage in northern Spain and southern France.
Figure 2. North Atlantic Oscillation Index, three-month moving average
2001–2011 (Source: Climate Prediction Center http://www.cpc.ncep.noaa.gov/)
The NAO is a climate signal that influences atmospheric
flow in the North Atlantic Ocean and drives much of the
weather variability over parts of Europe and North America.
(It is also responsible for the mild winter season this year
in the U.S.) The NAO is determined by the difference
in pressure between a subtropical high pressure system
located near Bermuda, the Azores High, and a subarctic low
pressure system located near Iceland, the Icelandic Low (see
Figure 3). The clockwise rotation of the Azores High and the
counterclockwise rotation of the Icelandic Low directs air
eastward to Europe. The strength of this flow depends on
the pressure difference between the two. A well-developed
Azores High (strong high pressure) and a well-developed
Icelandic Low (very low pressure) corresponds to the NAO
positive (NAO+) phase. The NAO negative (NAO-) phase
occurs when both systems are weak.
Figure 4. Monthly NAO during winter, 2009–2012 (Source: Climate Prediction
Center http://www.cpc.ncep.noaa.gov/)
This year, as shown in Figure 4, the NAO remained positive
through January, and daily indices are still showing this
trend. However, no significant storms have formed after
Andrea in early January, indicating that weather factors
other than the NAO are influencing storm activity. Indeed,
the low pressure system around Iceland has shifted
westward, and continental high pressure over Russia has
extended through central Europe and into parts of western
Europe. This weather phenomenon is called Scandinavian
blocking, whereby the high pressure that has moved over
Europe is blocking storms from tracking eastward into
Europe. This blocking is also causing extremely cold air
from the Arctic and Russia to flow into central and western
Europe. This resulted in the severe cold snap that gripped
the continent—the worst in several decades—that claimed
hundreds of lives since late January.
Nature Is not an Option: Modeling
Storm Clusters
Figure 3. The pressure difference between the Azores High and the Icelandic Low
determines the phase of the NAO and influences the flow of westerly winds that
direct storms toward Europe
In addition to influencing storm tracks, NAO+ phases are
also often associated with above average counts of storms,
particularly over northwestern Europe, as well as the
tendency for storms to follow closely on each other’s heels,
or cluster. Mailier et al (2005) showed that storm clustering
is statistically significant in northwestern Europe at the exit
region of the typical North Atlantic storm track, but not
at the entrance region near North America. This suggests
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AIRCurrents
02.12|a windy winter season
By Dr. Gerhard Zuba
Edited by Nan Ma
that storms form at regular intervals in the western North
Atlantic, but that the variability in a number of large-scale
climate patterns (including the North Atlantic Oscillation,
the East Atlantic Pattern, and the Scandinavian Pattern)
can influence their path and travel times over the Atlantic.
These climate fluctuations ultimately lead to the clustered
arrival of the storms in Europe.
The 2011–2012 season saw a succession of six damaging
storms within a six week period, interspersed among
numerous other named storms (see Table 1).
Table 1. Named storms from the 2011–2012 season; shaded rows indicate
damaging events (Source: Freie University of Berlin)
While this season has not packed the punch of past years
with major clustered storms (like 1999’s Anatol, Lothar,
and Martin), it is crucial for windstorm models to be able
to accurately identify individual storms, represent their size
realistically, and therefore capture their temporal occurrence
patterns. Clustering is a real and well-studied phenomenon,
one that should not be an optional setting that can be
turned on and off in the model at will.
AIR’s methodology identifies storms based on their vortex
centers and places them realistically within a seasonal
timeline using a block-bootstrapping technique. This
approach is consistent with reinsurance contracts that
adopt event-based occurrence definitions, which requires
clustered storms to be correctly separated according
to meteorological characteristics before applying hours
clauses (read the AIRCurrents article European Windstorms:
Implications of Storm Clustering on Definitions of
Occurrence Losses for a more detailed discussion). AIR’s
methodology is also ideally suited to assess the probability
of having a “surplus” of damaging events in a given
year because of storm clusters, compared to average
expectations. This is crucial to estimating the likelihood of
triggering reinstatements in reinsurance contracts.
Storm Name
Date named
Xaver (Berit)
November 22, 2011
Yoda
November 24, 2011
Zafer
November 28, 2011
Arno
November 29, 2011
Bob
December 1, 2011
Christoph
December 3, 2011
Danilo
December 4, 2011
Ekkehard
December 6, 2011
Friedhelm
December 7, 2011
Closing Thoughts
Günter
December 8, 2011
Hergen
December 11, 2011
Iven
December 13, 2011
Joachim
December 14, 2011
Klausdieter
December 17, 2011
Louis
December 19, 2011
Martin
December 21, 2011
The current winter storm season thus far is unlikely to go
down in history as an exceptional year. While several major
storms have struck Europe, none of them are likely to
have caused insured losses in excess of half a billion Euros.
Nevertheless, the clustered arrival of this season’s storms in
northern Europe, which was likely influenced by a phase
change in the North Atlantic Oscillation, reinforces the
importance of capturing realistic storm size, frequency, and
temporal occurrence patterns in a catastrophe model.
Norbert
December 22, 2011
Oliver
December 23, 2011
Patrick (Dagmar)
December 24, 2011
Quirin
December 25, 2011
Robert
December 26, 2011
Sebastian
December 27, 2011
Tilo
December 30, 2011
Ulli (Emil)
December 31, 2011
Andrea
January 3, 2012
Bibiana
January 4, 2012
...
...
Several smaller storms within a year can cause aggregate
losses that rival that of a single large storm (the so-called
20-, 50-, 100-, or 250-year, etc., events that companies
typically prepare for). In such situations, excess of loss
reinsurance contracts, which commonly allow for one
reinstatement, may easily be exhausted or may not
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AIRCurrents
02.12|a windy winter season
By Dr. Gerhard Zuba
Edited by Nan Ma
be triggered at all, leaving insurance companies with
inadequate protection. Further, small storms are not the
only ones to cluster. Storm clusters can contain one or more
major events (like 1990’s Daria, which was the strongest
among a cluster of eight storms), which together have
the potential to cause extreme windstorm losses with no
historical precedence.
The AIR Extratropical Cyclone Model for Europe simulates
realistic years of storm activity, faithfully preserving
intraseasonal storm patterns by date-stamping each storm.
Consistent with what has been observed in nature, AIR’s
model captures storm sizes that accurately reflect the
meteorological record and explicitly allows storms to cluster,
instead of treating separate clustered events as a single
large storm. This is essential in providing a realistic view of
occurrence and aggregate losses in accordance with actual
practices in the industry.
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