STORMS IN NORTHERN EUROPE

1
STORMS IN NORTHERN EUROPE CHANGES,
PERSPECTIVES, AND IMPACTS

• HANS VON STORCHInstitute of Coastal Research,
  (Helmholtz Zentrum Geesthacht), and
• KlimaCampus Hamburg, Germany

TOKIO MARINE'S 2011 SUMMIT ON GLOBAL WARMING AND
CLIMATE CHANGE, APRIL 29, 2011, ATLANTA USA
2
Counting storms in weather maps steady increase
of NE Atlantic storms since the 1930s .
3
Popular argument When it is getting warmer, then
we have more water vapor in the atmosphere, and
mid-latitude storms become more intense (more
fuel).
However at mid latitudes, intense storms are
baroclinic, and thus driven by temperature
differences explaining, why we have more
storms in the cold season that in the warm
season.
Unchanging extratropical storm conditions are not
inconsistent with a rise in temperature
4
Homogeneity of local observations(Wind speed
measurement SYNOP Measuring net of German
Weather service Coastal stations at the German
Bight Observation period 1953-2005)

• Causes of inhomogenities
• Changes in
• Instruments
• Sampling frequencies
• Measuring units
• Environments (e.g. trees, buildings)
• Station relocations (Dotted lines)

Lindenberg et al., 2011
5
Representativity of near surface wind speed
measurements
6

• The major problem for applying statistical
  analysis in climate change studies is related
• to the assumptions of a stationary sampling
  process, and
• to the assumption of representativity.
• The local data change their statistics because of
  changing observational practices and conditions
  (inhomogeneity) also their representativity for
  a larger area is often compromised.
• Usage of weather analyses, incl. re-analyses and
  proxies such as damages are generally not
  suitable.

7
For assessing recent and ongoing changes of
mid-latitude storm conditions, principally two
approaches are possible

• Use of proxies derived from air pressure
  readings.
• Simulation by empirical or dynamical downscaling
  of large scale information.

8

• A proxy of storminess Intraseasonal statistics
  of daily geostrophic winds (pressure gradients)

• Air pressure readings are mostly homogenous
• The scaled pressure gradient is called
  geostrophic wind.
• The tropospheric wind is to first order
  approximation proportional to atmospheric
  pressure gradient
• We use annual/seasonal percentiles (e.g., 95 or
  99iles ) of geostrophic wind derived from
  triangles of pressure readings as proxies for
  annual /seasonal storminess.

Schmidt and von Storch, 1993
9
Testing the robustness of the link seasonal
percentiles of geostrophic wind speed and of
surface wind speeds in the virtual reality of a
50 years regional simulation
Correlations between time series of geostrophic
wind speed percentiles and wind speed percentiles
Krueger and von Storch, 2011
Triangles used
10
Geostropic wind stats N Europe
99iles of annual geostrophic wind speeds for a
series of station triangles in the North Sea
regions and in the Baltic Sea region.
Alexandersson et al., 2002
11
Dynamical (process based model) downscaling
cascade for constructing variable regional marine
weather statistics

• Dynamical downscaling to obtain high-resolution
  (10-50 km grid 1 hourly) description of weather
  stream- with spectral nudging constraint- use of
  NCEP or ERA re-analysis allows reconstruction of
  regional weather in past decades (1948-2010)-
  when global scenarios are used, regional
  scenarios with better description of space/time
  detail can be derived.
• Meteorological data are fed into dynamical
  models of weather-sensitive systems, such as
  ocean waves, catchment hydrology, long-range
  pollution etc.

12
Extreme value analysis of wind speed at platform
K13 (southern North Sea) January 1980-January 1997
Weisse, pers. comm.
13
Stormcount 1958-2001
t T
t T
Weisse et al., J. Climate, 2005
Change of Bft 8/year
14
Skill in representing wave conditions
significant wave height
days
wave direction
days
Red buoy, yellow radar, blue wave model run
with REMO winds
Gerd Gayer, pers. comm., 2001
15
www.coastdat.de

• The CoastDat data set
• Long (50 years) and high-resolution
  reconstructions of recent offshore and coastal
  conditions mainly in terms of wind, storms,
  waves, surges and currents and other variables in
  N Europe
• Scenarios (100 years) of possible consistent
  futures of coastal and offshoreconditions
• extension ecological variables, SE Asia
• Clients
• Governmental various coastal agencies dealing
  with coastal defense and coastal traffic
• Companies assessments of risks (ship and
  offshore building and operations) and
  opportunities (wind energy)
• General public / media explanations of causes
  of change perspectives and options of change

16
Some applications of

• Ship design
• Navigational safety
• Offshore wind
• Interpretation of measurements
• Oils spill risk and chronic oil pollution
• Ocean energy
• Scenarios of storm surge conditions
• Scenarios of future wave conditions

Wave Energy Flux kW/m
Currents Power W/m2
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Range of changes in seasonal maximum wind (10 m)
in Northern Germany in a series of dynamically
downscaled scenarios (HadCM, MPI A2, B2)
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Polar lows

• Mesoscale (lt 1000 km) sized maritime storms
• intense/ strong winds , severe weather
• occur polewards of the Polar Fronts in both
  hemispheres during winter
• typically induced by disturbances in the air
  flow
• typically driven by convective processes
• Here only Northern North Atlantic

Spitz-bergen
Scandinavia
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Past annual frequencies of polar lows
PLS Polar Low Season (July-June)
Zahn and von Storch, 2008
20
Downscaling scenarios
Downscaling scenarios and simulations C20
(1960-1989) A2, B1, A1B1 (2070-2099) run with
ECHAM5/MPIOM1 downscaled with CLM, employing
spectral nudging.
21
Projected changes in polar low frequency and
vertical atmospheric stability
Differences of the area and time-averaged
ice-free SST and T500hPa over the maritime
northern North Atlantic as proxy for frequency of
favorable polar low conditions (CMIP3/IPCC AR4)
Zahn and von Storch, 2010
22
Past and future occurrence of North Atlantic
Polar Lows

• Past changes according to dynamical downscaling
  of NCEP/NCAR re-analysis 1949-2005
• Strong inter annual variability
• Frequency remains on a similar level no
  systematic trend
• Polar lows become less frequent in the Northern
  North Atlantic in the coming 100 years according
  to
• regional modeling (dynamical downscaling) -
  control and scenario simulations with one GCM,
  and by
• analysis of vertical stability in a large set
  of (CMIP3) global climate simulations all
  simulations show an increase in projected
  stability.
• The genesis regions shift northward.

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Conclusion Usage of proxies

1. Monitoring extra-tropical storminess may be based
   on air pressure proxies.
2. This allows assessments for 100 and more years.
3. Decades long upward and downwards trends have
   been detected in recent years in the Northern
   European and other regions.
4. These trends are not sustained and have show
   recent reversals in all considered regions.
5. Recent trends are not beyond the range of natural
   variations, as given by the historical past, but
   are more of intermittent character. Regional
   temperatures rose significantly at the same time.

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Conclusion Usage of dynamical downscaling

1. Dynamical downscaling for describing synoptic and
   mesoscale variability is doable.
2. Analysis of 60 year simulations point to strong
   year-to-year variability, to less
   decade-to-decade variability and no noteworthy
   trend in the region of the North
   Atlantic/European Sector both in terms of
   baroclinic storms and polar lows.
3. Similar study done for North Atlantic Polar Lows
   , E Asian typhoons and medicanes. (work in
   progress)

25
Mediterranean hurricanes are strong mesoscale
cyclones with some resemblance with tropical
cyclones and polar lows (warm core, cloud-free
eye, winds up to hurricane speed) that develop
occasionally over the Mediterranean Sea.
Medicanes
15 Jan 1995
Dynamically downscaled
Leone Cavicchia, pers. comm
26

• Take-Home Message
• Baroclinic storms and polar lows represent key
  climatic risks in Northern Europe.
• In the past, no noteworthy systematic changes
  have been detected, when homogeneous,
  sufficiently long times series of evidence has
  been considered.
• For the futures, scenarios describe the
  possibility for a weak intensification of strong
  wind cases (baroclinic storms) in winter, and a
  reduction and northward shift of polar low
  activity.
• Scenarios are consistent with ongoing change.
• The methodology, based on dynamical downscaling
  and the usage of proxies for storminess are well
  developed. They may be employed for analyzing
  conditions in other parts of the world.

27
Reserve
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How do size and surface conditions influence the
description of storm activity?
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Single-station based proxies
(median 0.198, 0.05-quantile -0.09)
correlations between annual 99th percentiles of
ground level wind speed and annual number of
pressure observations below 980 hPa
30
Local pressure stats since 1800
Stockholm Lund
Time series of pressure-based storminess indices
derived from pressure readings in Lund and
Stockholm. From top to bottom Annual number of
pressure observations below 980 hPa (Np980),
annual number of absolute pressure differences
exceeding 16 hPa/12 h (NDp/Dt), Intra-annual
95-percentile and 99-percentile of the pressure
differences (P95 and P99) in units of hPa. From
Bärring and von Storch, 2005 see also BACC 2008.
31
Special application assessing the lull wind
conditions, leading to insufficient energy
harvest by wind mills in Germany, winters
2008-2009
Weisse, pers. Comm.
32
Downscaling vs. obs
C0,72
NCEP-based downscaling (black) and observations
(red) of MetNo (Noer, pers comm)
Monthly comparison of NCEP-downscaling (in black)
with analysed observed data (in red
Blechschmidt, 2008)
33
Density of polar low genesis
Genesis in NCEP downscaling RCM simulation
Bracegirdle, T. J. and S. L. Gray, 2008