Forecasting extremes
Presentation
by
Anne Karin Magnusson
Norwegian Meteorological Institute
Marine forecasting center, Bergen
Marked.met.no
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Maxwave- EU project 2000-2003
A. THEME, “Extreme ocean
waves”.
Singular ocean waves sometimes
have damage impacts far beyond
what seems expected from the
general sea state.
The aim of Maxwave was to
enhance and put under scrutiny the
knowledge on these “maxwaves”
and to derive new safety tools for
marine design and operation.
Cross section of sea elevation
from SAR imagette.
B. EVIDENCE AND
CHARACTERISTICS
Evidence comes from 2Hz records by lasers
and waveriders. Abrupt extremes occur, or
groups of waves which seem to stay ‘in family’
for some time, finally to culminate in a
singular extreme event. There is some evidence
that ‘crossing seas’ produce outstanding
extremes. In the statistical sense, there is a
need to investigate further and conclude
whether such waves belong to their separate
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population. Three popular terms apply to
categories of extreme waves; “The White
Wall”, “Tower Waves”, and wave groups
Johannes Guddal & Anne Karin Magnusson sometimes named “Three Sisters”.
OGP, Paris 12.-16. September 2005
OUTCOMES AND IMPACTS FROM MAXWAVE
I. Two emerging new tools for recording
singular and extreme waves and wave
groups came out of Maxwave;
–
the application of nautical radar images
(sea clutters) to derive wave data
– the application of Synthetic Aperture
Radar (SAR) imagettes to produce the
same.
Further improved understanding is also
expected from laser arrays
II. Offshore construction designers have in
general compensated for singular
events by application of conservative
safety factors.
III. Offshore operations may benefit from
better forecasting of such events, but
forecasting techniques can only be
probability based.
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Development of criteria and forecasting methods
Measuring
techniques
Observations
Research
Forecast/Hindcast
Models
Tools
in design
Design
Environmental
database
Hindcast
Operation limits
User
Observations/
Validation
Forecast
Prognosis
(Operation
Leader)
Forecaster
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Elements of ”enhanced forecasting”
during extreme events
Measuring
techniques
Observations
Research
Forecast/Hindcast
Models
Tools
in design
Design
Environmental
database
Hindcast
Operation limits
User
Observations/
Validation
Forecast
Prognosis
(Operation
Leader)
Forecaster
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Forecasting extreme events (waves)
A]
Probability based
B]
– BFI index : validity not
verified with observations
– Crossing index :
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
BFI Benjamin-Feir Instability index (Janssen)
Olagnon and Magnusson, Isope 2004:
”Sensitivity study of sea state parameters in correlation to extreme wave occurences”
Analysis of 18.000 records
(or 3.3 106 waves) (Frigg data):
Number of cases as function of BFI,
defined as some measure of steepness divided by
some measure of spectral bandwidth.
Exhibit a high rate of false
alarms !
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Crossing index
• MeteoFrance started forecast of a crossing index during the
Maxwave period in the Mediterranean due to a specific highspeed craft accident in 2002
• Earlier reports (visual observations) have pointed at wave
directionality as cause in ”freak wave” occurences
@BBC
– Litterature (Monthly weather review / …)
– Visual reports (BBC production ’Rogue waves’)
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Aftermaths of Maxwave
• Rogues’04 in Brest (organized by Michel Olagnon)
• AHA’04 in Hawai (Peter Muller and Al Osborne)
• Extreme or freak waves are topic in other conferences
• New EU projects (proposals)
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
”The role of meteorological focusing in generating rogue wave conditions”
Donelan and Magnusson, 2005
Sharp increase in
the probability of
high wave crests for
a given significant
wave height when
sea is mixed
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Forecasting extreme events
A]
Probability based
– BFI index : validity not
verified with observations
– Crossing index :
B]
Highly qualified forecasters and
proper riskreducing procedures
•
EXWW (Ekofisk eXtreme Wave Warning)
14 years of experience with continuous
development
Similarity to:
• Hurricane actions in GoM ?
• Polar lows in Barents Sea ?
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
EXWW – forecasting an ”air gap”
• Combination of Hs and water level
forecast
• Criteria are direction dependent due to
sheltering effects of the tank
• Criteria are shown on the product -> easy
to interprete in a stressed situation
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Elements in enhanced forecasting
during extreme events
• Monitoring (QA-data, qualified personell, ..)
• Forecasting products comparable to operational criteria
(enhanced use of design aspects?)
• Up to date in last developments (models,
instrumentation, research…….)
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Response forecasting
From wave spectra via MIMOSA to respons
Geofjord
Geofjord
Minimum Hiv
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Monitoring from space
1. January 1995 (Draupner case)
A polar low developed in northerly
air stream in Norwegian Sea
and moved Southward in
North Sea
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Monitoring: space and ground data
15. dec. 2003 03 UTC . [NOAA_clouds + observations + MSLP]
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
HIRLAM-20km:
55-60 kts max
EC-40km :
45 kts max
UKFIN:
45 kts max
QSCAT: 60 kts max
Wind shift
west of Faeroes:
Confirm observations that
indicate a faster
eastward propagation.
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Scatterometer winds and ground radar data (precipitation)
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Monitoring
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Johannes Guddal & Anne Karin Magnusson
wind
Observations from several
platforms every 10 or 20
minutes:
waves
•
Wind
•
MSLP, Temperature (air/water),
Water level,
•
Wave parameters
•
Wave profiles
•
Wave 2D-data
OGP, Paris 12.-16. September 2005
Monitoring
Observations from several
platforms every 10 or 20
minutes:
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Johannes Guddal & Anne Karin Magnusson
•
Wind
•
MSLP, Temperature (air/water),
Water level,
•
Wave parameters
•
Wave profiles
•
Wave 2D-data
OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Comparing 1D spectra from WAM model and MIROS (-20/0/+20 min)
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Wave instrumentation on Ekofisk, central North Sea (56.5 N 3.2 E)
Waverider
WAMOS
Laser Flare North
Laser array
Laser Flare South
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Monitoring with new measuring techniques
The 4 lasers in an array at Ekofisk,
produce HR directional wave information
for rogue wave studies.
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
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OGP, Paris 12.-16. September 2005
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Means of improving forecasting
in extreme events
• Research
• Workshops / conferences
• User contact
• High competance !
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005
Relevance of Maxwave to offshore?
 Minor impact on present design procedures;
 Interest to investigate a possible separation of ‘abnormal’ conditions from
‘normal’, consequently to test validity of hypothesis of “stationarity”;
 Based on phenomenological studies: What will be the physical causes of
abnormal events?
 Provide high quality observation data for research and validation studies.
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Johannes Guddal & Anne Karin Magnusson
OGP, Paris 12.-16. September 2005