Johannes Guddal & Anne Karin Magnusson OGP, Paris 12.-16. September 2005 1 Forecasting extremes Presentation by Anne Karin Magnusson Norwegian Meteorological.

Johannes Guddal & Anne Karin Magnusson OGP, Paris 12.-16. September 20051

Forecasting extremes

Presentation

by

Anne Karin Magnusson

Norwegian Meteorological Institute

Marine forecasting center, Bergen

Marked.met.no


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 CHARACTERISTICSEvidence 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 population. Three popular terms apply to categories of extreme waves; “The White Wall”, “Tower Waves”, and wave groups sometimes named “Three Sisters”.

Maxwave- EU project 2000-2003


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.


Development of criteria and forecasting methods

Prognosis

Hindcast

ObservationsForecast/Hindcast

Models

Design

Operation limitsEnvironmental

database

Observations/Validation

Forecast

Tools in design

Measuring techniques

Research

User

(Operation Leader)

Forecaster


Elements of ”enhanced forecasting” during extreme events

Prognosis

Hindcast

ObservationsForecast/Hindcast

Models

Design

Operation limitsEnvironmental

database

Observations/Validation

Forecast

Tools in design

Measuring techniques

Research

User

(Operation Leader)

Forecaster


Forecasting extreme events (waves)

A] Probability based

– BFI index : validity not verified with observations

– Crossing index :

B]


BFI Benjamin-Feir Instability index (Janssen) Olagnon and Magnusson, Isope 2004: ”Sensitivity study of sea state parameters in correlation to extreme wave occurences”

Exhibit a high rate of false alarms !

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.


Crossing index

• MeteoFrance started forecast of a crossing index during the Maxwave period in the Mediterranean due to a specific high-speed craft accident in 2002

• Earlier reports (visual observations) have pointed at wave directionality as cause in ”freak wave” occurences – Litterature (Monthly weather review / …)

– Visual reports (BBC production ’Rogue waves’)

@BBC


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)


”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


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 ?


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


Elements in enhanced forecastingduring 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…….)


Response forecastingFrom wave spectra via MIMOSA to respons

Geofjord Geofjord

Minimum Hiv


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


15. dec. 2003 03 UTC . [NOAA_clouds + observations + MSLP]

Monitoring: space and ground data


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.


Scatterometer winds and ground radar data (precipitation)



Monitoring

Observations from several platforms every 10 or 20 minutes:

• Wind

• MSLP, Temperature (air/water), Water level,

• Wave parameters

• Wave profiles

• Wave 2D-data

wind

waves


Monitoring

Observations from several platforms every 10 or 20 minutes:

• Wind

• MSLP, Temperature (air/water), Water level,

• Wave parameters

• Wave profiles

• Wave 2D-data






Comparing 1D spectra from WAM model and MIROS (-20/0/+20 min)

27Johannes Guddal & Anne Karin Magnusson OGP, Paris 12.-16. September 200527

Laser Flare North

Laser Flare South

WAMOSWaverider

Wave instrumentation on Ekofisk, central North Sea (56.5 N 3.2 E)

Laser array


Monitoring with new measuring techniques

The 4 lasers in an array at Ekofisk, produce HR directional wave information for rogue wave studies.




Means of improving forecastingin extreme events

• Research

• Workshops / conferences

• User contact

• High competance !


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.

Johannes Guddal & Anne Karin Magnusson OGP, Paris 12.-16. September 2005 1 Forecasting extremes Presentation by Anne Karin Magnusson Norwegian Meteorological.

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