Forecasting extremes Presentation by Anne Karin Magnusson Norwegian

Forecasting extremes Presentation by Anne Karin Magnusson Norwegian Meteorological Institute Marine forecasting center, Bergen Marked. met. no 1 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 2 Hz 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 2 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 Johannes Guddal & Anne Karin Magnusson“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. 3 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 4 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 5 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 : 6 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 ! 7 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

Crossing index • Meteo. France 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’) 8 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) 9 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 10 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 e. Xtreme Wave Warning) 14 years of experience with continuous development Similarity to: • Hurricane actions in Go. M ? • Polar lows in Barents Sea ? 11 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 12 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……. ) 13 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

Response forecasting From wave spectra via MIMOSA to respons Geofjord Minimum Hiv 14 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 15 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] 16 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

HIRLAM-20 km: 55 -60 kts max EC-40 km : 45 kts max UKFIN: 45 kts max QSCAT: 60 kts max Wind shift west of Faeroes: Confirm observations that indicate a faster eastward propagation. 17 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

Scatterometer winds and ground radar data (precipitation) 18 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

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Monitoring 20 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 2 D-data OGP, Paris 12. -16. September 2005

Monitoring Observations from several platforms every 10 or 20 minutes: 21 Johannes Guddal & Anne Karin Magnusson • Wind • MSLP, Temperature (air/water), Water level, • Wave parameters • Wave profiles • Wave 2 D-data OGP, Paris 12. -16. September 2005

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Comparing 1 D spectra from WAM model and MIROS (-20/0/+20 min) 26 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 27 27 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. 28 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005

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Means of improving forecasting in extreme events • Research • Workshops / conferences • User contact • High competance ! 31 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. 32 Johannes Guddal & Anne Karin Magnusson OGP, Paris 12. -16. September 2005