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A Sensitivity Study on the NOAA/GLERL Wind Wave Model used in the Graphical Forecast Editor

A Sensitivity Study on the NOAA/GLERL Wind Wave Model used in the Graphical Forecast Editor. David Zaff (NWS) and Keith Jaszka (summer student) NOAA/National Weather Service Buffalo, NY Oct 08, 2008. Goal. Learn a bit about the NOAA/GLERL Wind Wave Model .

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A Sensitivity Study on the NOAA/GLERL Wind Wave Model used in the Graphical Forecast Editor

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  1. A Sensitivity Study on the NOAA/GLERL Wind Wave Model used in the Graphical Forecast Editor David Zaff (NWS) and Keith Jaszka (summer student)NOAA/National Weather ServiceBuffalo, NY Oct 08, 2008

  2. Goal Learn a bit about the NOAA/GLERL Wind Wave Model Fun Fact: The Great Salt Lake is the largest producer of brine shrimp in the world.

  3. Main variables • Only 3 variables are needed for the GLERL Wind Wave Model: • Air Temperature • Wind Speed • Lake Temperature

  4. Unstable or Stable Marine Boundary Layer • Cold lake, warm air • Inversion • Small waves • Warm lake, cold Air • Unstable • Friction • Big waves Climate/Temperature for Lake Erie

  5. Assumptions • Test for Lake Ontario • Set lake to 0 C for entire study and varied: • the air temperature • The wind speed. • Constant wind at 240 degrees • (typical direction) • Ran model until wave heights reached equilibrium state • No ice.

  6. 400 Simulations later… • Results strongly support rule of thumb: • Winds>18 kts (9.5 m/s) produce: • waves in excess of 4 ft (1.2 m) • Regardless of air/lake temperature difference SMALL CRAFT ADVISORY (18-33 kts and/or waves > 4 ft (3-5 ft)

  7. Lets see some data…

  8. Wind speed (0-10 kts) Small Craft Advisory (>= 4 ft/(1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  9. Wind speed (0-15 kts) Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  10. Wind speed (0-40 kts) Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  11. Wind speed (10-20 kts) • Focus in on area that straddles the SMALL CRAFT ADVISORY10-20 kts. • If you forecast 10-20 kts – should you issue a SCA? • Stable airmass – you’re OK • Marginally stable or unstable airmass? Don’t know Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  12. Wind speed (10-20 kts) THE FORECAST….SOUTHWEST WINDS 10 TO 20 KNOTS Small Craft Advisory (>= 4 ft/1.2m) Worse case scenario: 10kt winds 2.2 ft 20kt winds 6.5 ft (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  13. Case Study #1August 16, 2008 • Upper level low passing by to east • Lake temp: 73F/23C • Forecast High: 75F/24C (after a morning low of 56F/13C) 500mb Height Anomaly 500mb Height UNSTABLE in the Marine Boundary Layer

  14. Case Study #1August 16, 2008 850mb Temperature MSLP • The nearshore forecast (off lake Erie): • .TODAY...NORTHWEST WINDS 10 KNOTS OR LESS BECOMING SOUTHWEST AND INCREASING TO 10 TO 20 KNOTS. PARTLY SUNNY WITH AN ISOLATED SHOWER OR THUNDERSTORM POSSIBLE. ISOLATED WATERSPOUTS POSSIBLE. WAVES 2 FEET OR LESS BUILDING TO 2 TO 3 FEET.

  15. Case Study #1August 16, 2008 • Through 12Z: • SOUTHWEST WINDS 10 TO 20 KNOTS • WAVES 2 TO 3 FEET. Winds 10-15kts Waves below 2 ft

  16. Case Study #1August 16, 2008 • Through 16Z: • SOUTHWEST WINDS 10 TO 20 KNOTS • WAVES 2 TO 3 FEET. 3 TO 5 FEET SCA X 16Z: “What’s going on?” Winds estimated 15-20kts With waves near 3ft. VERY choppy waters A large no. of boats returning to harbor Winds 12G18kts Waves below 2 ft

  17. Case Study #1August 16, 2008 • End result (22Z)/Verification • SOUTHWEST WINDS eventually hit 20ktswith gusts near 25kts • WAVES approached 5ft.

  18. Wind speed (10-20 kts) • End result (22Z)/Verification • SOUTHWEST WINDS eventually hit 20ktswith gusts near 25kts • WAVES approached 5ft. Very close to 20kt Sensitivity study line Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  19. 1st Important Conclusion • Do not forecast 10-20kt wind forecasts when you have: • 1) a long fetch • 2) Winds to last several hrs • 3) an unstable or marginally unstable Marine Boundary Layer. • Rather – choose: • 10-15kts (No SCA) or • 15-20kts (SCA)

  20. Case Study #2(Questionnaire….) • What’s the highest wave height observed in Lake Ontario? • (Buoy data available from: • approx April-Nov 2002-2007 ) • A) 15ft • B) 20ft • C) 25ft • D) 30ft • E) 35ft November 13, 2003 Wind: 24345G59KT 3AM Forecast: STORM WARNING 50KT STORMS WAVES 10-15 FEET

  21. Wind speed (0-40 kts) Outside study area, But probably on target • End result/Verification for highest wave heights • On Lake Ontario (Apr-Nov 2002-2007) • SOUTHWEST WIND 45kts with gusts to 59kts • WAVES approached 25ft. Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  22. Case Study #3Sept 15, 2008 • The Remnants of Hurricane IKE passed right over Lake Ontario…. Sept 15, 2008 Sept 14, 2008 Sept 13, 2008

  23. Case Study #3Sept 15, 2008 346AM and 1006AM update: SW WIND TO 30KTS WAVES 5-7 FT 341PM: W WIND TO 30KTS WAVES 4-6 FT 545PM: S TO W GALES TO 40KTS WAVES 4-7 FT 607PM GALES TO 35KTS WAVES 7-10FT 704PM GALES TO 40KTS WAVE 7-10FT 1007PM GALES TO 35KTS WAVES 7-10FT

  24. Case Study #3Sept 15, 2008 • Verification: • SOUTHWEST WINDS eventually hit 39ktswith gusts to 50kts • WAVES hit 12ft.

  25. Wind speed (0-40 kts) Sensitivity Study would Have over forecast wave Heights by several feet. But this was a unique case with short lived Wind event • End result/Verification IKE • WEST WINDS 39kts with gusts to 50kts • WAVES hit 12ft. Small Craft Advisory (>= 4 ft/1.2m) (Cold Lake and Warm Airmass) (Warm Lake and Cold Airmass)

  26. Case Study #2 and #3 • Why did we under forecast wave heights? • Is it the formatter? • What did the “Grids” show?

  27. Stability Considerations • Lighter winds have more of an impact on wave height when factoring in instability • 3 “regimes” • Moderate/Stronger winds – less impact when considering instability • 2 “regimes” Wind speed (10-20 kts) End State Wave Height (ft)

  28. Summary • SCA Rule of thumb works: • Winds >18kts  Issue a SCA regardless of marine boundary layer • HOWEVER - • SCA conditions possible for wind as low as 14kts • unstable marine boundary layer • Forecasting 10-20kt winds can get you in trouble during summer/fall • Wave Heights can vary substantially (below or above SCA limits) within this range. • Waves more susceptible to stability considerations for lighter winds.

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