An operational wavemodel for the Faroe Shelf

1. An operational wavemodel for the Faroe Shelf Thesis defence 2/4-2007 Bárður A. Niclasen Náttúruvísindadeildin Statoil, Phillips, Enterprise og Veba

2. Tidal currents and their influence on sailing conditiones

3. Wave Current Effect of stationary currents on waves (size<20 km) (quasi-stationar approximation) Wave height and steepness increase while absolute period remains constant

4. Large breaking wave hit Sjóberinat ~18:30 on the 8/3-2007 Reason : current treveled in opposite direction of the wind

5. Existing operational wave forecast http://vs.en.sigling.is – based on ECMWF forecast Buoy  Time Hm0 Dir  North WV-3 12:04 9.7 338 West WV-2 12:22 5.4 359 East WV-1 11:51 5.6 17 South WVD-4 12:39 7.4 342 http://ocean.dmi.dk

6. Motivation for this project The impact of tidal currents on the sea state in the coastal sone of Faroe Islands are very important ...but these effests are not included in present operational wave models for the area • Need local wave model with increased resolution • that includes the effect from the tidal currents on the waves. • As this can improve sea safety in the long run

7. Outline • General information • Validation of wave forecasts from ECMWF • Optimal settings of a local wave model (SWAN) • SWAN runs including the effect from tidal currents on the sea state. • Conclusions/Outlook

8. Waves according to linear theory Steepness of the wave is H/λ

9. Wave measurement example Hm0 =2.0 m Tp =6.3 s Wave parameters: Hm0 = 2.0m wave height Tp = 6.3s peak-period Tm02 = 4.9s average-period ... T-1

10. How linear wave models represent real sea states

11. Governing equations in wave model Propagation: where, Dynamics:

12. WV-1, WV-2, WV-3 & F-8: Landsverk. WVD-4: DataQuality. K7: MetOffice. ECMWF validation • 4 hindcast periods spanning one month • All include one major storm (the 4 largest in 1999-2004 at WVD-4) • 2 summer events and 2 winter events

13. Time series from WVD-4 vs. ECMWF in Event 1

14. Measured Model Wave spectra at WVD-4. in Event 1

15. Statistical average from all events

16. measurements padded f -5 tail WVD-4 fhigh = 0.58 Hz, WV-2 fhigh = 0.50 Hz, K7 fhigh = 0.25 Hz Effect of high ferquency cut off Tm02 bias WVD-4: 0.5 s WV-2: 0.6s K7: -0.2s After correcting for different measurement range (padding f-5 tail) WVD-4: 0.8 s WV-2: 1.0s K7: 1.2s

17. Conclusions – on ECMWF validation • Overall it is found that ECMWF analysis wind and wave data are suited to force local wave model • WV-1 and WV-3 not suited for validation • Seasonal variation with less negative Hm0 bias in summer periods compared to winter periods • Smooth predicted spectra with some missing swell events • Slightly less energy in spectral tail compared to measurements (positive bias in Tm02)

18. Choosing a local model • SWAN was choosen because: • High resolution runs favour implicit propagation • Want to include unstationary currents • Less site specific tuning

19. Nested SWAN model domains

20. Different model physics/numerics • Dynamics

21. Different SWAN runs, Event 1, Nesting 1

22. Intermediate results from Nesting 1 • SWAN-Komen: Hm0 and Tp OK, but serious negative bias in Tm02 • SWAN-Janssen: Tp and Tm02OK, but negative bias in Hm0 • SWAN-1G and SWAN-2G: OK, but too much scatter in results • A need to ’retune’ source terms in SWAN

23. Whitecapping dissipation in SWAN SWAN-Komen (WAM3) : SWAN-Janssen (WAM4):

24. Least error if n=2.0-2.1 Least error if n=1.9 Sds,w’n’-tuningno f -5 tail with f -5 tail

25. ECMWF-averaged spectra close to the measured, but slight undershoot in the high frequencies Default SWAN poor fit to the measured spectra, but good fit with new ’n’ Default SWAN-Janssen poor fit to the measured spectra, but good fit with new limiter Time averaged spectra from modelled spectra vs. data from WVD-4 in Event 1

26. Waves If the nested models are compatible with the forcing they should have Hm0 and Tm02 close to the forcing model (black) Do the nested models accept the incoming sea state? Compatability at the boundary Default SWAN is not compatible with the forcing model, but retuned SWAN-n=2 and SWAN-Janssen with new limiter are compatible with the ECMWF-WAM4 forcing !!

27. Conclusions from 1.-nestings • Optimal nested-model performance with SWAN-Komen with n =1.9 (2.0-2.1) • SWAN-Komen with retuned ’n’ much better compatibility with ECMWF-WAM4 than default setup • Hersbach-Janssen limiter greatly improves the SWAN-Janssen runs, but runs are unstable in high resolution

28. U-component (East) of the tidal current ellipse in [m/s] for Event 1 2. and 3.-nestings with tidal currents Direct validation of the numerical tidal model not possible, but it can be compared against independent tidal predictions based on measurements One such example from a location close to the south buoy...

29. m Waves Hm0=2.5m Wind 10 m/s Influence from tidal currents on wave height in idealized test case(difference plot)

30. Waves Hm0=2.5m Wind 10 m/s Influence from tidal currents on wave steepness in idealized test case(difference plot) Offshore wave steepness ~ 0.058

31. Waves Hm0=2.5m Wind 10 m/s Influence from tidal currents on wave dissepation in idealized test case

32. 3.nesting with and without currentsexample where the tidal effect is clear at WVD-4

33. Conclusions from 2. and 3. nestings • SWAN capable of recreating most of the tidally induced variations seen in the measurements • SWAN-Komen with n=2 is a better option than default SWAN for deepwater areas with unstationar currents • DIA (Snl) counteracts variations in spectral shape (Tm02) forced by unstationar currents • Lag in measured Hm0 variations compared to relative current due caused by dynamical effects (up-wind slowing down of wave energy)

34. Suggested operational setup Reducing computational cost • 2.nesting only • 1.-order propagarion • Larger time step Comp. time v.s realtime Nest 3 setup extended to the entiere nest-2 domain: comp. ratio ~ 1.4 (i.e. 2-day forecast finnished after 2.8 days) Operational setup of nest-2 area: comp. ratio ~ 0.1 i.e. 2-day forecast in 5 hours if serial comp. or approx. 1.0-1.5 hours if paralell comp.)

35. Summary • Regional wave/wind model is vaildated for the Faroese area (ECMWF) • Local wave model is implemented, adjusted and verifyed • Operational setup for a local wave model including the effects from tidal currents is suggetsed. Is possible to run on the local linux-closter.

36. Outlook • Operational waveforecasts that include the effect of unstationary tidal currents are possible for the Faroe Shelf ... if $$! • Implementation of high-resolution in wave- and wind-models, inclusion of wave reflections, dynamic currrents (oceanic, wind, pressure)

37. Thank you

38. Don't be cruel ....pity me!!

39. Wave spectra at WVD-4, event 1, 3.-nesting

40. 3.nesting with and without currents

41. Intermediate conclusions from 3.nestings • Default SWAN-Komen better predictiones without including the effects of the current • SWAN-Komen with n=2 better predictiones when including the effects from the current • Variations in wave height and direction recreated to resonable extent when including the influence from the current • Modelled variations in Tm02 are missing ... ? • Modelled variations in Hm0 are lagging ... ?

42. 3.nestings with different source terms turned off

43. Numerical tidal model used in the 2. and 3.-nestings model vs. tidal ( prediction from measurements at WV-4) Estimated maxmum current strength

44. Effect of stationary currents on waves (size<20 km) (quasi-stationar approximation) Wave height and steepness change while absolute period remains constant