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Observation-Based Physics for the Third Generation Wave Models

Observation-Based Physics for the Third Generation Wave Models. Alexander Babanin, Ian Young, Erick Rogers and Stefan Zieger Centre for Ocean Engineering, Science and Technology Swinburne University, Melbourne Australian National University, Canberra Naval Research Laboratory, MS, USA.

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Observation-Based Physics for the Third Generation Wave Models

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  1. Observation-Based Physics for the Third Generation Wave Models Alexander Babanin, Ian Young, Erick Rogers and Stefan Zieger Centre for Ocean Engineering, Science and Technology Swinburne University, Melbourne Australian National University, Canberra Naval Research Laboratory, MS, USA Hobart, Australia May, 2014

  2. Radiative Transfer Equation is used in spectral models for wave forecast • Describes temporal and spatial evolution of the wave energy spectrum E(k,f,q,t,x) Stot– all physical processes which affect the energy transfer Sin – energy input from the wind Sds– dissipation due to wave breaking Snl– nonlinear interaction between spectral components Sbf– dissipation due to interaction with the bottom

  3. Motivation • physics (parameterisations of the source terms) was cursory • had not been updated for some 20 years • was not based on observations • bulk calibration • Requirements for the modern-day models: • more accurate forecast/hindcast • being used in the whole range of conditions, from swell to hurricanes • coupling with weather, ocean circulation and climate models

  4. Lake George experiment sponsored by ONR • 20 km x 10km • uniform finite water depth (0.3m - 2.2m) • steep waves fp > 0.3 Hz • strongly forced waves 1 < U/cp < 8 • source functions measured

  5. following the waves

  6. The full flow separation

  7. The parameterisation, growth rate γ

  8. Flow separation due to breaking (f) = 0(f)(1+bT)

  9. Wind Input – Sin Donelan, Babanin, Young and Banner (Part I, JTEC, 2005; Part II, JPO, 2006, Part III, JPO, 2007) • the parameterisation includes very strongly forced and steep wave conditions, the wind input for which has never before been directly measured in field conditions • new physical features of air-sea exchange have been found: • - full separation of the air flow at strong wind over steep waves • - the exchange mechanism is non-linear and depends on the wave steepness • - enhancement of the input over breaking wavesγ=γ0(1+bT) • - pressure-height decay is very rapid for strong winds – young seas combination

  10. Breaking Dissipation Sds • two passive acoustic methods to study spectral dissipation • segmenting a record into breaking and non-breaking segments • using acoustic signatures of individual bubble-formation events Babanin et al., 2001, 2007, 2010, Babanin & Young (2005), Manasseh et al. (2006), Young and Babanin (2006), Babanin (2011)

  11. White Cap Dissipation Sds 1) Spectrogram method Frequency distribution of dissipation due to dominant breaking. - Cumulative effect Young and Babanin, 2006, JPO

  12. White Cap Dissipation Sds 1) Segmenting the record Directional dissipation fp 2fp fp

  13. Manasseh et al., 2006, JTEC Sds Bubble-detection method Cumulative effect Dependence on the wind • two-phase behaviour of spectral dissipation: • linear dependence of Sdson the spectrum at the peak • cumulative effect at smaller scales • bTdepends on the wind for U10 > 14 m/s

  14. White Cap Dissipation Sds • Spectrogram method • Threshold behaviour of the dissipation Babanin and Young, WAVES-2005 Babanin & van der Weshuysen, JPO, 2008 Babanin et al., 2001, JGR • threshold behaviour is also predicted by the modulational instability

  15. Babanin, APS, 2009 White Cap Dissipation Sds • spectral dissipation was approached by two independent means based on passive acoustic methods • threshold: if the wave energy dissipation at each frequency were due to whitecapping only, it should be a function of the excess of the spectral density above a dimensionless threshold spectral level, below which no breaking occurs at this frequency. This was found to be the case around the wave spectral peak: dominant breaking • dissipation at a particular frequency above the peak demonstrates a cumulative effect, depending on the rates of spectral dissipation at lower frequencies • dimensionless saturation threshold value of • should be used to obtain the dimensional spectral threshold Fthr(f) at each frequency f • dissipation depends on the wind for U10 > 14 m/s

  16. Methodology Tsagareli et al, 2010, JPO Babanin et al., 2010, JPO • Important! • observe known physical constrains • calibrate the source functions, if necessary, separately

  17. Babanin, CUP, 2011 Swell attenuation b1=0.002 • Dissipation • volumetric • per unit of surface • per unit of propagation distance

  18. Young, Babanin, Zieger, JPO, 2013 Swell attenuation

  19. TESTING, CALIBRATION, VALIDATIONS

  20. Charles James, PIRSA-SARDI (UA), Spencer Gulf SWAN model, default and new physics (Rogers et al., 2012)

  21. Cyclone Yasi Deep water track, winds (top), waves (bottom) waves next to Townsville (ADCP) and Cape Cleveland (buoy)

  22. TEST451 validations/updates • if based on physics, incremental improvements of the model are possible • changing a source term does not require re-turning the other source terms • integrals of the sources can be used as fluxes for coupling with GCMs • Hindcast 2006 • Spatial bias in mean wave height • TEST451 with BETAmax=1.33 • BYDRZ swell parameter b1=0.25E-3 BYDRZ

  23. observation-based source terms Implemented in WAVEWATCH-III and SWAN • Wind input (Donelan et al. 2006, Babanin et al, 2010) • weakly nonlinear in terms of spectrum • slows down at strong winds (drag saturation) • constraint on the total input in terms of wind stress • Breaking dissipation (Babanin & Young 2005, Rogers et al. 2012) • threshold in terms of spectral density • cumulative effect away from the spectral peak • strongly nonlinear in terms of spectrum • Non-breaking (swell) dissipation (Babanin 2011, Young et al. 2013) • interaction of waves with water turbulence • Negative input (adverse or oblique winds, Donelan 1999, unpublished Lake George observations) • of principal significance for modelling waves in tropical cyclones • Physical constraints (Babanin et al. 2010, Tsagareli et al. 2010)

  24. Where to go?

  25. work in progress • Wave-bottom interactions (completed) • Boundary layer model instead of wind input parameterisation • New term for non-linear interactions (non-homogeneous, quasi-resonant, Stokes corrections, wave breaking) • Wave-current interactions • Wave-ice interactions

  26. Model Based on Full Physics Can be used for • prediction of adverse events (dangerous seas, freak waves, swells, breaking, steepness, PDF tail) • outputting the fluxes • coupling with extreme weather (hurricane) models • coupling with atmospheric and oceanic modules of GCMs, atmospheric boundary layer, ocean circulation, climate

  27. Input and total stress f-4 to f-5 JONSWAP DHH

  28. Whitecapping dissipation • now, coefficients a and b need to be found • Young and Babanin a = 0.0069 (only one record analysed) coeff. a and b based on the input/dissipation ratio Donelan (1998) showing the fraction of momentum (dashed line) and of energy (plain line) retained by the waves

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