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INTERNAL WAVE GENERATION , BREAKING, MIXING AND MODEL VALIDATION

INTERNAL WAVE GENERATION , BREAKING, MIXING AND MODEL VALIDATION. ALAN DAVIES (POL) JIUXING XING (POL) JARLE BERNTSEN (BERGEN). EXTERNAL FORCING. Tides (Moon , Sun ) , with stratification (T or S origin) + topography gives internal waves.

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INTERNAL WAVE GENERATION , BREAKING, MIXING AND MODEL VALIDATION

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  1. INTERNAL WAVE GENERATION , BREAKING, MIXING AND MODEL VALIDATION • ALAN DAVIES (POL) • JIUXING XING (POL) • JARLE BERNTSEN (BERGEN)

  2. EXTERNAL FORCING • Tides (Moon , Sun ) , with stratification (T or S origin) + topography gives internal waves. • Meterological , solar heating gives stratification , with wind forcing + stratification internal waves

  3. LOCAL MIXING INFLUENCE LARGE SCALE CIRCULATION • Significant Ocean circulation in lateral boundary layers • Topographic gradients + Density gradients in these regions , source of internal wave generation , + mixing which influences their lateral extent , Hence boundary layer flow.

  4. MIXING SOURCES • Energy cascade through breaking internal waves • Internal waves generated in one region propagate to another • Energy loss to mixing during propagation • Energy loss to mixing , due to non-linear processes giving rise to wave breaking

  5. HOW DO WE VALIDATE THAT WE HAVE CORRECT INTERNAL WAVE + MIXING • INTERNAL WAVE SPECTRA AT KEY LOCATIONS • DETAILED + COMPREHENSIVE TURBULENCE MEASUREMENTS

  6. MODEL NEEDS • DETAILED SMALL SCALE TOPOG. • PRECISE SPECTRA OF FORCING AND ITS AMPLITUDE • ACCURATE INITIAL STRATIFICATION AND DETAILS OF ITS EVOLUTION FOR VALIDATION

  7. HOW TO PARAMETERIZE AND UPSCALE TO LARGE AREA MODELS • Topographic gradients dh/dx • Details of stratification • Details of small scale wind forcing

  8. TWO EXAMPLES INTERNAL WAVE MIXING • Wind forced internal waves trapped in cold water dome • Tidally forced internal waves over a sill.

  9. Format • (A) Internal Wave trapping in Domes • (B) Mixing over abrupt topog. • Conclusions and future Developements

  10. BAROCLINIC IRISH SEA MODEL • Simulation 3D baroclinic model • Dome formation and breakdown • Dome circulation published JPO

  11. Non-Linear effects on Inertial Oscillations • Unbounded Ocean Eqts • Effect of external shear is to change Amp. + Freq. of I.O. • Frontal Shear Changes I.O. amp./Freq at depth so conv/divg. Gives internal wave at level of thermocline. • Freq. int. wave above inertial propogates away , if below trapped

  12. Super-inertial wind forcing

  13. Wavelength λf from Dispersion Relation • ωf = forcing frequency • So λf/Leff gives nodal structure where Leff is “effective length” of dome

  14. Sub-inertial wind forcing

  15. CONCLUSIONS • 1. Non-linear effects associated with along frontal flows produce near-inertial internal waves in presence of wind forcing • 2. Super-inertial internal waves propagate away from generation region (front) • 3. Sub-inertial are trapped and enhance mixing in frontal region • 4. In a cold water bottom dome, super-inertial internal waves are trapped as standing waves, can modify GM spectrum • 5. Response in centre of dome different from 1D model, must account for internal wave • 6. Sub-inertial wave confined to front, and response in centre of dome as in 1D model

  16. TIDAL MIXING AT SILLS • Idealized Loch Etive • Recent measurements Inall et al • Non-hydrostatic model • High resolution • Idealized M2 forcing + idealized T profile • Example of internal tidal mixing

  17. Initial Conditions

  18. Influence of small scale topog. • Lee wave characteristics influenced by • Buoyancy frequency • Velocity over sill….. Froude Number • Fourier transform of topog. • So How small scale effect mixing ?????

  19. CONCLUSIONS….. Sill • Internal tide little mixing • Lee Wave not advected back over sill • Lee Wave major source of mixing • Lee wave distribution influenced by non-hydro. nature of model • Lee wave spectrum/mixing influenced by small scale topog. • Assumptions in b.b.l. also infulence lee wave hence mixing

  20. Future • Role surface stratification / fresh water , wind mixing • Detailed distribution of Topog. • Sill b.b.l effects • Lateral + across sill form drag

  21. Model Skill Assessment • Model Validation in highly variable undersampled domain. • Spectral Decompostion.. Hans van Haren

  22. SPECTRA

  23. Conclusions • Details of wind field frequency composition • Precision in stratification • Accurate tidal forcing • Precise small scale topog. Variations. • MAJOR PROBLEMS IN VALIDATION • HOW TO UPSCALE WITHOUT LOOSING ACCURACY !!!!!!!!!

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