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Vertical Mixing from ROMS: Spectral Response of Velocities

Vertical Mixing from ROMS: Spectral Response of Velocities. Dr. Robin Robertson School of PEMS UNSW@ADFA Canberra, Australia. Indonesian Throughflow. Domain Transects. Elevation Fields. Rms differences: M 2 7.6 cm S 2 5.8 cm K 1 11.7cm O 1 8.8 cm

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Vertical Mixing from ROMS: Spectral Response of Velocities

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  1. Vertical Mixing from ROMS: Spectral Response of Velocities Dr. Robin RobertsonSchool of PEMSUNSW@ADFACanberra, Australia

  2. Indonesian Throughflow Domain Transects

  3. Elevation Fields Rms differences: M2 7.6 cm S2 5.8 cm K1 11.7cm O1 8.8 cm With site 8 excluded

  4. Isotherm Fluctuations

  5. Vertical Mixing • Internal Waves are a significant mixing mechanism • 3.2 TW (TW=1012 W) [Garrett, 2003] • Interest in estimates of vertical mixing • Measurements • Difficult and expensive • Small scale • Episodic • From Models • Depends on the parameterization used

  6. Vertical Mixing ParameterizationsIn ROMS • Many available in Roms • Mellor-Yamada 2.5 level turbulence closure (MY2.5) • Kpp – Large-McWillamns-Doney (LMD) • Brunt-Väisälä Frequency Based (BVF) • Pacanowski-Philander (PP) • General Ocean Mixing (GOM) • Generic Length Scale (GLS) • - • - • -l • generic

  7. Evaluated the Performance of theseMixing Parameterizations • Internal Tidal model for Fieberling Guyot • Accessible data set for both velocities and dissipation • Good agreement for major axes of semidiurnal tidal ellipses between model estimates and observations

  8. Domain

  9. Comparison to Observations: Observations Model Results rms: 3.7 cm s-1 rms: 6.5 cm s-1 rms: 7.1 cm s-1

  10. Baroclinic Tides

  11. Velocity Dependence:Vertical Mixing Parameterization

  12. Diffusivity of Momentum:Vertical Mixing Parameterization

  13. Vertical Diffusivities Averaged over the Region • MY25* 1.1x10-4 m2 s-1 • GOM 7.0x10-5 m2 s-1 • BVF 3.4x10-3 m2 s-1 • LMD* 1.9x10-4 m2 s-1 • LMD-SCCW 1.7x10-4 m2 s-1 • PP 4.8x10-5 m2 s-1 • GLS- kkl 1.7x10-3 m2 s-1 • GLS- k 1.7x10-3 m2 s-1 • GLS- k* 9.9x10-5 m2 s-1 • GLS- gen* 8.9x10-5 m2 s-1

  14. Is there really no difference in the velocities? • Maybe the tidal frequencies aren’t the place to look • Vertical mixing transfers energy from tides to high frequencies, especially harmonics

  15. Spectral Response • Background Location • Follows Garrett-Munk spectra (N=5 cph)

  16. Leads to 2 questions • Why do the spectra follow Garrett-Munk? • Which one is the best performer?

  17. Garrett-Munk and Vertical Diffusion • Vertical Diffusion plays a big role in spectral shape • Is not responsible for the transfer of energy between frequencies or energy cascade • Does remove the higher frequency energy • Background location • Black – with vertical diffusion • Red – without vertical diffusion

  18. What about Higher Frequencies? • Diurnal and Semidiurnal tidal peaks clearly visible • 0.04 hr-1 (24 hr) • 0.08 hr-1 (12 hr) • GOM has increased energy at highest frequency (midlevel)

  19. Focusing on Higher Frequencies • Internal Wave Generation Site

  20. Normalizing by MY25 • Internal Wave Generation Site

  21. Focusing on Higher Frequencies • Background Site

  22. Normalizing by MY25 • Background Site

  23. Observations • Correlation between high vertical diffusivity and low spectral energy • GOM showed enhanced energy at high frequencies; exceeded the 95% confidence • PP shows very weak vertical diffusivity • BVF, GLS-kkl, and GLS- have high average KM • MY25 has bump in energy at high frequency end • Differences at tidal frequencies are small • Differences occur in high frequencies.

  24. Summary • All 10 vertical mixing schemes generated spectra roughly following G-M • Believed to be due to non-linear interactions and the vertical mixing parameterization • No answer as to which is “best” • Best performers are: GLS-, GLS-gen then MY25, LMD • Based on matching • tidal velocities • Dissipation: both with depth and area average • Spectral response

  25. The End:Time for Discussion

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