Vertical Tidal Mixing in ROMS: A Reality Check

Transcription

1 Vertical Tidal Mixing in ROMS: A Reality Check Dr. Robin Robertson School of PEMS UNSW@ADFA Canberra, Australia Adjunct at Lamont-Doherty Earth Observatory of Columbia University

2 Vertical Tidal Mixing Internal Tides = significant mixing mechanism 3.2 TW (TW= 2 W) [Garrett, 23] Measurements Difficult and expensive Small scale... Hard to locate Episodic Harder to be there at right time From Models Model needs to reproduce the energy transfers from the tides to dissipation Depends on the vertical mixing parameterization used Need high resolution and good bathymetry

3 Why get Vertical Mixing Right? Important for: Water mass formation and transformation Indonesian Throughflow Polar regions Structure of the water column Biological studies Diffusion of momentum Non-linear wave interactions

4 Location of INSTANT Moorings

5 M 2 Internal Tides for Transect across the INSTANT Moorings

6 A Region of Inactive/Background Internal Tides Model spectrum match of that the observations within 9% confidence levels for.2-.33cph Differ at high and low frequency ends Energy levels match Garrett- Munk (dashed green line (N=2 cph))

7 A Region of Active Internal Tides Harmonics peak up for both the model and the observations (red and black) Energy levels above Garrett-Munk (dashed green line at high frequencies Differ at high and low frequency ends

8 Vertical Mixing Parameterizations In ROMS Mellor-Yamada 2.5 level turbulence closure (MY2.5) Kpp Large-McWilliams-Doney (LMD) Brunt-Väisälä Frequency Based (BVF) Pacanowski-Philander (PP) General Ocean Mixing (GOM) Generic Length Scale (GLS) κ-ω κ-ε κ-κl Generic -old Generic -new

9 Fieberling Guyot Water Depth B Latitude F2 R2 P F4,F5 F3 C R B3 5 (m) Longitude

10 Evaluated the Performance of these Mixing 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

11 Semidiurnal Tidal Velocity Dependence: Vertical Mixing Parameterization Depth (m) -2-4 MY Bottom (cm s - ) LMD -2-4 GLS: k-kl GLS: k- ω Depth (m) -2-4 GOM GLSgen-new GLS: k- ε Depth (m) -2-4 BVF PP GLS: gen Latitude Latitude Latitude

12 Diffusivity of Temperature: Vertical Mixing Parameterization Observations (K ρ ) from Kunze and Toole (995; 997) Depth (m) Depth (m) Depth (m) Obs. MY GOM P, 997 GW, 24 (m 2 s - ) E-3 E-4 E-5 Bottom LMD GLSgen-new -2-4 GLS: k-kl GLS: k- ω GLS: k- ε Depth (m) -2-4 BVF PP GLS: gen Latitude Latitude Latitude

13 Vertical Diffusivities Averaged over the Region MY25*.4x -3 m 2 s - GOM 9.x -4 m 2 s - BVF.3x -2 m 2 s - (way too big) LMD*.x -3 m 2 s - LMD-SCCW.x -3 m 2 s - PP 8.x -5 m 2 s - (too small) GLS- kkl 3.8x -3 m 2 s - GLS- kω 3.7x -3 m 2 s - GLS- kε*.9x -3 m 2 s - GLS- gen*.8x -3 m 2 s - GLS- gen2* 3.7x -4 m 2 s- (looks best; good value)

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 Depth (m) Depth (m) Depth (m) MY GOM BVF Bottom (cm s - ) LMD GLSgen-new PP -2-4 GLS: k- GLS: k- GLS: gen Latitude Latitude Latitude GLS: k-kl

15 Spectral Response Background Location Follows Garrett-Munk spectra (N=5 cph) Spectral Density ((cm s - ) 2 /cph) Spectral Density \((cm s - ) 2 /cph) Spectral Density ((cm s - ) 2 /cph) m 73 m 48 m U obs V obs U model V model GM Spectrum... Frequency (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 all of the transfer of energy between frequencies or energy cascade Does induce non-linear interactions and transfer energy from low to high frequencies Does remove the higher frequency energy and contribute to the slope Background location Black with vertical diffusion Red without vertical diffusion Spectral Density ((cm s - ) 2 /cph) Spectral Density ((cm s - ) 2 /cph) Spectral Density ((cm s - ) 2 /cph)... E-5 E-6... E-5 E-6 E-7... E-5 E-6 Surface.. Mid-Water Column.. Bottom.. Frequency (cph)

18 What about Higher Frequencies? Diurnal and Semidiurnal tidal peaks clearly visible.4 hr- (24 hr).8 hr- (2 hr) GOM has increased energy at highest frequency (midlevel) Upper Water Column (layer 55 of 6) Spectral Density ((cms - ) 2 /cph) Mid-Water Column (layer 3 of 6) Spectral Density ((cms - ) 2 /cph) Lower Water Column (layer 7 of 6) Spectral Density ((cms - ) 2 /cph) Spectra at 3 Levels Site in Internal Wave Generation Region East-West Velocity... Frequency (cph)... Frequency (cph)... Frequency (cph) MY 2.5 GOM BVF LMD LMD-SSCW PP GLS-kkl GLS-kω GLS-kε GLS-gen North-South Velocity... Frequency (cph)... Frequency (cph)... Frequency (cph)

19 Focusing on Higher Frequencies Internal Wave Generation Site. Frequency (cph) Frequency (cph) Frequency (cph). Frequency (cph). Frequency (cph). Upper Water Column (layer 55 of 6) Spectral Density ((cms - ) 2 /cph) Upper Water Column (layer 7 of 6) Upper Water Column (layer 3 of 6) Upper Water Column (layer 55 of 6).. Spectra at 3 Levels Site in Internal Wave Generation Region East-West Velocity North-South Velocity Frequency (cph) MY 2.5 LMD GLS-kkl GLS-kω GLS-kε GLS-gen Lower Water Column (layer 7 of 6) Spectral Density ((cms - ) 2 /cph) Mid-Water Column (layer 3 of 6) Spectral Density ((cms - ) 2 /cph)

20 Normalizing by MY25 Internal Wave Generation Site Frequency (cph) 4 Frequency (cph) 3 2 Upper Water Column (layer55 of 6) Normalized Spectral Density (-) Normalized Spectral Density (-) Normalized Spectral Density (-) 6 Frequency (cph) Normalized Spectral Density (-) 3 2 Frequency (cph) Frequency (cph) Mid-Water Column (layer 3 of 6) Normalized Spectral Density (-) Normalized Spectra at 3 Levels Site in Internal Wave Generation Region East-West Velocity North-South Velocity MY 2.5 LMD 4 GLS-kkl GLS-kω GLS-kε 3 GLS-gen Frequency (cph) Bottom Boundary Layer (layer 7 of 6) Normalized Spectral Density (-)

21 Conclusions??? ROMS can reproduce tidal energy and the harmonics both in regions of active and inactive internal tides (resolution important) Vertical mixing parameterizations 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-κω have high average K M MY25 has bump in energy at high frequency end Differences at tidal frequencies are very minor Small differences occur in high frequencies.

22 Summary All vertical mixing schemes generated spectra roughly following G-M Believed to be due to non-linear interactions and the vertical mixing parameterization Best performers are: GLS-gen(new) GLS-κω, GLS-gen then MY25, LMD Based on matching tidal velocities Dissipation: both with depth and area average Spectral response of velocities

23 Still in Search of a Definitive Answer/ Where to go from here This study has some shortcomings Done at a low resolution (2km) Done with old version of ROMS Presently repeating the analysis km resolution with 6 levels New version of ROMS (29) Looking at the frequency response of the vertical diffusivity

24 Questions?