1 / 30

ISMAR

ISMAR. Influence of different vertical mixing schemes and wave breaking parameterization on forecasting surface velocities S. Carniel 1 , J.C. Warner 2 , R.P. Signell 2 , J. Chiggiato 3 , P.-M. Poulain 4 1 CNR-ISMAR, Venice, Italy 2 USGS, Woods Hole, USA 3 SMR-ARPA-EMR, Bologna, Italy

liriano
Download Presentation

ISMAR

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ISMAR Influence of different vertical mixing schemes and wave breaking parameterization on forecasting surface velocities S. Carniel1, J.C. Warner2, R.P. Signell2, J. Chiggiato3, P.-M. Poulain4 1 CNR-ISMAR, Venice, Italy 2USGS, Woods Hole, USA 3 SMR-ARPA-EMR, Bologna, Italy 4 OGS, Trieste, Italy ROMS-Oct’04

  2. Motivations • Most 3D circulation models compute subgrid scale momentum and tracer mixing by means of a two equation turbulence closure scheme, TCMs (e.g Mellor-Yamada 2.5 or k-ε). • These closure schemes fail, however, in wave affected surface layers, and eddy viscosity “errors” produce unrealistic velocities.

  3. Motivations • These models are tuned to treat the sea-surface as a solid boundary and therefore, during events of strong wind, reproduce a log velocity profile in the proximity of the surface. • This is in contradiction with recent studies and measurements: during breaking wave conditions, the near-surface mixing is higher and the velocity shear lower than those modeled by usual TCMs.

  4. Current picture Air-sea interface is not rigid, therefore OML cannot be patterned after a solid boundary (e.g. law of the wall) A) Near surface region “breaking layer”, O(Z0S), all mixed; B) region adjacent to air-sea interface, O(10 Z0S), turb. diss. rate decays with a power law -4 (l.o.w.: -1). (Kantha&Clayson, 2004; Drennan, 1996; Terray 1996…) C) l.o.w. valid again at a certain distance from the surface …most of wave generated TKE dissipated in the O(SWH) Highly desirable to inlcude wave-breaking to explore near-surface distributions of T, S, velocities (S&R, oil-spill predictions, etc.)

  5. k –  p=3, m=1.5, n=-1  = cp km n k – k p=0, m=1, n=1 Two-Equations 2nd mom. TCMs Following Reynolds’ approach, 2nd moment quantities are computed as u’w’= KM (U/ Z). Thus two extra prognostic eqs. are required: 1st for the transport of the TKE, k (or q2); 2nd for the transport of turbulence length scale, . ...integrating them, the Eddy Viscosity (Diffusivity) coeff.for mometum (scalar) at (t,z) is KM (KH) k Ref:Kantha, 2004. The length scale equation in turbulence models. Nonlinear processes in Geophysics, 7, 1-12 Kolmogoroff relation ( k3/2  -1) allows the choices of several … giving the name to the 2nd mom-2 eqs TCM: k-, k-k, k- (turb. Freq. k1/2 -1)…i.e. generally km n (Generic Length Scale approach) Ref: Umlauf and Burchard, 2003. A generic length-scale equation for gephysical turbulence. J. Marine Research, 61(2), 235-265

  6. Tools – Numerical Models How: integrating Umlauf & Burchard (UB 2003) GLS method, allowing to choose among different parameterisations of vertical mixing processes, into Regional Ocean Model System (ROMS), a 3-D finite-difference hydrodynamical model (Warner et al., 2005) Where: a) idealized 20 m deep basin b) Adriatic Sea When: Febraury 2003 (bora event)

  7. Sensitivity Tests 1-D ROMS Idealized basin 20 m deep 1000x1000 m 100 stretched levels Wind stress u-direction: 1 N/m2 (approx. 20 m/s) Periodic BCs NESW …vertical resolution…

  8. TKE Surface B.C.: (Craig and Banner, 1994) 100-150 …at z=0, =f(Z0s). k=0.4, but… Z0s =f(sea state) Charnock formula (1955) fully developed sea: const Turbulence and Wave-breaking a: 1400 (CB 1994; GOTM 1999, etc.) • …Navier-Stokes eqs. describe all hydrodynamic processes, but the real world has large range of spatial/temporal scales… • D.N.S.= accurate modeling of flows where all turbulent scales are resolved. No closure assumptions required. Applied numerically to idealised, small-scale problems. Demanding very large computer resources. • L.E.S.= predict large scale turbulent structures as large energy-containing eddies, while small scales into which the KE is transferred are parameterised.

  9. Sensitivity Tests 1-D ROMS -Test 1 C&B All GLS with K-w, NO C&B GEN, NO C&B (UB 2003) GEN, CB Lsft=0.2, a=1400 A …C&B increases surface TKE

  10. Sensitivity Tests 1-D ROMS C&B All GLS with K-w, NO C&B GEN, NO C&B GEN, CB Lsft=0.2, a=1400 …C&B shows minor surface velocities A

  11. Sensitivity Tests 1-D ROMS All GLS with C&B K-w K-eps GEN Lsft=0.2, a=1400 B …all including C&B... showing differences among TCMs…

  12. Sensitivity Tests 1-D ROMS All GLS with C&B K-w K-eps GEN Lsft=0.2, a=1400 B …all including C&B.. see difference among TCMs…

  13. Sensitivity Tests 1-D ROMS All GLS with C&B K-w K-eps GEN Lsft=0.2, a=1400 B B …differences due to…

  14. Sensitivity Tests 1-D ROMS All GLS-GEN with C&B Lsft=0.2, a=1400 Lsft=0.2, a=14000 Lsft=0.4, a=1400 Lsft=0.4, a=14000 C

  15. Sensitivity Tests 1-D ROMS All GLS-GEN with C&B Lsft=0.2, a=1400 Lsft=0.2, a=14000 Lsft=0.4, a=1400 Lsft=0.4, a=14000 C

  16. Sensitivity Tests 1-D ROMS All GLS-GEN with C&B Lsft=0.2, a=1400 Lsft=0.2, a=14000 Lsft=0.4, a=1400 Lsft=0.4, a=14000 NO C&B C …value of alpha to be used?

  17. TKE Surface B.C.: (Craig and Banner, 1994) 100-150 …at z=0, =f(Z0s) Z0s =f(sea state) Charnock formula (1955) fully developed sea: const Turbulence and Wave-breaking a=1400? …in order to to have Z0s=O(SWH), use O(105) (KC 2004, Stacey 1999) • …Navier-Stokes eqs. describe all hydrodynamic processes, but the real world has large range of spatial/temporal scales… • D.N.S.= accurate modeling of flows where all turbulent scales are resolved. No closure assumptions required. Applied numerically to idealised, small-scale problems. Demanding very large computer resources. • L.E.S.= predict large scale turbulent structures as large energy-containing eddies, while small scales into which the KE is transferred are parameterised.

  18. Sensitivity Tests 1-D ROMS All GLS-GEN with C&B Lsft=0.2, a=1400 Lsft=0.4, a=100000 Lsft=0.2, a=100000 C NO C&B … …

  19. Surface Wind from LAMI model LAMI: 3-D finite-difference, non hydrostatic, 7 km resolution, Forecast output every 3 hours

  20. Surface Currents from ROMS model ROMS: 3-D primitive eqs, hydrostatic, sigma level, finite difference These are surface currents (0.5 m) from the GLS GEN (UB 2003) case

  21. 3-D ROMS in the Adriatic Bora Velocity at 5-m depth(m/s)

  22. Floaters release from ROMS model GEN, No C&B GEN, C&B Z0S= f(Charnok), L_sft=0.2, a=1400 Drifters data Floaters kept at 0.5 m… (modification to floats.in file to the trajectory type file in order to keep them at a fixed depth…)

  23. GLS as k-e vs GEN KEPS C&B Z0S= f(Charnok), L_sft=0.2, a=14000 GEN C&B Z0S= f(Charnok), L_sft=0.2, a=14000 Drifters data

  24. GLS as k-e vs GEN GEN wave-breaking Z0S= f(Charnok) L_sft=0.4, a=100000 KEPS wave-breaking Z0S= f(Charnok) L_sft=0.4, a=100000 Drifters data

  25. Message • Recently it has become possible to modify two equation turbulence models in order to account for wave-breaking effects. • When wave-effects are included, near-surface shears are significantly reduced, better matching observations, surface currents are diminished (and are virtually less sensitive to the near-surface grid resolution!) • First simulations incorporating wave-enhanced mixing point out how model results (e.g. velocities) are sensitive to how we parameterize the roughness scale.

  26. Message • How to handle the length scale near the surface (i.e what is it at z=0) is still an open issue • In real-life situations the choice of correct parameters appear to be more important than the TCM selected (at least for this data-set and within the GLS set)

  27. EOP

  28. 3-D ROMS in the Adriatic Scirocco Velocity at 5-m depth(m/s)

  29. “S3” Seasonal Evolution Forcings: Wind: 1 hour S: restoration Run P1: k- Run P2: one-eq (length scale prescribed algebraically) Run P3: GLS Run P4: k- SURFACE BOTTOM

  30. Where does turbulence come from? R.A.N.S.= (still) the most convenient way to describe complex flow situations, where all turbulent motions are parameterised by a sub-scale turbulencemodel in a statistical sense. adopting Reynolds’ approach: ... are new unknowns for which transport equations can be written but contain thirdmoment covariances… ad infinitum Equations not closed at any level! Turbulence is an unresolved problem in physics! • …Navier-Stokes eqs. describe all hydrodynamic processes, but the real world has large range of spatial/temporal scales… • D.N.S.= accurate modeling of flows where all turbulent scales are resolved. No closure assumptions required. Applied numerically to idealised, small-scale problems. Demanding very large computer resources. • L.E.S.= predict large scale turbulent structures as large energy-containing eddies, while small scales into which the KE is transferred are parameterised.

More Related