INTERNAL WAVE GENERATION, BREAKING, MIXING AND MODEL VALIDATION ALAN DAVIES (POL) JIUXING XING (POL)...

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.

• Meterological , solar heating gives stratification , with wind forcing + stratification internal waves

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.

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

HOW DO WE VALIDATE THAT WE HAVE CORRECT INTERNAL WAVE + MIXING

• INTERNAL WAVE SPECTRA AT KEY LOCATIONS

• DETAILED + COMPREHENSIVE TURBULENCE MEASUREMENTS

MODEL NEEDS

• DETAILED SMALL SCALE TOPOG.

• PRECISE SPECTRA OF FORCING AND ITS AMPLITUDE

• ACCURATE INITIAL STRATIFICATION AND DETAILS OF ITS EVOLUTION FOR VALIDATION

HOW TO PARAMETERIZE AND UPSCALE TO LARGE AREA MODELS

• Topographic gradients dh/dx

• Details of stratification

• Details of small scale wind forcing

TWO EXAMPLES INTERNAL WAVE MIXING

• Wind forced internal waves trapped in cold water dome

• Tidally forced internal waves over a sill.

Format

• (A) Internal Wave trapping in Domes

• (B) Mixing over abrupt topog.

• Conclusions and future Developements

BAROCLINIC IRISH SEA MODEL

• Simulation 3D baroclinic model

• Dome formation and breakdown

• Dome circulation published JPO

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

Z

UV

AZ

VfY

UV

X

UU

T

U

Z

VV

AZ

UfY

VV

X

VU

T

V

Super-inertial wind forcing

• Wavelength λf from Dispersion Relation

• ωf = forcing frequency

• So λf/Leff gives nodal structure where Leff is “effective length” of dome

2f

K21C2f2

fω

fλπ2

fK

2/1h2h1hg

1C

Sub-inertial wind forcing

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

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

Initial Conditions

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 ?????

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

Future

• Role surface stratification / fresh water , wind mixing

• Detailed distribution of Topog.

• Sill b.b.l effects

• Lateral + across sill form drag

Model Skill Assessment

• Model Validation in highly variable undersampled domain.

• Spectral Decompostion.. Hans van Haren

SPECTRA

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 !!!!!!!!!