internal wave generation , breaking, mixing and model validation
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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 !!!!!!!!!