A coherent framework for forecasting currents, waves and drift:

1. What we do at SHOM

2. Hydrodynamics theory

3. First results

4. Perspectives on remote sensing

Dr. Fabrice ARDHUIN, Nicolas Rascle

(SHOM/CMO, Brest, France)Dr. Alastair Jenkins,

(Bjerknes Center, Bergen, Norway)Dr. Bertrand Chapron (Ifremer/DRO/OS)

Funding from: Aurora program (Norway-France), and project MERCATOR

Plateaucontinental

Açores

Madère

EspagnePortugal

France

GrandeBretagne Plateau

continental

1998: first forecast of the SOPRANE system

Espagne

Portugal

France

GrandeBretagne

Climatology (1990)

Barotropic streamfunction

Ardhuin and others - 3Wave effects in the upper ocean – STW-SAR, Brest 2004

Center for Military Oceanographyfrom the early 1990s to today

Ardhuin and others - 3Wave effects in the upper ocean – STW-SAR, Brest 2004

Ocean circulation modelling today

HYCOM2004: Mercator, and …

2006: coastal model demonstrator : 1-2 km resolution from Dover Straits to Gibraltar(this is a 10 M€ program: field data, computer, HF radars, contribution to Mercator …)

Ardhuin and others – 4Wave effects in the upper ocean – STW-SAR, Brest 2004

+ RCC Corsen

http://surfouest.free.fr/

One basic problem of today’s current models:

-Surface drift is about 2-3% of wind speed

-Turbulence is very strong near the surface -> uniform profiles -> small surface velocity (0.5 – 1% of wind)

(Agrawal et al. 1992, Craig and Banner 1994, Mellor and Blumberg 2004)

But what velocity do we talk about ?

-> ad hoc empirical fix (e.g. MOTHY …)

Ardhuin and others – 6Wave effects in the upper ocean – STW-SAR, Brest 2004

Motions at the air-sea interface

Ingredients of surface drift:

Ardhuin and others – 7Wave effects in the upper ocean – STW-SAR, Brest 2004

Motions of air and water (no oil, no ship …) Wave breaking

Langmuir circulations

eddy mixing

(internal wave breaking ...)Eddy viscosity

Kz

z = 0

z = - Hs ~ z0

total wind stress (momentum flux between atmosphere and ocean)

wave induced stress (wind input): in tangentialwind stress a - in

z = - <h>z = - <h> +

Sediments

Drift velocityue+ust

net momentum uptake for waves : in - dis

(growth-dissipation)

wave dissipation stress ("virtual wave stress" + breaking) : dis

Mixed boundary layerwaves+current

z = - <h> +

Mixing processes

thermocline

radiationstress

radiationstress

I

satellite

Ardhuin and others – 8Wave effects in the upper ocean – STW-SAR, Brest 2004

A general 3D formalism (Ardhuin & Jenkins submitted to JFM 2004,

extension of Mellor, JPO 2003): Mellor used a vertical coordinate transform from z to :

with due to waves

This can be re-derived from the GLM of Andrews & McIntyre 1978):

Ardhuin and others – 9Wave effects in the upper ocean – STW-SAR, Brest 2004

Mixing parameterization: a GLM-average TKE equation

TKE production by waves:

TKE production due to « Stokes drift shear »

Used by Tolman & Chalikov 1996

Ardhuin and others – 10Wave effects in the upper ocean – STW-SAR, Brest 2004

Application to swell dissipation(Ardhuin et Jenkins, submitted to JPO, 2004)

Ardhuin and others – 11Wave effects in the upper ocean – STW-SAR, Brest 2004

First application: the surface mixed layer

What is the vertical profile of Tdis ? (we are working on this, paper in preparation for J. Geophys. Res.)

Tdis is the momentum flux from waves to currents due to wave dissipation (viscosity, breaking, wave-turbulence interaction).

+ b.c. on momentum: - in Wind stress – wave-induced stress

+ b.c. on TKE flux(e.g. Mellor and Blumberg 2004, Janssen & al. 2004)

Coriolis force: waves and mean flow Vertical mixingWave dissipation stress

- Tdis (z)

Ardhuin and others - 9Ardhuin and others – 12Wave effects in the upper ocean – STW-SAR, Brest 2004

1D Mixed layer. No stratification (Craig and Banner, 1994)Kz = Sm q l, l=max[z0 ,0.41(z0-z)] based on Mellor-Yamada 2.5

q = sqrt(b) from TKE equation : db/dt = production + transport - dissipation

With P-M wave spectrum, z0= Hs

(Mellor and Blumberg 2004) U10=10 m/s

uL = U = û + Us = ue + ust

“Classical”, no waves z0=0.1 m. U10=10 m/s

Wind stress

Ardhuin and others – 13Wave effects in the upper ocean – STW-SAR, Brest 2004

HYCOM with wave forcing

1st realistic application : Prestige oil spill hindcast

HYCOM 1/3 degré ATL+MED, assimilating altimetry, forcing: ARPEGE winds + WAM (ECMWF) waves

Standard HYCOM

Ardhuin and others – 14Wave effects in the upper ocean – STW-SAR, Brest 2004

Wave heights from same image(tiled “imagettes” processed as level 2)

New observation methods: Doppler signal from Synthetic Aperture Radars, ATI and/or Doppler centroïd: here Envisat images in VV polarization

processing: Ifremer – Boost Technologies

Verfication of theory: perspectives on remote sensing

Ardhuin and others - 12Wave effects in the upper ocean – STW-SAR, Brest 2004

Ardhuin and others – 15Wave effects in the upper ocean – STW-SAR, Brest 2004

Doppler velocity UD (m/s)

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HF radars measure « surface drift »: Sea trials with Uni. Toulon, 2003 (VHF)

Deployment of a HF-radar system, 2005 (funding: DGA research programs)

Ardhuin and others – 17Wave effects in the upper ocean – STW-SAR, Brest 2004

ardhuin@shom.frhttp://www.shom.fr/fr_page/fr_act_oceano/vagues/vagues.htm

http://www.shom.fr/fr_page/fr_act_oceano/vagues/

PLUS/PUBLIS/index_f.html http://surfouest.free.fr/WOO2003/

Conclusions:

- Surface velocities are not fully understood (work under

way)

- Today’s models are not coherent

- New remote sensing data (that we hope to further validate in the next 2 years)

APPENDIX: example of coastal wave forecast validation at Blancs Sablons Beach, just south of RCC Corsen (measurements: March-April 2004)

Observations

NB: none of the models include tidesas yet (that will be “Surfouest V2”).

CREST (ray-tracing), Initialized with WW3(“surfouest V1”)

REF-DIF initialized with WW3(“surfouest V0”)

Offshore waves