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

Plateau continental Açores Madère Espagne Portugal France Grande Bretagne Plateau continental 1998: first forecast of the SOPRANE system Espagne Portugal France Grande Bretagne Climatology (1990) Barotropic streamfunction Ardhuin and others - 3Wave effects in the upper ocean – STW-SAR, Brest 2004 Center for Military Oceanography from the early 1990s to today

Ardhuin and others - 3Wave effects in the upper ocean – STW-SAR, Brest 2004 Ocean circulation modelling today HYCOM 2004: 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

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 …) 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 T dis ? (we are working on this, paper in preparation for J. Geophys. Res.) T dis 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 mixing Wave dissipation stress - T dis (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) K z = S m q l, l=max[z 0,0.41(z 0 -z)] based on Mellor-Yamada 2.5 q = sqrt(b) from TKE equation : db/dt = production + transport - dissipation With P-M wave spectrum, z 0 = H s (Mellor and Blumberg 2004) U 10 =10 m/s u L = U = û + U s = u e + u st “Classical”, no waves z 0 =0.1 m. U 10 =10 m/s Wind stress Ardhuin and others – 13Wave effects in the upper ocean – STW-SAR, Brest 2004

HYCOM with wave forcing 1 st 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 2004Ardhuin and others – 15Wave effects in the upper ocean – STW-SAR, Brest 2004

Doppler velocity U D (m/s) ENVISAT ASAR, wide swath, VV complex, Doppler centroïd analysis © ESA, Boost Technologies ( )

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

PLUS/PUBLIS/index_f.html 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 tides as yet (that will be “Surfouest V2”). CREST (ray-tracing), Initialized with WW3 (“surfouest V1”) REF-DIF initialized with WW3 (“surfouest V0”) Offshore waves