NEMO developments for regional modelling at MERCATOR-OCEAN G. Reffray, J. Chanut, B. Levier and S. Cailleau and the modelling team of Mercator-Ocean

Regional activities at MERCATOR-OCEAN carried out since 2005 Projects: (SHOM): Model intercomparison study on the Biscay domain for 2004 (NEMO, MARS and SYMPHONIE) => EPIGRAM (ECOOP)/ (EASY): regional R&D – improvement of the customers services (new OpenDAP for ECOOP) : (INFOCEAN): Development of a relocatable tool to set up a regional configuration wherever in the global ocean. Applications in the Gulf of Guinea, Indonesia and Canarian islands are expected (MyOcean – WP8 IBI): Development of an operational system on the IBI region (France, Ireland, Portugal, Spain) at the 1/36° including high frequency processes. Reference: Mercator Ocean Quaterly Newsletter #30, July Towards North East Atlantic Regional modelling at 1/12° and 1/36° at Mercator Ocean - Model intercomparison in the Bay of Biscay during 2004: the front of Ushant and the cold water pool

What is the “Regional version of NEMO” developed and used at Mercator Ocean ? Current version (NEMO version 2.3): -Explicit tidal forcing -Explicit free surface with time-splitting -Non-linear free surface -New OBC package -Model of turbulence : TKE2 and GLS -New advection schemes for tracer (QUICKEST+ULTIMATE, PPM) -BDY package to introduce river runoffs as lateral point sources -High frequency meteorological fields (3h) including diurnal cycle -Ocean colour dependant short wave penetration (on going work)

Tidal forcing Tidal amplitude (m) and phase (°) of M2 Currently, the considered tidal spectrum is composed of 11 components. Harmonic analysis of M2: FES2004NEATL36

Non linear free-surface (variable volumes key_vvl) Relax η<<H hypothesis. Update the vertical coordinate at each time step. Important for non linear tidal waves (for ex: M4), residual currents, internal waves induced mixing or river plumes spreading. Annual mean sea surface salinity for 2004 (config BISCAY) In presence of tidal amplitudes, if the hypothetis of the fixed volumes is kept, the spreading of the river plumes is restricted. SYMP CLIM NEMO MARS NEMO VVL Tracers conservation with time splitting is still problematic with tides. Example in the Bay of Mont Saint-Michel where the tidal range is greater than 10 meters. As a consequence, a spurious « salinity plume » is induced cause to this numerical artefact

Time splitting scheme update (1) Problem: The NEMO standard scheme (Griffies et al, 2001) does not conserve tracers locally. Maybe acceptable for climate oriented modeling, but not for shelf applications where the sea surface elevation is high. Annual mean sea surface salinity for 2004 (NEATL12) Are they realistic salinity plumes ? (Our) solution: Replace existing scheme by ROMS UCLA generalized forward-backward (Shchepetkin and McWilliams, OM, 2005): conservative and cheaper !

Time splitting scheme update (2) Sea surface salinity differences between the standard time- splitting scheme in NEMO and the new one (NEATL12). The values exceed 10 PSU closed to the coast ! Standard NEMO scheme New scheme Standard - new

New OBC package More flexible: allow several segments for each open boundary type. Clarified time splitting case. Fixed open boundary corner dilemma. Added flow relaxation.Added state of the art open boundary schemes: Barotropic mode: Specified, Neumann, characteristic method (Blayo and Debreu 2005), Flather, Normal/tangential implicit radiation, Sommerfeld radiation. Specific to tidal/surge modeling: Blumberg and Mellor (1987), Clamped elevation. Baroclinic mode: Specified, Neumann, Normal/tangential implicit/explicit radiation. Tracers: Specified, Neumann, upwind advection.

Bdy package for rivers inputs Climatological river flows in NEATL36 model Add special mode in bdy package for river inflows case (nbdy_dta=2). Simplify input data files: only flowrates at selected u-v cells are needed instead of velocity profiles. Set S=0 and zero temperature/baroclinic velocity fluxes. Advantages: Input of momentum 3D structures: the salinity plume has its own inertia and the interaction with the circulation is more realistic Numerically robust even for extreme events especially with refined surface vertical mesh Possibility to specify rivers temperature and salinity (for ex: St-Laurent).

Turbulence update Upgraded 1.5 TKE closure: The spatial discretization of the shear term in the turbulent kinetic energy equation has been modified to ensure numerical stability. No need to horizontally average the viscosities and diffusivities to ensure the numerical stability (nave=0 !). The routine step.F90 has been reorganized for energy consistency. (available in NEMO 3, key_zdftke2) Generic length scale (GLS) model (Umlauf and Burchard, 2003): Resolves two prognostic equations for turbulent kinetic energy and a generic length scale. Thanks to the latter, commonly used closures can be retrieved: k-ε, k- , Mellor Yamada... The viscosities and diffusivities also depend of stability function (Canuto et al 2001, Kantha-Clayson 1994, Galperin 1983). For turbulence sensitivity tests, this model has been implemented. Surface boundary condition: Mellor-Blumberg (2003) (simplification of the Craig- Banner BC 1994) => impact of the waves Bottom boundary condition: Balance between production and dissipation The mixing length is limited by the value at the bottom: l min =Kz b with z b = m => l min = m

Tidal front k-  TKE2 MODIS Mean SST July 2004 First run - TKE JULY NEATL12

Radiative fluxes vs cloud cover Winter Summer Radiatives fluxes Cloud cover Seasonal biases between MODIS and NEATL12 (OBS-MOD). The biaises are surestimated with the CEP radiative fluxes in summer

Future work (for July !) NEATL36 : Run 06/2007 => 06/2009 with updates of the model and forcings. September : validation of this simulation Update of the bathymetry : SST May 2008 Actual bathymetry New bathymetry The irregularities of the bathymetry signe the SSTs fields. The tidal front should be better represented with the new bathymetry (thanks Florent Lyard for his contribution).

Other regional applications (INFOCEAN project) 1/12° configuration : extraction of the ORCA12 bathymetry, initialized and forced at the open boundaries by the MERCATOR reanalysis GLORYS (1/4°). Preliminary results, no validation done yet ! Gulf of Guinea (after 4 months of run …) : Surface temperatureSurface salinity

Other regional applications (INFOCEAN project) Indonesia : first simulation (no validation done yet !) Mean surface fields for 2006 (after 2 years of spin-up) Temperature Salinity Velocity It runs ! Are the results goods ? Need validation …

Conclusion and perpectives The model is numericaly stable for several applications : Bay of Biscay, NEATL12, NEATL36, Gulf of Guinea and Indonesia We think that the major numerical development is finished Validation needed ! Real time production of NEATL36 at the end of 2010

Thank you !

Impact of the non linear free-surface Tidal amplitude (m) and phase (°) of M4 (non linear wave from M2). (config NEATL36) Non linear free surface Linear free surfaceMOG2D