Configuration Description

ORCA is the generic name given to global ocean configurations. Its specificity lies on the horizontal curvilinear mesh used to overcome the North Pole singularity found for geographical meshes. LIM (Louvain-la-Neuve sea-ice model, multi-category model LIM3 is used) is a thermodynamic-dynamic sea ice model specifically designed for climate studies. A brief description of the model is given here.

=Space-time domain=

  The horizontal resolution available through the standard configuration is ORCA2. it is based on a 2 degrees Mercator mesh, (i.e. variation of meridian scale factor as cosinus of the latitude). In the northern hemisphere the mesh has two poles so that the ratio of anisotropy is nearly one everywhere. The mean grid spacing is about 2/3 of the nominal value: for example it is 1.3 degrees for ORCA2. Other resolutions (ORCA4, ORCA05 and ORCA025) are running or under development within specific projects. In the coarse resolution version (i.e. ORCA2 and ORCA4) the meridional grid spacing is increased near the equator to improve the equatorial dynamics. Figures in pdf format of mesh and bathymetry can be found and downloaded here. The sea-ice model runs on the same grid.[[BR]]


 The vertical domain spreads from the surface to a depth of 5000m. There are 31 levels, with 10 levels in the top 100m. The vertical mesh is deduced from a mathematical function of z (Madec and Imbard 1996). The ocean surface corresponds to the w-level k=1, and the ocean bottom to the w-level k=31. The last T-level (k=31) is thus always in the ground.The depths of the vertical levels and the associated scale factors can be viewed .Higher vertical resolution is used in ORCA025 and ORCA12 ( see [https://www.drakkar-ocean.eu DRAKKAR ] project). [[BR]]


 The time step depends on the resolution. It is 1h36' for ORCA2 so that there is 15 time steps in one day.[[BR]]


=Ocean Physics (for ORCA2)=

    horizontal diffusion on momentum: the eddy viscosity coefficient depends on the geographical position. It is taken as 40000. m2/s, reduced in the equator regions (2000 m2/s) excepted near the western boundaries.[[BR]]
    isopycnal diffusion on tracers: the diffusion acts along the isopycnal surfaces (neutral surface) with a eddy diffusivity coefficient of 2000. m2/s.[[BR]]
    Eddy induced velocity parametrization with a coefficient that depends on the growth rate of baroclinic instabilities (it usually varies from 15 m2/s to 3000m2/s).[[BR]]
    lateral boundary conditions : zero fluxes of heat and salt and no-slip conditions are applied through lateral solid boundaries.[[BR]]
    bottom boundary condition : zero fluxes of heat and salt are applied through the ocean bottom. The Beckmann [19XX] simple bottom boundary layer parameterization is applied along continental slopes. A linear friction is applied on momentum.[[BR]]
    convection: the vertical eddy viscosity and diffusivity coefficients are increased to 1 m2/s in case of static instability.[[BR]]
    forcings: the ocean receives heat, freshwater, and momentum fluxes from the atmosphere and/or the sea-ice. The solar radiation penetrates the top meters of the ocean. The downward irradiance I(z) is formulated with two extinction coefficients [Paulson and Simpson, 1977], whose values correspond to a Type I water in Jerlov's classification (i.e the most transparent water)[[BR]]


=ORCA2_LIM is a reference configuration with the following characteristics:=

    global ocean configuration,[[BR]]
    based on a tri-polar ORCA grid, with a 2° horizontal resolution,[[BR]]
    31 vertical levels,[[BR]]
    forced with climatological surface fields,[[BR]]
    coupled to the sea-ice model LIM[[BR]]


=AGRIF demonstrator:=

From the ORCA2_LIM configuration, a demonstrator using AGRIF nesting can be activated. It includes the global ORCA2_LIM configuration and a nested grid in the Agulhas region.
To set up this configuration, after extracting NEMO (see http://www.nemo-ocean.eu/Using-NEMO/User-Guides/Basics/NEMO-Quick-Start-Guide):[[BR]]

    Build your AGRIF configuration directory from ORCA2_LIM, with the key_agrif cpp key activated:[[BR]]


{{{
cd NEMOGCM/CONFIG ; ./makenemo -r ORCA2_LIM -n AGRIF add_key "key_agrif"
}}}


    Using the ORCA2_LIM input files and namelist, AGRIF test configuration is ready to run[[BR]]