2D interpolations and regriddings

More details on the 2D interpolations and regriddings available and related options are provided here.

• 2D nearest neighbour (element nneighbour2D): an inverse-distance weighted nearest-neighbour interpolation (the great circle distance on the sphere is used):
  • The number N of source neighbours can be specified (element nbr_neighbours).
  • The distance can be weighted by a Gaussian (element gaussian_variance)
  • If some or all of the N nearest neighbours are masked, different options are available (element if_masked)
  • This interpolation is available for all types of 2D grid supported by OASIS4 (see section 5.3.1).

• Bilinear (element bilinear): an interpolation based on a local bilinear approximation :
  • If some or all of the 4 bilinear neighbours are masked, different options are available (element if_masked).
  • This interpolation is available for all types of 2D grid supported by OASIS4 (see section 5.3.1)

• Bicubic (element bicubic): an interpolation based on a local bicubic approximation :
  • Two bicubic methods are available (element bicubic_method): either gradient i.e. the 4 enclosing source neighbour values and gradient values based on the 12 additional enclosing neighbours are used (only for PRISM_reglonlatvrt and PRISM_irrlonlat_regvrt grids, see section 5.3.1) or sixteen i.e. the 16 enclosing source neighbour values are used (this second method assumes that the source points are located 4 by 4 at the same latitude and is therefore valid only for PRISM_reglonlatvrt and PRISM_gaussreduced_regvrt grids, see section 5.3.1).
  • If some or all of the 16 bilinear neighbours are masked, different options are available (element if_masked).

• 2D conservative (element conservativ2D): the weight of a source cell is proportional to area of the source cell intersected by target cell.
  • Currently, only the first order conservative remapping is available.
  • Different types of normalization can be applied (element methodnorm2D)
  • This remapping is available for all types of 2D grid supported by OASIS4 (see section 5.3.1).
  • The following considerations must be taken into account when choosing the 2D conservative remapping:
    • Using the divergence theorem, the SCRIP library evaluates the cell intersections with the line integral along the cell borders enclosing the area. As the real shape of the borders is not known (only the location of the 4 corners of each cell is defined with the prism_set_corners call, see 5.3.2), the library assumes that the borders are linear in latitude and longitude between two corners. In general, this assumption is not really valid close to the poles. For latitudes above the north_thresh or below the south_thresh values specified in oasis4/lib/common_oa4/include/psmile.inc, the library evaluates the intersection between two border segments using a Lambert equivalent azimuthal projection. Problems have been observed in some cases for the grid cell located around this north_thresh or south_thresh latitude.

    • Another limitation of the SCRIP conservative remapping algorithm is that is also supposes, for line integral calculation, that $sin(latitude)$ is linear with respect to the longitude on the cell borders which again is in general not valid close to the pole.

    • For a proper consevative remapping, the corners of a cell have to coincide with the corners of its neighbour cell.

    • Duplicated cells (e.g. when a periodic grid overlaps to itself) are not allowed. In general, duplicated cells should be excluded from the valid shape (see 5.3.1); if it is not possible, dupliczted cells should then be masked.

    • A target grid cell intersecting no source cell (either masked or non masked) at all i.e. falling in a ``hole'' of the source grid will not be treated and will not receive any value

    • If a target grid cell intersects only masked source cells, it will be given the psmile_dundef value (=-280177.0).