The grid types for the transformations

As described in section 6, the different transformations in OASIS3 support different types of grids. The characteristics of these grids are detailed here.

1. Grids supported for the INTERP interpolations (see section 6.4)
   • `A' grid: this is a regular Lat-Lon grid covering either the whole globe or an hemisphere, going from South to North and from West to East. There is no grid point at the pole and at the equator, and the first latitude has an offset of 0.5 grid interval. The first longitude is 0$^o$ (the Greenwhich meridian) and is not repeated at the end of the grid ( $CPER = P and $NPER= 0). The latitudinal grid length is 180/NJ for a global grid, 90/NJ otherwise. The longitudinal grid length is 360/NI.

   • `B' grid: this is a regular Lat-Lon grid covering either an hemisphere or the whole globe, going from South to North and from West to East. There is a grid point at the pole and at the equator (if the grid is hemispheric or global with NJ odd). The first longitude is 0$^o$ (the Greenwhich meridian), and is repeated at the end of the grid ($CPER = P and $NPER= 1). The latitudinal grid length is 180/(NJ-1) for a global grid, 90/(NJ-1) otherwise. The longitudinal grid length is 360/(NI-1).

   • `G' grid: this is a irregular Lat-Lon Gaussian grid covering either an hemisphere or the whole globe, going from South to North and from West to East. This grid is used in spectral models. It is very much alike the A grid, except that the latitudes are not equidistant. There is no grid point at the pole and at the equator. The first longitude is 0$^o$ (the Greenwhich meridian) and is not repeated at the end of the grid ($CPER = P and $NPER= 0). The longitudinal grid length is 360/NI.

   • `L' grid: this type covers regular Lat-Lon grids in general, going from South to North and from West to East.. The grid can be described by the latitude and the longitude of the southwest corner of the grid, and by the latitudinal and longitudinal grid mesh sizes in degrees.

   • `Z' grid: this is a Lat-Lon grid with non-constant latitudinal and longitudinal grid mesh sizes, going from South to North and from West to East. The deformation of the mesh can be described with the help of 1-dimensional positional records in each direction. This grid is periodical ($CPER = P) with $NPER overlapping grid points.

   • `Y' grid: this grid is like `Z' grid except that it is regional ($CPER = R and $NPER = 0).

2. Grids supported for the SCRIPR interpolations
   • `LR' grid: The longitudes and the latitudes of 2D Logically-Rectangular (LR) grid points can be described by two arrays longitude(i,j) and latitude(i,j), where i and j are respectively the first and second index dimensions. Streched or/and rotated grids are LR grids. Note that A, B, G, L, Y, or Z grids are all particular cases of LR grids.

   • `U' grid: Unstructured (U) grids do have any particular structure. The longitudes and the latitudes of 2D Unstructured grid points must be described by two arrays longitude(nbr_pts,1) and latitude(nbr_pts,1), where nbr_pts is the total grid size.

   • `D' grid The Reduced (D) grid is composed of a certain number of latitude circles, each one being divided into a varying number of longitudinal segments. In OASIS3, the grid data (longitudes, latitudes, etc.) must be described by arrays dimensioned (nbr_pts,1), where nbr_pts is the total number of grid points. There is no overlap of the grid, and no grid point at the equator nor at the poles. There are grid points on the Greenwich meridian.