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MARS/Dissemination
Specifies a rotation for of the target grid.
rotation=<value> |
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The values for this keyword consist of the latitude and longitude of the southern pole A rotation is requested by specifying the position of the South Pole of the rotated grid as latitude/longitude in degrees, e.g. rotation = -30.0/10.0.
Fields on spherical harmonics or Gaussian grid can be rotated with the directive rotation. The rotation is performed prior to any other conversion. Therefore:
- Spherical harmonics fields can be retrieved as rotated spherical harmonics or converted to rotated latitude/longitude or Gaussian grid fields.
- Gaussian grid fields can be retrieved as rotated latitude/longitude or rotated Gaussian grid fields.
If the input is a spectral field and the output is a rotated spectral field, a file of rotation coefficients is generated and used in the processing. The convergence of the algorithm for rotating the spectral fields is sensitive to the spectral truncation. For some high resolutions and some large angles of rotation, it is necessary to split the rotation into three steps (two forwards and one backwards!). The size of the rotation coefficient files can be very large.
If the input is a spectral field and the output is a rotated grid point field, the spectral field is interpolated to a non-rotated Gaussian grid which is then transformed to the required rotated grid point field using a 12-point interpolation based on the FULL-POS scheme used in the ECMWF forecast model.
| Value | Comment |
|---|---|
rotation = off | no rotation (default) |
rotation = a/b | a is the latitude of the South Pole of rotation in deg. b is the longitude of the South Pole of rotation in deg. |
Rotation is strictly linked to the keyword GRID:
| GRID specification | target grid |
|---|---|
grid=lat/lon | rotated regular lat/lon |
grid=F... | rotated full Gaussian grid |
grid=N... | rotated reduced Gaussian grid |
grid=O... | octahedral (reduced) Gaussian grid |
and usually specified together with an AREA. Target fields in spherical harmonics are not supported. The default interpolation method used is linearA rotated grid-point field is created from an input grid-point field by finding for each rotated grid-point its nearest four neighbours in the input field and carrying out a bilinear interpolation.
Examples:
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Dissemination
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