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MARS

Specifies a rotation for the target grid.

rotation=<value>

The values for this keyword consist of the latitude and longitude of the southern pole 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.

A 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:

...      # Switch land-sea mask processing on for 
lsm=ON   # the current retrieve
retrieve,
lsm=OFF, # Switch land-sea mask processing off for
...      # the following retrieve

Dissemination

Not supported


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