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Any requested data manipulation or post-processing is carried out by the MARS client, except in the case of a local Member State's the Web API client where data is first processed at ECMWF prior to its transmission over the network. The post-processing is carried out by MIR, a set of routines present in the EMOSLIB library. Please, refer to the Field interpolation software routines for in-depth details about such post-processing.library of routines for Meteorological Interpolation and Regridding (see ECMWF newsletter no.152 for a description). For details on the various operations please see also the relevant post-processing keyword descriptions.
Sub-area extraction
Most of the data at ECMWF is global. A sub-area can be created using the area keyword by defining its latitude and longitude boundaries: North/West/South/East.
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The truncation can be controlled using the keywords truncation and intgrid. Default truncations are detailed in the following table.Users wanting to post-process at the full archived resolution can specify truncation = none in the request. Note that high resolutions might need more resources to carry out post-processing.
| Legacy emoslib default Default spectral truncation wave number for given lat/lon grid increment ∆. | |
| Grid increment [deg] | Truncation |
| 2.5 ≤ ∆ | T63 |
| 1.5 ≤ ∆ < 2.5 | T106 |
| 0.6 ≤ ∆ < 1.5 | T213 |
| 0.4 ≤ ∆ < 0.6 | T319 |
| 0.3 ≤ ∆ < 0.4 | T511 |
| 0.15 ≤ ∆ < 0.3 | T799 |
| 0.09 ≤ ∆ < 0.15 | T1279 |
| 0.0 ≤ ∆ < 0.09 | T2047 |
Users wanting to post-process at the full archived resolution can specify resol = av in the request. Note that high resolutions might need more resources to carry out post-processing.
Rotation
Fields on spherical harmonics or Gaussian grid can be rotated with the keyword 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.
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Retrieval efficiency
To retrieve data efficiently please follow the hints below:
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