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The GRIB Filter icon is used to filter GRIB data from either a MARS Retrieval or a GRIB file icon. You can then obtain just the field(s) you want from a large request/file containing a wide variety of other field(s). The filter also has post-processing options such as Grid and Area, for regridding and sub-area extraction respectively; these use the same interpolation routines as the standard ECMWF MARS client.

You may ask yourself, why filter a MARS Retrieval instead of retrieving from the database exactly the data you want, thus avoiding the extra filtering step? The usefulness depends on what you want to do :

It may be more efficient to retrieve (and save) a large chunk of data with a single MARS Retrieval and then filter fractions of the large dataset with GRIB Filter. Remember that the large chunk of data, once retrieved is cached (or you may have saved it) and further access to it via GRIB Filter is much faster than making many separate individual MARS Retrievals.