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.
From Macro/Python the icon can be called as read().
The GRIB Filter Editor
With the exception of a few parameters which specify the data to be filtered, all other parameters found in the GRIB Filter editor can also be found in the MARS Retrieval editor.
Source
Specifies the GRIB file path.
Data
Drop a MARS Retrieval or a GRIB file icon inside this icon field. Note that you should specify either Source or Data, not both, otherwise the process will fail.
Order
Specifies whether you want to sort the files you have filtered (Sorted) or to obtain them in the order they are on the original file (As Is). The sorting is the same as that carried out by MARS on retrieving data (date, time, forecast step, level, parameter), hence this has no effect if you are filtering a MARS request (or a GRIB file containing the result of a single MARS retrieval).