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Retrieved data can of course be manipulated in any way outside MARS, e.g. using Fortran programs. Python scripts etc. To users with less programming experience some tools available at ECMWF might be of particular interest:
The GRIB ecCodes package offers handy tools to examine the content and to manipulate GRIB files on the command line.
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offers a GUI to some functionality of GRIB APIecCodes's examination tools.
CDO is a collection of Climate Data Operators, including, e.g. vertical interpolation, which can by applied to meteorological fields in various formats, including GRIB. This tool box is installed at ECMWF but has been developed and is maintained by MPI, Hamburg. Please direct all queries to them. For more details refer to the CDO home page.
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