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The OpenIFS model outputs files in GRIB format. These files are a mix of GRIB 1 & GRIB 2 format messages. They also contain multiple vertical coordinates: pressure levels, sub-surface levels etc as well as fields on reduced Gaussian grids. Unfortunately these features can cause problems with 3rd party GRIB software.
The ICMGG files contain all the gridpoint fields. The ICMSH files are the spherical harmonics of the wind fields, pressure and temperature.
CDO commands
These instructions assume the use of a recent version of Climate Data Operators (CDO) (available from Max-Planck-Institut, Germany).
CDO does not understand GRIB2 messages. In order to work around this, use the grib_set command (from the grib_api installation) to change the GRIB edition number for all messages. Note this will only work if you are using less than 128 model levels and it does not actually convert the message to GRIB1.
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grib_set -s editionNumber=1 <input grib2 file> <output grib1 file> |
Then CDO can be used to interpolate from the Gaussian grid to a regular grid for plotting:
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cdo -R -f nc copy <input grib1> <output netcdf> |