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This article show different approaches to convert GRIB to netCDF for use in other analysis and visualisation software. It also describes different methods to interpolate the reduced Gaussian grid to regular grids. |
What are GRIB filesGRIB files are the file format used at ECMWF. GRIB is a WMO standard and consists of GRIB Edition 1 and Edition 2. The OpenIFS/IFS models output in GRIB format. These files are a mix of GRIB-1 & GRIB-2 messages, the multi-level fields are encoded as GRIB-2, whereas surface fields are GRIB-1. GRIB files can also contain multiple vertical coordinates: pressure levels, model levels, sub-surface levels etc. This can cause a problem with some 3rd party tools, as the same GRIB variable code is used for each axis. The instructions below show how to work around this by splitting the file to separate fields on different vertical axes. OpenIFS model outputThe OpenIFS model outputs two types of files: those beginning with ICMSH contain fields represented as spherical harmonics, those that begin ICMGG contain gridpoint fields. The ICMSH files are the spherical harmonics of the wind fields, pressure and temperature and require a spectral transform to convert to gridded data. Also see controlling the OpenIFS output for more details of output fields and options. Retrieving data from MARS archivePlease note that if using the MARS archive (apps.ecmwf.int) (e.g. for reanalysis products such as ERA-Interim/ERA-5), it is possible to download the files in netCDF format as well as GRIB. /grib-api infoPlease note that grib-api is being phased out in preference for ecCodes. All the grib_* commands shown below are still available with ecCodes.grib_to_netcdfThis command will convert one or more GRIB files to netCDF and is available with the ECMWF ecCodes /grib-api software (for grib-api, versions above 1.11. 0 are recommended). For more details please see: grib_to_netcdf description. grib_to_netcdf only works correctly when a single level coordinate is present in the GRIB file. Often the model output files have fields on multiple level types (ie. hybrid model levels and pressure levels).
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| title | How to split GRIB file into separate level types |
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| grib_copy ICMGGftkm+001440 ICMGG_[typeOfLevel].grb |
In this example, the GRIB model output file ICMGGftkm+001440 contains a number of different model level types. The special square bracket "[ ]" syntax is recognised by grib_copy (and other grib commands such as grib_filter) and can contain any valid GRIB key. This example will copy the original file, separating the level types into their own file: ICMGG_hybrid.grb, ICMGG_isobaricInhPA.grb, ICMGG_surface ...and so on. grib_to_netcdf can then be used on the individual files:
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| title | Convert GRIB file to netCDF |
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| grib_to_netcdf ICMGG_hybrid.grb -o ICMGG_hybrid.nc
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By default, grib_to_netcdf will pack the data into scaled integers with an offset to optimize space. If you prefer data stored as floats then use: | Code Block |
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| language | bash |
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| title | Convert GRIB to netCDF storing values as floats rather than scaled integers... |
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| grib_to_netcdf -D NC_FLOAT ICMGG_hybrid.grb -o ICMGG_hybrid.nc |
To convert from vorticity and divergence to wind u and v, please see CDO instructions below. Note that grib_to_netcdf does not do any regridding. If the fields use a reduced Gaussian latitude grid, they will not be converted to a regular grid. Use CDO instead to do this as described below. |
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The MetView Metview application is available for analysis and visualisation of OpenIFS output and will both convert spectral to gridded parameters and plot reduced Gaussian grid directly.
Please see 'Using Metview with OpenIFS' for more details. |
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NCAR command language (NCL)
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CDO supports GRIB-2 but needs either the grib_api or ecCodes library from ECMWF to be included. This is necessary in order to work correctly with OpenIFS model output.
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| language | bash |
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| title | Check CDO is compiled with grib-api (or ecCodes) |
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cdo --version
Climate Data Operators version 1.79.26 (http://mpimet.mpg.de/cdo)
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Libraries: HDF5/1.10.4 proj/5.2 xml2/2.9.4
Filetypes: srv CDIext libraryieg versiongrb1 : 1.7.2 of Aug 10 2016 09:22:26
CGRIBEX grb2 nc1 nc2 nc4 nc4c nc5
CDI library version : 1.79.56
ofcgribex Junlibrary version 3 2016 14:44:00
GRIB_API: 1.9.2
ecCodes library version : 12.1512.05
NetCDF library version : 4.46.12 of AugFeb 14 32019 2016 1110:1013:4927 $
......... hdf5 library version : 1.10.4 threadsafe
exse library version : 1.4.1
FILE library version : 1.8.3
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If you find the CDO commands below do not work, you can either build CDO yourself, making sure a recent version of grib_api or ecCodes is used, or you can use the workaround below.
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If you have files with a mix of GRIB-1 and GRIB-2, then either split the file first or compile cdo with "--disable-cgribex --with-grib_apieccodes=yes" If variable names are lost, add the "-t ecmwf" option. |
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Another way to split the file would be using the grib_copy command from the ecCodes (or grib_api) software as software as shown above:
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grib_copy ICMSHg4a4+000000.grb ICMSHg4a4+000000_[typeOfLevel].grb |
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There are various commands for inspecting the contents of a GRIB file. The GRIB_API installation as part of ecCodes software installation required for OpenIFS has useful commands like grib_ls and grib_dump.
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cdo commands can be combined into a single command for greater efficiency. |
Using EMOSLIB to interpolate to regular grid
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Possible problems and solutions
cdo -R option does not work with GRIB-2 fields to convert to regular grid
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However, if this does not work for any reason, a workaround is to temporarily change the edition number to 1 of the GRIB file. Use the grib_set command (from ecCodes or grib_api) to change the GRIB edition number for all messages:
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cdo vct in.grb > pv.txt |
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Parameter names are lost for GRIB-1 fields
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A workaround is to use the -t ecmwf option. The -t option tells CDO to use the predefined ECMWF parameter tables (see CDO documentation for more details).
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cdo -t ecmwf -f nc copy mygrib1.grb mygrib1.nc |
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Do not use this option for GRIB-2 fields. It sets the GRIB table default to be specific to the GRIB-1 ECMWF tables. If problems persist, we recommend using grib_to_netcdf to convert to netCDF. |
Acknowledgements
This will make CDO use ECMWF parameter table 128 for the definitions of variables but 128 is only for GRIB 1. This will work for the surface fields which are encoded by IFS as GRIB 1, but not for the multi-level fields which are encoded in GRIB 2. Variable names become 'unknown' if -t ecmwf is used with the multi-level fields. It is best to omit it completely, even for surface fields, and only use it for GRIB 1 data if you get missing variable names after CDO has converted the data (see CDO documentation for more details).
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cdo -t ecmwf -f nc copy mygrib1.grb mygrib1.nc |
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Do not use this option for GRIB-2 fields. It sets the GRIB table default to be specific to the GRIB-1 ECMWF tables. If problems persist, we recommend using grib_to_netcdf to convert to netCDF. |
Acknowledgements
Thanks to Paul Dando of User Support for help with the contents of this page.
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The excerpt below includes a Google analytics tracker, do not remove! |
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