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NCAR command language (NCL)
NCL provides a tool to convert both GRIB1 & GRIB2 to netCDF called ncl_convert2nc.
NCL example scripts to convert to netCDF are also available.
CDO
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: Climate Data Operators
These instructions assume the use of a recent version of Climate Data Operators (CDO) (available from Max-Planck-Institut, Germany).
CDO supports GRIB2 but needs the GRIB_API library from ECMWF in order to work correctly with OpenIFS model output.
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cdo --version
Climate Data Operators version 1.7.2 (http://mpimet.mpg.de/cdo)
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CDI library version : 1.7.2 of Aug 10 2016 09:22:26
CGRIBEX library version : 1.7.5 of Jun 3 2016 14:44:00
GRIB_API library version : 1.15.0
NetCDF library version : 4.4.1 of Aug 3 2016 11:10:49 $
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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 a workaround below. Use the grib_set command (from the grib_api installation) to change the GRIB edition number for all messages.
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grib_set -s editionNumber=1 <input grib2 file> <output grib1 file>
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Note this will only work if you are using less than 128 model levels as it does not actually convert the message to GRIB1 if there are any GRIB2 specific elements in the messages. |
Interpolate from reduced to regular grid
CDO CDO can be used to interpolate from the reduced Gaussian grid to a regular Gaussian grid (model IFS/OpenIFS resolutions T21 & T42 use a regular Gaussian grid)
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If the -R option does not work, use ' |
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Steps to convert GRIB to netCDF
Split z axis
Before converting to netCDF, separate the different vertical axes using the generic command: cdo splitzaxis <input file> <output file pattern>.
This will ensure the following commands work correctly.
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cdo splitzaxis <input file> <output file> |
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ICMSHg4a4+000000.grb ICMSHg4a4+000000_split
ls ICMSH*split*
ICMSHg4a4+000000_split01.grb
ICMSHg4a4+000000_split02.grb
ICMSHg4a4+000000_split03.grb
ICMSHg4a4+000000_split04.grb |
In this example, the different files might contain '01' fields on pressure levels, '02' fields on model levels, '03' hybrid levels, '04' surface and so on. The number of files created depends on the number of different types of levels. Use the command 'grib_ls' to inspect the contents of each split file.
Note that the filename 'ICMSH' in this example, indicates it contains spectral and not gridpoint fields.
Another way to split the file would be using the grib_copy command from the grib-api (or ecCodes) library:
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grib_copy ICMSHg4a4+000000.grb ICMSHg4a4+000000_[typeOfLevel].grb |
In this case 'typeOfLevel' is a GRIB key. The square brackets is a special syntax to the grib_copy command. This approach works with any GRIB key.
Convert spectral to gridpoint
CDO supports two options for converting spectral to gridpoint data:
- sp2gp - converts spectral to a quadratic Gaussian grid.
- sp2gpl - converts spectral to a linear Gaussian grid, appropriate for IFS data.
Both produce a regular Gaussian grid. For more details please see the description of these operators on the CDO homepage.
Convert the resulting output files to netcdf.
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cdo -t ecmwf -f nc copy <input grib> <output netcdf> |
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cdo -t ecmwf dv2uvl <input file> <output file> |
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cdo -t ecmwf ml2pl,92500,85000,50000,20000 <input file> <output file> |
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In this example a single GRIB file with model level data, on a regular Gaussian grid is converted to netCDF with vorticity & divergence replaced by u & v and model level data interpolated to pressure levels.
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grib_set -s editionNumber=1 file file.grb cdo -t ecmwf -f nc copy file.grb file.nc cdo -t ecmwf dv2uvl file.nc file_uv.nc cdo -t ecmwf sp2gpl file_uv.nc file_uv_gg.nc cdo -t ecmwf ml2pl,92500,85000,50000,20000 file_uv_gg.nc file_uv_gg_pl.nc |
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The following script shows how to loop over several output files using the steps above to convert to netCDF. Note the spherical harmonic spectral parameter files are also converted but the extra command using the sp2gpl operator is first used to convert the variables from spherical harmonics spectral to gridpoint.
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for i in 000 024 048 072 do grib_set -s editionNumber=1 ICMGGfpp4+000${i} ICMGGfpp4+000${i}.grb cdo -R copy ICMGGfpp4+000${i}.grb ICMGGfpp4+000${i}_R.grb cdo splitzaxis ICMGGfpp4+000${i}_R.grb ICMGGfpp4+000${i} cdo -t ecmwf -f nc copy ICMGGfpp4+000${i}01.grb ICMGGfpp4+000${i}_PLGG.nc cdo -t ecmwf -f nc copy ICMGGfpp4+000${i}02.grb ICMGGfpp4+000${i}_MLGG.nc grib_set -s editionNumber=1 ICMSHfpp4+000${i} ICMSHfpp4+000${i}.grb cdo splitzaxis ICMSHfpp4+000${i}.grb ICMSHfpp4+000${i} cdo sp2gpl ICMSHfpp4+000${i}01.grb ICMSHfpp4+000${i}01_GP.grb cdo sp2gpl ICMSHfpp4+000${i}03.grb ICMSHfpp4+000${i}03_GP.grb cdo sp2gpl ICMSHfpp4+000${i}04.grb ICMSHfpp4+000${i}04_GP.grb cdo -t ecmwf -f nc copy ICMSHfpp4+000${i}03_GP.grb ICMSHfpp4+000${i}_ML.nc cdo -t ecmwf -f nc copy ICMSHfpp4+000${i}01_GP.grb ICMSHfpp4+000${i}_PL.nc cdo -t ecmwf -f nc copy ICMSHfpp4+000${i}04_GP.grb ICMSHfpp4+000${i}_surf.nc done |
Possible problems and solutions
Workaround if CDO does not understand GRIB2 messages
If you are using an old version of CDO, or one that does not use the grib-api or ecCodes libraries, Use the grib_set command (from the grib_api installation) to change the GRIB edition number for all messages.
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grib_set -s editionNumber=1 <input grib2 file> <output grib1 file>
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Note this will only work if you are using less than 128 model levels as it does not actually convert the message to GRIB1 if there are any GRIB2 specific elements in the messages. |
Parameter names are lost
The -t option tells CDO to use the predefined ECMWF parameter tables (see CDO documentation for more details). Without this option parameter names are not preserved. If the - t option doesn't work this is usually because CDO is using an old version of the ECMWF GRIB_API software.
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