Model output
The instructions in this article show how to convert OpenIFS GRIB output to netCDF for use in other analysis and visualisation software.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. 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.
The ICMGG files contain all the gridpoint fields. The ICMSH files are the spherical harmonics of the wind fields, pressure and temperature.
Grib-api tools
grib_to_netcdf
This command will convert one or more GRIB files to netCDF and is available with the ECMWF grib_api software (versions above 1.11.0 are recommended. It is not available in versions less than 1.10.0).
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).
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 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 ...etc.
The same square bracket syntax is also understood by the grib_filter command.
grib_to_netcdf can then be used on the individual files:
grib_to_netcdf ICMGG_hybrid.grb -o ICMGG_hybrid.nc
To convert from vorticity and divergence to wind u and v, please see CDO instructions below.
Please see 'Using Metview with OpenIFS' for more details.
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 commands
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.
If you find the CDO commands below do not work, you can either build CDO yourself, making sure a recent version of grib_api 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.
grib_set -s editionNumber=1 <input grib2 file> <output grib1 file>
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.
CDO can be used to interpolate from the reduced Gaussian grid to a regular Gaussian grid (model resolutions T21 & T42 use a regular Gaussian grid)
cdo -R copy <input grib> <output grib>
If the -R option does not work, use 'cdo setgridtype,regular' instead.
Before converting to netCDF separate the different vertical axes:
cdo splitzaxis <input file> <output file>
Convert the resulting output files to netcdf. 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.
cdo -t ecmwf -f nc copy <input grib> <output netcdf>
If you prefer wind components u & v instead of vorticity and divergence, use the cdo command:
cdo -t ecmwf dv2uvl <input file> <output file>
To interpolate to pressure levels instead of model levels, use this cdo command for example:
cdo -t ecmwf ml2pl,92500,85000,50000,20000 <input file> <output file>
There are various commands for inspecting the contents of a GRIB file. The GRIB_API installation as part of OpenIFS has useful commands like grib_ls and grib_dump. Here is another example using the cdo command:
$ cdo sinfo ICMSHfrq2+000000.grb
File format: GRIB
-1 : Institut Source Param Time Typ Grid Size Num Levels Num
1 : ECMWF unknown 11.3 var P16 65792 1 60 1
2 : ECMWF unknown 39.3 var P16 65792 1 60 1
3 : ECMWF unknown 43.3 var P16 65792 1 60 1
4 : ECMWF unknown 44.3 var P16 65792 1 60 1
5 : ECMWF unknown 152.128 var P16 65792 1 1 2
6 : ECMWF unknown 6.3 var P16 65792 1 1 2
Horizontal grids :
1 : spectral > size : dim = 65792 truncation = 255 spc = 32896
complexPacking = 1
Vertical grids :
1 : hybrid level : 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
52 53 54 55 56 57 58 59 60
2 : hybrid level : 1
Time axis : 1 step
RefTime = 1999-12-24 12:00:00 Units = hours Calendar = PROLEPTIC
YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss YYYY-MM-DD hh:mm:ss
1999-12-24 12:00:00
cdo sinfo: Processed 6 variables over 1 timestep. ( 0.04s )
Example 1 - Converting a single file
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.
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
Example 2 - Converting series of files
The following script shows how to loop over several output files using the steps above to convert to netCDF. Note the spherical harmonic files are also converted but the extra command using the sp2gpl operator is first used to convert the variables from spherical harmonics to gridpoint.
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