It also describes different methods to interpolate the reduced Gaussian grid to regular grids.
What are GRIB files
GRIB 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 output
The 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 archive
Please 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.
ecCodes tools
grib_to_netcdf
This command will convert one or more GRIB files to netCDF and is available with the ECMWF ecCodes software. 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 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:
grib_to_netcdf ICMGG_hybrid.grb -o ICMGG_hybrid.nc
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:
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.
Please see 'Using Metview with OpenIFS' for more details.
NCAR command language (NCL)
NCL provides a tool to convert both GRIB-1 & GRIB-2 to netCDF called ncl_convert2nc.
NCL example scripts to convert to netCDF are also available.
CDO: 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 GRIB-2 but needs either the ecCodes library from ECMWF to be included. This is necessary in order to work correctly with OpenIFS model output.
cdo --version
Climate Data Operators version 1.9.6 (http://mpimet.mpg.de/cdo)
...........
Libraries: HDF5/1.10.4 proj/5.2 xml2/2.9.4
Filetypes: srv ext ieg grb1 grb2 nc1 nc2 nc4 nc4c nc5
CDI library version : 1.9.6
cgribex library version : 1.9.2
ecCodes library version : 2.12.5
NetCDF library version : 4.6.2 of Feb 14 2019 10:13:27 $
hdf5 library version : 1.10.4 threadsafe
exse library version : 1.4.1
FILE library version : 1.8.3
.........
If you find the CDO commands below do not work, you can either build CDO yourself, making sure a recent version of ecCodes is used, or you can use the workaround below.
Interpolate from reduced to regular grid
CDO can be used to interpolate from the reduced Gaussian grid to a regular Gaussian grid:
cdo -R copy <input grib> <output grib>
The -R option only works for GRIB edition 1 (GRIB 1) data as it uses the CGRIBEX decoder.
For GRIB-2 messages it's better to use the setgridtype operator:
cdo setgridtype,regular <input grib> <output grib> cdo -f nc setgridtype,regular <input grib> <output netcdf>
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-eccodes=yes"
If variable names are lost, add the "-t ecmwf" option.
If these steps do not work, see workarounds below.
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 steps work correctly:
cdo splitzaxis 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 ecCodes software as shown above:
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:
- sp2gpl - converts spectral to a linear Gaussian grid, appropriate for IFS data.
- sp2gp - converts spectral to a quadratic Gaussian grid (only use this for T21/42 resolutions)
Both produce a regular Gaussian grid with equal number of longitudes on each latitude row. Remember that a Gaussian grid has irregularly spaced latitudes (but nearly regular). For more details please see the description of these operators on the CDO homepage.
cdo -f nc -sp2gpl <input spectral coefficent grib> <output gridded netcdf>
Convert the resulting output files to netcdf
After any operation converting a GRIB file to any other GRIB file, the conversion to netCDF is just:
cdo -f nc copy <input grib> <output netcdf>
Convert vorticity and divergence to wind
To convert wind components u & v instead of vorticity and divergence, use:
cdo dv2uvl <input file> <output file>
Interpolate to pressure from model levels
To interpolate to pressure levels from model levels, use:
cdo ml2pl,92500,85000,50000,20000 <input file> <output file>
Viewing contents of GRIB files
There are various commands for inspecting the contents of a GRIB file. The ecCodes software installation required for OpenIFS has useful commands like grib_ls and grib_dump.
grib_ls ICMSHfrq2+000000.grb grib_dump ICMSHfrq2+000000.grb
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 )
Examples
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.
cdo -f nc copy file.grb file.nc cdo dv2uvl file.nc file_uv.nc cdo sp2gpl file_uv.nc file_uv_gg.nc cdo ml2pl,92500,85000,50000,20000 file_uv_gg.nc file_uv_gg_pl.nc
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 spectral parameter files are also converted but the extra command using the sp2gpl operator is first used to convert the variables from spectral to gridpoint on a regular, linear, Gaussian grid.
In this example, we use the pipelining feature of CDO to do both the 'copy' and 'sp2gpl' steps in one command, keeping the entire operations in memory for efficiency.
for i in 000 024 048 072
do
cdo -R copy ICMGGfpp4+000${i}.grb ICMGGfpp4+000${i}_R.grb
cdo splitzaxis ICMGGfpp4+000${i}_R.grb ICMGGfpp4+000${i}
cdo -f nc copy ICMGGfpp4+000${i}01.grb ICMGGfpp4+000${i}_PL_GP.nc # pressure level gridpoint fields
cdo -f nc copy ICMGGfpp4+000${i}02.grb ICMGGfpp4+000${i}_ML_GP.nc # model level gridpoint fields
cdo splitzaxis ICMSHfpp4+000${i}.grb ICMSHfpp4+000${i}
cdo -f nc copy -sp2gpl ICMSHfpp4+000${i}01.grb ICMSHfpp4+000${i}01_PL_GP.grb # spectral fields on press levels transformed to gridpoint
cdo -f nc copy -sp2gpl ICMSHfpp4+000${i}03.grb ICMSHfpp4+000${i}03_ML_GP.grb # spectral fields on model levels transformed to gridpoint
cdo -f nc copy -sp2gpl ICMSHfpp4+000${i}04.grb ICMSHfpp4+000${i}04_surf_GP.grb # spectral fields on surface levels transformed to gridpoint
done
cdo commands can be combined into a single command for greater efficiency.
Using EMOSLIB to interpolate to regular grid
The ECMWF interpolation software library EMOSLIB also provides the capability to interpolate spectral data to regular gaussian grids or regular lat-lon grids, and interpolate regular gaussian grids to regular lat-lon grids.
The EMOSLIB library provides a Fortran library to enable users to write their own interpolation software. Please see the EMOSLIB website for more details and examples.
EMOSLIB also provides two command-line tools for using interpolation. Assuming a recent version of the EMOS library, there are two tools that can be used.
In the 'bin' directory there is a command 'emos_tool' which can be used as:
emos_tool --regular=256 [--area="40/-10/31/20"] in.grib out.grib
The grid specified for the --regular option follows the grid naming convention for EMOSLIB here: Reduced Gaussian Grids.
Another command can be found in the 'tools' directory. This allows the interpolation function to be specified and is more flexible:
tools/int --INTOUT:gridname=F256 --INTOUT:area="40/-10/31/20" --input=1.grib --output=2.grib --intf2
Here 'F256' means 'full-grid' not reduced, and --intf2 is the EMOSLIB interpolation function to be used.
Possible problems and solutions
cdo -R option does not work with GRIB-2 fields to convert to regular grid
CDO's -R option, to convert from reduced Gaussian grid to regular Gaussian grid, only works with GRIB 1 format as the CGRIBEX decoder this uses does not work with GRIB-2 (see cdo man page). This is a problem as multi-level output from OpenIFS is encoded as GRIB-2 data.
The preferred way to deal with this is to use the setgridtype operator as described above:
cdo -f nc setgridtype,regular <input grib> <output netcdf>
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) to change the GRIB edition number for all messages:
grib_set -s editionNumber=1 in.grb in_1.grb cdo -R copy in_1.grb in_1_R.grb grib_set -s editionNumber=2 in_1_R.grb in_2_R.grb
This does not actually convert the message to GRIB-1, it only changes the editionNumber. Any GRIB-2 specific elements are unaltered.
It's recommended that the editionNumber is changed back to 2 to ensure that the parameter names remain correct in any subsequent cdo / grib commands.
Workaround if using more than 128 levels
The workaround above will fail if the grib file contains field with more than 128 levels. This is because there are not enough bits available in the GRIB-1 parameter to encode the total number of model half levels (which would exceed 255).
In this case, the 'pv' GRIB parameter causes the problem (nothing to do with potential vorticity!). This is an array holding the half level values of the model's A & B coefficients that define the location of the levels.
The workaround above can be extended to delete this array while setting the edition number to 1, like this:
grib_set -s deletePV=1,editionNumber=1 in.grb in_1.grb cdo -R copy in_1.grb in_1_R.grb grib_set -s editionNumber=2 in_1_R.grb in_2_R.grb
The deletePV option is known as a 'concept' rather than a parameter contained in the GRIB file itself. It ensures the PV array is deleted correctly by grib_set. Now the cdo regridding will work correctly, but remember to reset the editionNumber back to 2.
The A & B half level coefficients can be recovered from the original file by:
cdo vct in.grb > pv.txt
Parameter names are lost for GRIB-1 fields
ECMWF GRIB-1 use keys which may not be recognised by CDO because they are locally defined (e.g. the shortName key) and not defined in the WMO GRIB tables that CDO uses. This can cause parameter names to be lost or not recognised when using the CDO commands.
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).
cdo -t ecmwf -f nc copy mygrib1.grb mygrib1.nc
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.