Section

Column

GRIB files

Excerpt
The instructions in this article show how ways to convert GRIB to netCDF for use in other analysis and visualisation software.

GRIB files are the standard format used at ECMWF. Files retrieved from the MARS archive, or from the public data server for reanalysis products such as ERA-Interim are all GRIB format. GRIB is a WMO standard.

The OpenIFS/IFS models output fields in GRIB format. These files are a mix of GRIB-1 & GRIB-2 format 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.

Considering the output from OpenIFS, the ICMGG files contain all the gridpoint fields. The ICMSH files are the spherical harmonics of the wind fields, pressure and temperature. See controlling the OpenIFS output for more details of output fields and options.

Grib-api tools

Please 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_netcdf

This 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. 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).

Code Block

title	Split GRIB file into separate level types

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:

Code Block

title	Convert GRIB file to netCDF

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.

Note that grib_to_netcdf does not perform 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.

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Info
The MetView application is available for analysis and visualisation of OpenIFS output. Please see 'Using Metview with OpenIFS' for more details.

NCAR command language (NCL)

Panel

title	See also...

ecCodes - grib tools

grib-api - deprecated grib tools

MARS user documentation

NCAR command language (NCL)

NCL provides a tool to convert both GRIB-1 & GRIB-2 NCL provides a tool to convert both GRIB1 & GRIB2 to netCDF called ncl_convert2nc.

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These instructions assume the use of a recent version of Climate Data Operators (CDO) (available from Max-Planck-Institut, Germany).

CDO supports GRIB2 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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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 as shown above:

Code Block
grib_copy ICMSHg4a4+000000.grb ICMSHg4a4+000000_[typeOfLevel].grb

...

To interpolate to pressure levels instead of from model levels, use this cdo command, for example:

Code Block

title	Interpolate model levels to pressure levels

cdo ml2pl,92500,85000,50000,20000 <input file> <output file>

...

Code Block

title	Example grib-api command for inspecting a file

grib_ls ICMSHfrq2+000000.grb
grib_dump ICMSHfrq2+000000.grb

Here is another example using the cdo command:

Code Block

title	Example cdo command for inspecting GRIB file

$ 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

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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.

Code Block
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.

Code Block

title	Sample script to convert OpenIFS GRIB output to netCDF

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_PLGGGP.nc		# pressure level gridpoint fields
cdo -f nc copy ICMGGfpp4+000${i}02.grb ICMGGfpp4+000${i}_ML_MLGGGP.nc

cdo   splitzaxis ICMSHfpp4+    # 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_PL.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_ML.grb
cdo -f nc copy -	# spectral fields on model levels transformed to gridpoint
cdo -f nc copy -sp2gpl ICMSHfpp4+000${i}04.grb ICMSHfpp4+000${i}04_surf_GP_surf.grb	# spectral fields on surface levels transformed to gridpoint

done

Possible problems and solutions

Workaround if CDO does not understand

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GRIB-2 messages

If you are using an old version of CDO, it may not understand GRIB-2 messages. CDO must be compiled with either grib-api or ecCodes library in order to handle GRIB-2.

A workaround is to use or one that does not use the grib-api or ecCodes libraries, Use the grib_set command (from the grib_api/ecCodes installation) to change the GRIB edition number for all messages.

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Note
Note this will only work if you are using less than 128 model levels as it does not actually convert the message to GRIB1 GRIB-1 if there are any GRIB2 GRIB-2 specific elements in the messages.

Parameter names are lost

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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 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.

.

Code Block
cdo -t ecmwf -f nc copy mygrib1.grb mygrib1.nc

Warning
Do not use this option for GRIB-2 fields. It sets the GRIB table default to be specific to the GRIB-1 ECMWF tables.

Acknowledgements

Thanks to Paul Dando of User Support for help with the contents of this page.

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Versions Compared

Old Version 48

New Version 49

Key

GRIB files

Grib-api tools

grib_to_netcdf

NCAR command language (NCL)

NCAR command language (NCL)

Examples

Converting a single file

Converting series of files

Possible problems and solutions

Workaround if CDO does not understand

GRIB-2 messages

Parameter names are lost

for GRIB-1 fields

Acknowledgements

Space shortcuts

Page tree

Page History

Versions Compared

Old Version 48

New Version 49

Key

GRIB files

Grib-api tools

grib_to_netcdf

NCAR command language (NCL)

NCAR command language (NCL)

Examples

Converting a single file

Converting series of files

Possible problems and solutions

Workaround if CDO does not understand

GRIB-2 messages

Parameter names are lost

for GRIB-1 fields

Acknowledgements