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/grib-api tools

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). 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.