• The following examples use the CDO tool, which is free to download from:
  https://code.mpimet.mpg.de/projects/cdo
• In the terminal, we will convert the grib file from reduced Gaussian to regular lat/lon. In order to this we need to know the number of regular latitude points correspoding to the linear reduced Gaussian grid (T799~N400), which can be found at:
  https://www.ecmwf.int/en/forecasts/documentation-and-support/gaussian_n400
• Given there are 1600 regular latitude points (and 800 longitude points), we can convert our reduced Gaussian grid to regular lat/lon:

module load cdo

• To convert to regular lat/lon
  cdo -R remapcon,r1600x800 -setgridtype,regular h14_2019053000.grib h14_2019053000_r.grib

• Then convert the file from grib to netcdf:

cdo -f nc copy h14_2019053000_r.grib h14_2019053000.nc

• Then extract the Italian domain from the global netcdf file (lon0=5, lon1=19, lat0=36, lat1=48):

cdo -sellonlatbox,5,19,36,48 h14_2019053000.nc h14_2019053000_Italy.nc

#Note that the size of the file is vastly reduced (h14_2019053000.nc ~ 25 mb, h14_2019053000_Italy.nc ~ 70 kb)!

• Size of netCDF files can be further reduced by compression e.g.
  cdo -f nc4c -z zip_6 copy h14_2019053000_r.grib h14_2019053000_compressed.nc
• To convert multiple files from bash to netCDF, you can loop over all the files in the directory and apply the commands above e.g.

  
  

for file in *.grib; do cdo -R remapcon,r1600x800 -setgridtype,regular "$file" "$file".reg; done #To convert to regular grid

for file in *.reg; do cdo -f nc copy "$file" "$file".nc; done #To convert regular grib files to netCDF 

for file in *.nc; do cdo -sellonlatbox,5,19,36,48 "$file" "$file"_italy; done #To extract Italian domain