Writing GRIB2 Files

wgrib2 and consequently pywgrib2_xr does not support creating GRIB2 files “from scratch”. Rather, an existing GRIB2 message is used as a template, the newly created message shares, by default, both metadata and grid values. New field values can be set via RPNRegister, metadata can be updated with the help of over 60 commands -set_X where X is the parameter to set. The function write_msg() hides implementation details. See wgrib2 -set_metadata documentation for detailed desctiption of all possible options.

Examples:

• Modify model issue time, preserve data values:

  # Add 1 day + 12 hours to date code for each message in input file
  date_inc = timedelta(days=1, hours=12)
  for item in pywgrib2.inv_from_grib(in_file):
      pywgrib2.write_msg(out_file, in_file, item, date=item.end_ft + date_inc)
  

  The keyword argument date=date_str is passed to wgrib as 'set_date', date_str.

• Add calculated wind speed at isobaric levels to a GRIB2 file:

  gribfile = 'gfs_4_20180930_1800_072.grb2'
  windfile = 'wind.grb2'
  
  def pred_wind_isobaric(item):
      return item.varname in ('UGRD', 'VGRD') and item.level_code == 100
  
  inv = pywgrib2.load_or_make_inventory(gribfile, save=True)
  tmpl = pywgrib2.make_template(gribfile, pred_wind_isobaric, vertlevels='isobaric')
  ds = pywgrib2.open_dataset(gribfile, tmpl, chunks={'isobaric1': 1})
  uv = ds['UGRD.isobaric'] + 1j * ds['VGRD.isobaric']
  ds['WIND.isobaric'] = np.abs(uv)
  # See https://github.com/pydata/xarray/issues/2609
  ds['WDIR.isobaric'] = (90.0 - xr.ufuncs.angle(uv, deg=True)) % 360.0
  # Not needed in this example
  # ds['WIND.isobaric'].attrs =
  # ds['WDIR.isobaric'].attrs =
  meta = next((i for i in inv if pred_wind_isobaric(i)))
  for p in ds['WIND.isobaric'].coords['isobaric1'].values:
      level = '{:d} mb'.format(p // 100)
      wspd = ds['WIND.isobaric'].sel(isobaric1=p)
      wdir = ds['WDIR.isobaric'].sel(isobaric1=p)
      pywgrib2.write_msg(windfile, gribfile, meta, data=wind.values, var='WIND',
                         lev=level, grib_type='same', append=True)
      pywgrib2.write_msg(windfile, gribfile, meta, data=wind.values, var='WDIR',
                         lev=level, grib_type='same', append=True)
  

  File windfile may now be concatenated to gribfile. The argument chunks in the call to open_dataset can be omitted when memory is not a concern.