Using the DWD's ICON forecast grib files with wgrib2 updated 12/2019, 4/2020
The DWD is making global forecasts using the ICON model. This model uses a triangular
mesh, and the forecast quantities are valid for the center of the triangles. The DWD
opendata server is distributing data in grib format for the forecast values from
the center of the triangles. This note shows how to process the grib data using
wgrib2.
Basics that DWD may change:
https://opendata.dwd.de/weather/nwp/icon/grib/HH
HH = 00, 06, 12 or 18
Step 1. Download the CLAT and CLON file
CLAT=latitude of the center of the triangles
CLON=longitude of the center of the triangles
https://opendata.dwd.de/weather/nwp/icon/grib/00/clat/icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLAT.grib2.bz2
https://opendata.dwd.de/weather/nwp/icon/grib/00/clon/icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLON.grib2.bz2
Step 2: Download some forecast files
Example
https://opendata.dwd.de/weather/nwp/icon/grib/00/t_2m/icon_global_icosahedral_single-level_YYYYMMDDHH_000_T_2M.grib2.bz2
https://opendata.dwd.de/weather/nwp/icon/grib/00/t_2m/icon_global_icosahedral_single-level_YYYYMMDDHH_001_T_2M.grib2.bz2
..
https://opendata.dwd.de/weather/nwp/icon/grib/00/t_2m/icon_global_icosahedral_single-level_YYYYMMDDHH_180_T_2M.grib2.bz2
Step 3: Uncompress the data
bunzip2:
Step 4: Combining the files
Bash:
cat icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLAT.grib2 \
icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLON.grib2 \
icon_global_icosahedral_single-level_YYYYMMDDHH_006_TMAX_2M.grib2 >icon.grb
Windows:
copy /b icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLAT.grib2 +
icon_global_icosahedral_time-invariant_YYYYMMDDHH_CLON.grib2 +
icon_global_icosahedral_single-level_YYYYMMDDHH_006_TMAX_2M.grib2 icon.grb
(all of above on one line)
Contents of icon.grb
$ wgrib2 icon.grb
1:0:d=2019040900:GEOLON:surface:anl:
2:5898409:d=2019040900:GEOLAT:surface:anl:
3:11796818:d=2019040900:TMP:2 m above ground:0-360 min max fcst:
Comment:
Regridding takes a long time for the first field because wgrib2 searches
each grid point to find the nearest neighbor. The rest of the fields
is much faster because wgrib2 retains a list of the nearest neighbors.
So processing is faster if all the fields that need regridding are put
into one file. (The unix cat command works for grib files.) This
slow first field behavior also works for the -lon option. The
nearest neighbor search is faster when using multiple cores and the
OpenMP version of wgrib2.
Example 1: Obtaining values for (10E, 20N) and (10W, 30S)
wgrib2 v2.0.9 (in development)
v2.0.9 adds -else, -elseif and -endif
v2.0.9 updates -grid_def to use GEOLAT and GEOLON
$ wgrib2 icon.grb -if "^(1|2):" -grid_def -else -s -lon 10 20 -lon 15 -30 -endif
1:0
2:5898409
3:11796818:d=2019040900:TMP:2 m above ground:0-6 hour max fcst::lon=9.968750,lat=20.012680,
val=296.588:lon=15.078125,lat=-30.069351,val=290.953
wgrib2 v2.0.6 - v2.0.8 (earlier versions of wgrib2 had a bug in -grid_def)
$ wgrib2 icon.grb \
-if ":GEOLAT:" -set center 7 -set_var NLAT -fi \
-if ":GEOLON:" -set center 7 -set_var ELON -fi \
-grid_def -s \
-not_if "^(1|2):" -lon 10 20 -lon 15 -30 -fi
1:0:d=2019040900:ELON:surface:anl:
2:5898409:d=2019040900:NLAT:surface:anl:
3:11796818:d=2019040900:TMP:2 m above ground:0-360 min max fcst::lon=9.968750,lat=20.012680,val=296.588:
lon=15.078125,lat=-30.069351,val=290.953
Example 2: a 1x1 degree global grid by nearest neighbor interpolation
wgrib2 v2.0.9 (in development)
$ wgrib2 icon.grb -if "^(1|2):" -grid_def -else -s -lola 0:360:1 -90:181:1 1x1.grb grib -endif
1:0
2:5898409
3:11796818:d=2019040900:TMP:2 m above ground:0-6 hour max fcst:
wgrib2 v2.0.6 - v2.0.8 (earlier versions of wgrib2 had a bug in -grid_def)
$ wgrib2 icon.grb \
-if ":GEOLAT:" -set center 7 -set_var NLAT -fi \
-if ":GEOLON:" -set center 7 -set_var ELON -fi \
-grid_def -s \
-not_if "^(1|2):" -lola 0:360:1 -90:181:1 1x1.grb grib
1:0:d=2019040900:ELON:local level type 1 0:anl:
2:5898409:d=2019040900:NLAT:local level type 1 0:anl:
3:11796818:d=2019040900:TMP:local level type 103 2:0-6 hour max fcst:
Example 3: Making a netcdf file
The raw ICON grib files do not have latitude and longitude information. By prepending
the CLON and CLAT files, the file has the longitude and latitude information. However,
the wgrib2 cannot make a netcdf file because the data are not on a lat-lon grid. One
could update the netcdf converter to output the ICON data on a trianglular mesh, but
how many visualization codes could read that netcdf file and make a plot?
The suggested method to make a netcdf file using wgrib2 is by making a lat-lon grib
file. See example 2. Once you have made the lat-lon file, you can make a netcdf
file using the grib2->netcdf utility of your choice.
The conversion from the trianglar mesh to a lat-lon grid is slow because a linear search
is used to find the nearest neighbor. The conversion can be made faster by using more cores
and setting the appropriate number of cores to use (export OMP_NUM_THREADS=n). This is why
you want more cores!