wgrib2 usage and examples to use it with weather and climate grib2 files

1. wgrib2
Arulalan T
Scientist
India Meteorological Department

2. Introduction
• conda install wgrib
• conda install wgrib2
• Its free and open source scientific software to manage grib1 and grib2
(weather and climate data files)
• wgrib –v infile.grib1
• wgrib2 –v infile.grib2 (try V, v2)

3. (1) How do I convert the entire file to binary?
$ wgrib2 grib_file -bin out.bin
(2) How do I get a verbose listing of the 20th message only?
$ wgrib2 -V -d 20 grib_file
If grib message 20 has submessages, then d 20 will get the first field of
the grib message. I.e, this will be the same as -d 20.1
(3) How do I extract the 20th and 30th grib message?
$ wgrib2 grib_file -match "(^20:^30:)" -bin binary_file
(4) How do I extract the first 10 records as a grib file?
$ wgrib2 grib_file -for_n 1:10 -grib new_grib_file

4. (5) How do I extract records 34-66 from a file as a grib file?
$ wgrib2 grib_file -for 34:66 -grib new_grib_file
(6) How do I extract grib messages 10,12,19 from a grib file ?
$ wgrib2 old_grib -match '^(1|12|19):' -grib new_grib
(7) How do I extract a range of dates from a file?
Example 1. Analysis file $ wgrib2 z500.gdas.199704.f06
1:0:d=1997040100:HGT:500 mb:6 hour fcst:
2:228571:d=1997040106:HGT:500 mb:6 hour fcst: ...
120:27173104:d=1997043018:HGT:500 mb:6 hour fcst:
The file contains the 500 mb geopotential height for the April 1997. To obtain
the data for a range of date:
$ wgrib2 z500.gdas.199704.f06 | awk '{d=substr($3,3); if (d >=1997042400 &&
d <=1997042512) print $0}' FS=':' | wgrib2 z500.gdas.199704.f06 -i -grib
z500subset.grb

5. Example 2. Extract a range of forecast times
$ wgrib2 t250.03.2015080900.daily.grb2
1:0:d=2015080900:TMP:250 mb:6 hour fcst:
2:20236:d=2015080900:TMP:250 mb:12 hour fcst: ..
456:8689473:d=2015080900:TMP:250 mb:2736 hour fcst:
To extract a range of forecasts, you can put the verification time into the inventory
$ wgrib2 t250.03.2015080900.daily.grb2 -vt -s
1:0:vt=2015080906:d=2015080900:TMP:250 mb:6 hour fcst:
2:20236:vt=2015080912:d=2015080900:TMP:250 mb:12 hour fcst: ...
Modifying example 1
$ wgrib2 t250.03.2015080900.daily.grb2 -vt -s | awk '{d=substr($3,4); if (d
>=2015081000 && d <= 2014081012) print $0}' FS=':' | wgrib2
t250.03.2015080900.daily.grb2 -i -grib subset.grb
4:60273:d=2015080900:TMP:250 mb:24 hour fcst:
5:80131:d=2015080900:TMP:250 mb:30 hour fcst:
6:99799:d=2015080900:TMP:250 mb:36 hour fcst:

6. (8) CMC files have some variables which don't have a name. wgrib2 doesn't
include the local extensions from CMC. AFAIK, the CMC grib tables follows the
NCEP grib table except where they don't. (How does that helps me?) So you
may be able to get most of the fields right by using nt the NCEP tables. This can
be done changing the center flag to NCEP (7).
$wgrib2 -set center 7 infile.grib2 (In memory)
$wgrib2 -set center 7 infile.grib2 -grib_out outfile.grib2 (In outfile)
(9) Use append option to append on existing grib2 files
$wgrib2 -set center 7 infile.grib2 -append -grib_out outfile.grib2

7. (10) I am generating a text dump (CSV) and the size of the file is really large. If
you set the grid point values to "undefined", they won't print out.
$wgrib2 grib_file -undefine out-box 30:40 -10:10 -csv out.txt
The grid points outside of the box are set to undefined. The -csv options tries to
print the entire grid but finds only the values between 30N-40N and 10W-10E
are defined.
(11) I am generating a text dump (CSV) and the size of the file is really large, I
am only interested in variable XYZ when it has values larger than 10
$wgrib2 grib_file -match ':XYZ:' -undefine_val -9e20:10" -csv output.txt
(12) How do I extract small region from big domain in grib2 file
$wgrib2 -small_grib LonW:LonE LatS:LatN file_name
$wgrib2 fcst.grb2 -small_grib 10:20 -20:20 small_out.grb

8. (13) I tried using the small_grib option but the files are bigger than the original.
By default, The small_grib option writes a regional subset in simple packing.
This requires less CPU resources but produces larger files than jpeg or complex
packing.
To use the same packing as the original file, use the "-set_grib_type same"
option. OR –set_grib_type jpeg OR –set_grib_type complex OR –set_grib_type
complex2 Or –set_grib_type complex3
(14) How do I extract a few variables and write out into new grib2 file ?
$wgrib2 infile.grib2 -match "(VIS:surface:60 min fcst:|HGT:cloud ceiling:60 min
fcst:)" – grib out.grib2
$wgrib2 infile.grib2 -match "(VIS:surface:60 min fcst:|HGT:cloud ceiling:60 min
fcst:)" –grib_out out.grib2
$wgrib2 infile.grib2 -match "(VIS:surface:60 min fcst:|HGT:cloud ceiling:60 min
fcst:)" –append –grib_out out.grib2

9. (15) I want to modify some existing grib files. For example, I want to make sure
the humidity (RH, SPFH) is positive.
$wgrib2 test.grb -rpn "0:max" -set_grib_type c2 -grib_out new.grb
-rpn 0:max, replaces the grid point values with max(0,grid_point_value)
(16) My software won't handle the level "30-0 mb above ground". How do I
change the level to "15 mb above ground"?
IF structure
$wgrib2 in.grb -if ":30-0 mb above ground" -set_lev "15 mb above ground" -
endif -grib out.grb
(17) How do I combine multiple grib2 files
$ cat infile1.g2 infile2.g2 infile3.g2 infile3.g2 > outfile.grib2

10. (18) Masking data: I want set all oceanic values to undefined. Suppose LAND is a
grib file with the land mask (land=1, water=0) with the same grid as the data file
(data.old) Make a new file with the land mask as the first record.
$ cat LAND data.old > data.tmp
$ wgrib2 data.tmp
-if '^1:’ -rpn '1:==:sto_1’ -fi
-not_if '^1:' -rpn 'rcl_1:mask' -set_grib_type same -set_scaling same
-grib_out data.new
(19) My grib file uses the special value of -999.9 as the undefine value. (Grid
points with a value of -999.9 are considered to be undefined). Using special
values as undefined values is outside of the grib standard and should be
avoided. To convert a file that uses -999.9 to the grib standard, you should do
$wgrib2 IN.grb -undefine_val 999.9 -set_grib_type same -grib_out OUT.grb

11. (20) The -merge_fcst option will try to combine adjacent messages by
increasing the time period for forecast average, accumulation, minimum
and maximum. For example,
$ wgrib2 prate.201404.ts -for 1:4
1:0:d=2014040100:PRATE:surface:0-1 hour ave fcst:
2:13083:d=2014040100:PRATE:surface:1-2 hour ave fcst:
3:26587:d=2014040100:PRATE:surface:2-3 hour ave fcst:
4:40437:d=2014040100:PRATE:surface:3-4 hour ave fcst:
$ wgrib2 preas.201404.ts -for 1:4 -merge_fcst 4 all.grb (listing of input)
$ wgrib2 all.grb
1:0:d=2014040100:PRATE:surface:0-4 hour ave fcst:
Similar way, automatically compute accumulation, minimum, maximum based
on input meta information available in the input.grib2 file.

12. (20) The -merge_fcst - make daily accumulated rainfall from four 6-hourly accumulated
rain
$ cat fhr06 fhr12 fhr18 fhr24 > IN.grb
$ wgrib2 IN.grb | sort -t: -k4,4 -k5,5 -k6,6n | wgrib2 -i IN.grb -grib OUT1.grb
$ wgrib2 OUT1.grb -merge_fcst 4 OUT2.grb
$ wgrib2 OUT1.grb -if ":APCP:” -merge_fcst 4 OUT2.grb
(21) how to change the date code in a grib2 file ex. change the analysis/initial time date to
12Z of 5th Jan 2023
$wgrib2 -match "^[0-9]*(:|.1)" IN.grb -set_date 2023010512 -GRIB OUT.grb
(22) How do I extract a particular lat, lon values? get values at (30N, -90W) and (40N, -80N)
$wgrib2 IN.grb -lon -90 30 > location1.txt
$wgrib2 IN.grb -lon -80 40 > location2.txt
$wgrib2 IN.grb -lon -90 30 -80 40 > locations.txt

13. (23) The GFS model has output every 3 hours in the files: gfs.t00z.pgrb2.1p00.fHHH HHH=000, 003, 006, 009, etc
The APCP (accumulated precip) fields of the following format
gfs.t00z.pgrb2.1p00.f003: has APCP:surface:0-3 hour acc fcst:
gfs.t00z.pgrb2.1p00.f006: has APCP:surface:0-6 hour acc fcst:
gfs.t00z.pgrb2.1p00.f009: has APCP:surface:6-9 hour acc fcst:
gfs.t00z.pgrb2.1p00.f012: has APCP:surface:6-12 hour acc fcst:
gfs.t00z.pgrb2.1p00.f015: has APCP:surface:12-15 hour acc fcst: ..
I want a file with the 3 hourly accumulations like
1:0:d=2017041800:APCP:surface:0-3 hour acc fcst:
2:97943:d=2017041800:APCP:surface:3-6 hour acc fcst:
3:195886:d=2017041800:APCP:surface:6-9 hour acc fcst:
4:301974:d=2017041800:APCP:surface:9-12 hour acc fcst:
5:383627:d=2017041800:APCP:surface:12-15 hour acc fcst: ..
You can use the -ncep_norm to make the 3 hourly accumulations of the APCP by
$ cat gfs.t00z.pgrb2.1p00.f0?? | wgrib2 - -match ':APCP: ' -if ':APCP:' -ncep_norm apcp.grb2
Results are $ wgrib2 apcp.grb2
1:0:d=2017041800:APCP:surface:0-3 hour acc fcst:
2:97943:d=2017041800:APCP:surface:3-6 hour acc fcst:
3:195886:d=2017041800:APCP:surface:6-9 hour acc fcst:
4:301974:d=2017041800:APCP:surface:9-12 hour acc fcst:

14. (24) In the operational CFSv2 time series, V does not follow U and the forecasts are not in order. When using -
new_grid, the vector fields must be adjacent (u-component then v-component).
$wgrib2 IN.grb | sed -e 's/:UGRD:/:UGRDa:/' -e 's/:VGRD:/:UGRDb:/' | sort -t: -k3,3 -k6n,6 -k5,5 -k4,4 | wgrib2 -
i IN.grb -grib OUT.grb
(25) Compute wind speed and direction and append into same input file. We need to make sure both u-wind
and v-wind are aligned properly
$wgrib2 -match "(UGRD|VGRD)" infile.grb2 -wind_speed tmpfile -wind_dir tmpfile
$cat tmpfile >> infile.grb2
(26) How do I interpolate/extrapolate to different grid resolution of winds ?
$wgrib2 IN.grb | sed -e 's/:d=([0-9]*):([A-Z])([^:]*)/:d=1:3_2:/' | sort -t: -k3,3 -k5,5 -k6,6 -k4,4 | wgrib2
-i IN.grb -new_grid_winds earth -new_grid latlon 0:144:2.5 -90:73:2.5 OUT.grb
(27) I have grib2 files in WE:NS order, how do I put them into WE:SN order?
Use -new_grid to a new grid. Just make the new grid the same as the old grid except for a WE:SN order.

15. (28) I wanted to find the min TMP2M for a given year from CFSR. IN.grb = time series of TMP2m (only) for the
given year # create an inventory where record 1 is last
$wgrib2 IN.grb -for 2:: > junk.inv
$ wgrib2 IN.grb -d 1 >> junk.inv # process the file with record 1 last
$wgrib2 -i <junk.inv IN.grb -if "^2:" -rpn "sto_1" -fi -rpn "rcl_1:min:sto_1" -if "^1:" -set_ave "0-1 year min anl" -
grib_out OUT.grb
(29) How do I convert grib2 to netcdf format ?
$ wgrib2 infile.grib2 -netcdf outfile.nc
$ wgrib2 infile.grib2 -nc_nlev 4 -netcdf outfile.nc # to bring proper 4 dimensional axis
(30) The -lola option with nlat=nlon=1 and dlat=dlon=1 (or any non-zero value)
binary: $ wgrib2 IN.grb -no_header -lola "10:1:1" "20:1:1" grid.bin bin
text: $ wgrib2 IN.grb -lola "10:1:1" "20:1:1" grid.txt text
CSV: $ wgrib2 IN.grb -lola "10:1:1" "20:1:1" grid.grb grib (converting to grib)
$ wgrib2 grid.grb -csv grid.csv (converting grib to CSV)

16. (31) I have a file with var246 that I want to change to the standard UGRD at 100 m above
ground.
$ wgrib2 IN.grb -if ":var246:" -set_var UGRD -set_lev "100 m above ground" -fi -grib OUT.grb
(32) I have 80 ensemble members that have WIND (wind speed) at 10m. I want to find the
probability the the WIND is greater or equal to 15 m/s based on the percentile of ens
members >= 15 m/s.
ensemble data is at $input/flx.memNNN.YYMMDDHH where NNN=001..080 WIND is only
defined at 10 meters above ground
$ cat flx.mem??? | wgrib2 - -match ":WIND:" -rpn "15:>=" -ens_processing ensstat.grb x
$wgrib2 ensstat.grb -match ":ens mean" -set_grib_type c3 -set_prob 1 1 1 15 15 -grib
wind_ge_15.grb

17. g2ctl.pl
For analyses:
$ g2ctl -O grib2_file > grib2_file.ctl
$ gribmap -O -i grib2_file.ctl
$ grads
ga-> open grib2_file.ctl
For forecasts (end of averaging period):
$ g2ctl grib2_file > grib2_file.ctl
$ gribmap -i grib2_file.ctl
$ grads
ga-> open grib2_file.ctl
For forecasts (start of averaging period):
$ g2ctl -b grib2_file > grib2_file.ctl
$ gribmap -b -i grib2_file.ctl
$ grads
ga-> open grib2_file.ctl
-verf .. use end of ave/acc period or
fcst time (default)
run gribmap with no option
-0 .. use analysis times
run gribmap with gribmap -0
-b .. use start of
averaging/accumulation period
run gribmap with gribmap -b
-365 .. use 365 day calendar
-no_profile .. no z coordinate
-raw .. use a raw grid
-ens "e1,..,en" .. a list of quoted
ensemble names

18. References
https://www.cpc.ncep.noaa.gov/products/wesley/wgrib2/
https://www.ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/tricks.w
grib2
https://www.ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/tricks.ch
eap
https://www.cpc.ncep.noaa.gov/products/wesley/g2ctl.html