CDO

The GRIB file contains:

cdo sinfon st4_conus.2023121800.06h.grb2

   File format : GRIB2
    -1 : Institut Source   T Steptype Levels Num    Points Num Dtype : Parameter name
     1 : unknown  unknown  v accum         1   1    987601   1  P21  : tp            
   Grid coordinates :
     1 : projection               : points=987601 (1121x881)
                          mapping : polar_stereographic
                                x : 0 to 5334000 by 4762.5 m
                                y : 0 to 4191000 by 4762.5 m
   Vertical coordinates :
     1 : surface                  : levels=1
   Time coordinate :
                             time : 1 step
     RefTime =  2023-12-17 18:00:00  Units = hours  Calendar = proleptic_gregorian
  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss
  2023-12-18 00:00:00

When you convert the data from GRIB to netCDF the coordinates will be kept, not remapped to a lonlat grid. This can be done with one of the remap operators, e.g.

gridfile.txt

gridtype  = lonlat
gridsize  = 987601
xsize     = 1121
ysize     = 881
xname     = lon
xunits    = "degrees_east" 
yname     = lat
yunits    = "degrees_north" 
xfirst    = -130.
xinc      = 0.06
yfirst    = 20.
yinc      = 0.06

cdo -f nc -remapbil,grid_deg.txt st4_conus.2023121800.06h.grb2 outfile.nc

But I want the Lat / Lon of the original grid in Polar Stereographic projection, which would be 2-dimensional for either Lat or Lon. Is there a way to get it?

Another issue I have is that, I realize the grid information in some of my grib2 files are "wrong".

For the file "ST4.2009113018.06h.grb2", the grid information using "cdo griddes" is:
gridtype = projection
gridsize = 987601
xsize = 1121
ysize = 881
xname = x
xunits = "m"
yname = y
yunits = "m"
xfirst = 0
xinc = 4763
yfirst = 0
yinc = 4763
grid_mapping = Polar_Stereographic
grid_mapping_name = polar_stereographic
standard_parallel = 60.
straight_vertical_longitude_from_pole = -105.
latitude_of_projection_origin = 90.
earth_radius = 6367470.
false_easting = 1901416.77987822
false_northing = 7613145.49149694
longitudeOfFirstGridPointInDegrees = 240.977
latitudeOfFirstGridPointInDegrees = 23.117

For the file "st4_conus.2023121800.06h.grb2", the grid information using "cdo griddes" is:
gridtype = projection
gridsize = 987601
xsize = 1121
ysize = 881
xname = x
xunits = "m"
yname = y
yunits = "m"
xfirst = 0
xinc = 4762.5
yfirst = 0
yinc = 4762.5
grid_mapping = Polar_Stereographic
grid_mapping_name = polar_stereographic
standard_parallel = 60.
straight_vertical_longitude_from_pole = 255.
latitude_of_projection_origin = 90.
earth_radius = 6371200.
false_easting = 1902531.67435698
false_northing = 7617604.93005048
longitudeOfFirstGridPointInDegrees = 240.976992
latitudeOfFirstGridPointInDegrees = 23.117

Based on my knowledge, the "xinc"/"yinc"/"earth_radius" in "ST4.2009113018.06h.grb2" is wrong, and they should be the same as those in "st4_conus.2023121800.06h.grb2".

I think this error in its grid description will cause problems when I either interpolate other data into the grid represented by "ST4.2009113018.06h.grb2" or interpolate data in "ST4.2009113018.06h.grb2" to other grids.

How could I make corrections to the grid description in "ST4.2009113018.06h.grb2".

Use the correct grid description file with one of the remap operators to fix this issue.

Could you give an example of the command line?

The command is similar to the command obove, but instead of using the grid description file you can use the file with the correct grid directly.

cdo -remapbil,st4_conus.2023121800.06h.grb2 ST4.2009113018.06h.grb2 outfile.grb2

In this way, the CDO will ignore the grid description of "ST4.2009113018.06h.grb2", but only use its data?

No, it remaps the data to the new grid.

If you want to exchange the grid description you can set the correct grid before you copy the data.

cdo -copy -setgrid,st4_conus.2023121800.06h.grb2 ST4.2009113018.06h.grb2 outfile.grb2

I think I did not explain my issue clear.

"ST4.2009113018.06h.grb2"'s grid description is wrong.

"st4_conus.2023121800.06h.grb2"'s grid description is right.

These two files should be on the same grid, and should have the same grid description when I use "cdo griddes". But in reality, "ST4.2009113018.06h.grb2"'s grid description is wrong.

If I use the command "cdo -remapbil,st4_conus.2023121800.06h.grb2 ST4.2009113018.06h.grb2 outfile.grb2", it will interpolate data from "ST4.2009113018.06h.grb2"'s grid, which has a wrong grid description, to the "st4_conus.2023121800.06h.grb2"'s grid, then "outfile.grb2" will be wrong, because NO interpolation is actually needed since the data in reality is on the same grid.

You were too fast or I was too slow. See my last answer that only exchange the grid description.

Just saw you post after I did my last post. I think your suggestion "setgrid" is what I need. Thanks.