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cdo: map from regular Gaussian to curvilinear grid

Added by Jia Wang 8 months ago

I try to map data from a regular Gaussian to a curvilinear grid.

This is my command:
cdo remapbil,target.grb aa.grb bb.grb

This is the warning message:
cdi warning (cgribexDefGrid): CGRIBEX library doesn't support generic grids, grid information will be lost!

But the source file "aa.grb" is on a regular Gaussian grid. This is the output from cdo grids:
gridtype = gaussian
gridsize = 13107200
xsize = 5120
ysize = 2560
xname = lon
xlongname = "longitude"
xunits = "degrees_east"
yname = lat
ylongname = "latitude"
yunits = "degrees_north"
numLPE = 1280
xfirst = 0
xinc = 0.0703125
yvals =

The target grid file "target.grb" is on a curvilinear grid:
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

The output file "bb.grb" does not have the correct grid information. This is the output from cdo griddes bb.grb
gridtype = generic
gridsize = 987601


Replies (12)

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

Hi Jia,

without the data file its not really possible to help.

I made a simple test with the topo data on a regular gaussian grid and it works as expected. The file target.txt contains your target grid description from above.

cdo -f nc -remapbil,F180 -topo topo_gaussianF180.nc

cdo remapbil,target.txt topo_gaussianF180.nc outfile.nc

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Jia Wang 7 months ago

Hi, Karin,

I have attached the data files.

These are the steps I did:

cdo griddes target_grid.grb > target.txt
cdo remapbil,target.txt -setgridtype,regular org.grb out.grb

This is the output from the last command:
cdo(1) setgridtype: Process started
cdo remapbil: Bilinear weights from gaussian (1616x800) to projection (1121x881) grid
cdi warning (cgribexDefGrid): CGRIBEX library doesn't support generic grids, grid information will be lost!
cdo remapbil: Processed 24563200 values from 1 variable over 19 timesteps [2.06s 293MB]

The output has no grid information, using the command "cdo griddes out.grb ":
gridtype = generic
gridsize = 987601
cdo griddes: Processed 1 variable [0.05s 27MB]

Download all files
target_grid.grb (199 KB) target_grid.grb
target.txt (563 Bytes) target.txt
org.grb (35.6 MB) org.grb
out.grb (53.7 MB) out.grb

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

Try

cdo -f nc -remapbil,target.txt -setgridtype,regular org.grb outfile.nc

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Jia Wang 7 months ago

It works now. Thanks a lot.
But why it is not working when the output is in the grib format?

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

I don't know why it is not working for GRIB1 output format, but for GRIB2 output format it works

cdo -f grb2 -remapbil,target.txt -setgridtype,regular org.grb outfile.grb2

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

I guess that the grid type projection is only available for GRIB2 files.

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Jia Wang 7 months ago

I have tried these two ways:
1st:
cdo -f nc -remapcon,target.txt -setgridtype,regular org.grb aa.nc

2nd:
cdo -f grb2 -remapbil,target.txt -setgridtype,regular org.grb outfile.grb
cdo -f nc copy outfile.grb bb.nc

Then I found out that there are differences in aa.nc and bb.nc.
Which way could better preserve the precision?

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Jia Wang 7 months ago

My mistake.

But even if I use the same interpolation method, there are still some differences:

Date     Time   Level Gridsize    Miss    Diff : S Z  Max_Absdiff Max_Reldiff : Parameter ID
     1 : 2024-01-29 12:00:00       0   987601       0  505282 : F T   3.7253e-09     0.49929 : 228.128    
     2 : 2024-01-30 00:00:00       0   987601       0  432881 : F T   3.7253e-09     0.49927 : 228.128    
     3 : 2024-01-30 12:00:00       0   987601       0  967290 : T T   3.7253e-09     0.99988 : 228.128

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

This may be due to the different precision when writing the data to the corresponding format.

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Jia Wang 7 months ago

Which way would better preserve the precision?

RE: cdo: map from regular Gaussian to curvilinear grid - Added by Karin Meier-Fleischer 7 months ago

I recommend to use the direct way to generate the output file format without copy.

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