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The file tz_an_pl.grib1 contains parameters T and Z on six pressure levels.
How could you use grib_copy to create two files, one containing all of the pressure levels for parameter T, the other for parameter
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Q ? Check the content of the new files with grib_ls.
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To copy only those messages for parameter T from the GRIB file tztq_an_pl.grib1 grib2 use grib_copy with the -w option to specify a shortName=t: | No Format |
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$> grib_copy -w shortName=t tztq_an_pl.grib1grib2 t_an_pl.grib1grib2 |
Similarly to copy only those messages for parameter ZQ, use: | No Format |
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$> grib_copy -w shortName=zq tztq_an_pl.grib1grib2 zq_an_pl.grib1grib2 |
Or, more simply, and and because the file contains only parameters T and ZQ, one can use: | No Format |
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$> grib_copy tztq_an_pl.grib1grib2 "[shortName]_an_pl.grib1grib2" |
Using grib_ls of the two files confirms that the contents are correct. For example: | No Format |
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$> grib_ls -p centre,paramId,typeOfLevel,level,date,dataType,time,shortName t_an_pl.grib1grib2
t_an_pl.grib1grib2
centre paramId typeOfLevel level date dataType time shortName
ecmf 130 isobaricInhPa 850 1000 20251013 20170219 fc an 0 1200 t
ecmf 130 isobaricInhPa 300 850 20251013 20170219 fc an 0 1200 t
ecmf 130 isobaricInhPa 7001000 20251013 20170219 fc an 0 1200 t
ecmf 130 isobaricInhPa 700 500 20251013 20170219 fc an 0 1200 t
ecmf 130 isobaricInhPa 500 400 20251013 20170219 fc an 0 1200 t
ecmf 130 isobaricInhPa 400 300 20251013 20170219 fc an 0 1200 t
6 of 6 messages in t_an_pl.grib1grib2
6 of 6 total messages in 1 files |
You can take this further and split the file tztq_an_pl.grib1 into separate files for each parameter/pressure level combination with: | No Format |
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$> grib_copy tztq_an_pl.grib1grib2 "[shortName]_[level].grib[edition]"
$> ls ?_*0.grib1grib?
tq_1000.grib1grib2 tq_400.grib1grib2 tq_700.grib1.grib2 q_an_pl.grib2 zt_1000300.grib1grib2 zt_400500.grib1grib2 zt_700850.grib1grib2
tq_300.grib1grib2 tq_500.grib1grib2 tq_850.grib1 z_300.grib1grib2 t_1000.grib2 zt_500400.grib1grib2 zt_850700.grib1grib2 |
Each file contains one message only, e.g.: | No Format |
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$> grib_ls -p centre,paramId,typeOfLevel,level,date,dataType,time,shortName t_1000.grib1grib2
t_1000.grib1grib2
centre paramId typeOfLevel level date dataType time shortName
ecmf 130 isobaricInhPa 1000 20251013 20170219 fc an 0 1200 t
1 of 1 messages in t_1000.grib1grib2
1 of 1 total messages in 1 files |
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The field in the file tp.grib is supposed to contain total precipitation, created using MARS compute to sum the convective and large-scale (stratiform) precipitation.
What is the parameter set to in the file tp.
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grib2 ? How could you use grib_set to change it is set correctly to total precipitation (shortName=tp, paramId=228) ?
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| title | Click here to see solution... |
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Use grib_ls to inspect the contents of the file tp.grib1grib2, printing also the paramId with the –P option: | No Format |
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$> grib_ls -P paramId tp.grib1grib2
tp.grib1grib2
paramId edition centre date typeOfLevel level dataType dataDategridType stepRangetypeOfLevel level dataType shortName stepRange packingType shortName gridType packingType
142 12 ecmf surface20241216 fc 0 regular_ll 20170221 surface 24 0 fc 0-24 lsp grid_simple regular_ll
1 of 1 messages in tp.grib1grib2
1 of 1 total messages in 1 files |
Alternatively, you could inspect the parameter namespace with grib_ls: | No Format |
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$> grib_ls -n parameter tp.grib1grib2
tp.grib1grib2
centre paramId unitsshortName units name name shortName
ecmf 142 mlsp m Large-scale precipitation lsp Large-scale precipitation
1 of 1 messages in tp.grib1grib2
1 of 1 total messages in 1 files |
The shortName is set to lsp and paramId to 142, i.e., large-scale precipitation. | Info |
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The field was created using MARS compute to sum the convective and large-scale (stratiform) precipitation. When using MARS compute, the output field maintains the information from the GRIB header of the first input field so, in this case, we see that the field has paramId 142 even though the message is now supposed to contain the total precipitation. |
To change the parameter to total precipitation, use grib_set as follows: | No Format |
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$> grib_set -s shortName=tp tp.grib1grib2 tp_new.grib1grib2 |
Check with grib_ls: | No Format |
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$> grib_ls -n parameter tp_new.grib2
tp_new.grib2
centre paramId | | No Format |
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$> grib_ls -P paramId tp_new.grib1
tp_new.grib1
paramId edition centre typeOfLevel level dataDate stepRange dataType shortName packingType gridType
228 1 ecmf shortName surface units 0 name
ecmf 20170221 24 228 tp fc m tp Total grid_simple regular_ll precipitation
1 of 1 messages in tp_new.grib1grib2
1 of 1 total messages in 1 files |
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Note that changing the shortName has also changed the value of paramId. The same could be achieved by setting paramId=228: | No Format |
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$> grib_set -s paramId=228 tp.gribgrib2 tp_new.gribgrib2 |
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To convert the GRIB messages in file.grib1grib2 to netCDF with data type NC_SHORT and store in a file named out1.nc use: | No Format |
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$> grib_to_netcdf -o out1.nc file1.grib1grib2
grib_to_netcdf: Version 2.3044.20
grib_to_netcdf: Processing input file 'file1.grib1grib2'.
grib_to_netcdf: Found 45 GRIB fields in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out1.nc'
grib_to_netcdf: NetCDF library version: 4.9.13 of FebMar 14 92025 2023 1307:5427:0922 $
grib_to_netcdf: Creating large (64 bit) file format.
grib_to_netcdf: Defining variable 't2m'.
grib_to_netcdf: Done. |
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Using ncdump to print the values for the variable t2m shows: | No Format |
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$> module load netcdf4
$> ncdump -v t2m out1.nc
netcdf out1 {
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data:
t2m =
20497-29628, -80928125,
21957, 21552,
-1631215905, -32766,
23701, 21176,
3276728426, 704232767,
-131225246, -1804123499,
1716810705, -122074529,
-414827955, -2839229374,
19382-32625, -1082732683,
-250630518, -2540629616 ;
} |
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The data values appear as integers because of the NC_SHORT data format. These values need to be unpacked using the scale_factor and add_offset NetCDF netCDF attributes. |
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How could you change the the grib_to_netcdf command to create a netCDF output with the values stored as floating point numbers ?
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To store the values in the netCDF file as floating point numbers, use the grib_to_netcdf tool with the data format to NC_FLOAT or NC_DOUBLE with the -D option. For example: | No Format |
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$> grib_to_netcdf -D NC_FLOAT -o out2.nc file1.grib1grib2
grib_to_netcdf: Version 2.3044.20
grib_to_netcdf: Processing input file 'file1.grib1grib2'.
grib_to_netcdf: Found 45 GRIB fields in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out2.nc'
grib_to_netcdf: NetCDF library version: 4.9.13 of FebMar 14 92025 2023 1307:5427:0922 $
grib_to_netcdf: Creating large (64 bit) file format.
grib_to_netcdf: Defining variable 't2m'.
grib_to_netcdf: Done. |
Checking with ncdump, we see the values are now stored as floating point values: | No Format |
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ncdump -v t2m out2.nc
netcdf out2 {
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data:
t2m = t2m =
282.9801, 283.1394,
288.4472, 288.4043,
281284.16664345, 277282.99436475,
275288.6859632, 273288.2363644,
282289.99371328, 279289.16325929,
277288.91947958, 275288.42856107,
280287.6712547, 276286.29716001,
277289.49710829, 273289.88722333,
281282.00076624, 276282.50266563,
277289.74163545, 274289.33182589 ;
} |
In this case, the data values do not need further processing. |
Try using grib_to_netcdf to convert the GRIB messages in file2.grib to netCDF. What happens....and why ?
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How would you fix it ?
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Converting file2.grib grib2 to netCDF gives the following error: | No Format |
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$> grib_to_netcdf -o out2.nc file2.grib1grib2
grib_to_netcdf: Version 2.3044.20
grib_to_netcdf: Processing input file 'file2.grib1grib2'.
grib_to_netcdf: Found 15 GRIB fieldfields in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out2out3.nc'
grib_to_netcdf: NetCDF library version: 4.9.13 of FebMar 14 92025 2023 1307:5427:0922 $
grib_to_netcdf: Creating large (64 bit) file format.
ECCODES ERROR : Grid type = reduced_gg
ECCODES ERROR : First GRIB is not on a regular lat/lon grid or on a regular Gaussian grid. Exiting. |
The conversion fails because the GRIB data is represented on a reduced Gaussian grid (gridType=reduced_gg). Conversion to NetCDF netCDF is possible only for GRIB data with gridType=regular_ll or gridType=regular_gg. To fix this it would be necessary to re-retrieve the data on a regular grid or interpolate to a regular grid. |
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