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Comment: Confirmed: Latest edition of course

1. The file file1.grib1 contains parameters T and Z on six pressure levels.

  • Use grib_copy to create two files, one containing all the pressure levels for parameter T, the other for Z. Check the content of the new files with grib_ls
Expand
titleSolution...

To copy only those messages for parameter T from the GRIB file file1.grib1 use grib_copy with the -w option to specify a shortName=t:

Code Block
   % grib_copy -w shortName=t file1.grib1 t.grib1

Similarly to copy only those messages for parameter Z, use:

Code Block
   % grib_copy -w shortName=z file1.grib1 z.grib1

Or, more simply, and because the file contains only parameters T and Z, one can use:

Code Block
  % grib_copy file.grib1 “[shortName]_an_pl.grib1”

Using grib_ls of the two files confirms that the contents are correct. For example:

Code Block
% grib_ls -p centre,shortName,typeOfLevel,level,dataDate t.grib1
  t.grib1
  centre       shortName    typeOfLevel  level        dataDate     
  ecmf         t            isobaricInhPa  1000        20190201     
  ecmf         t            isobaricInhPa  850         20190201     
  ecmf         t            isobaricInhPa  700         20190201     
  ecmf         t            isobaricInhPa  500         20190201     
  ecmf         t            isobaricInhPa  400         20190201     
  ecmf         t            isobaricInhPa  300         20190201 
  6 of 6 grib messages in t.grib1
 
  6 of 6 total grib messages in 1 files
  • Repeat but output the messages so the levels in the new files are in increasing numerical order
Expand
titleSolution...

To reorder the messages in order of ascending pressure level, use, e.g.:

Code Block
% grib_copy -B “level:i asc” file.grib1 “[shortName]_ordered.grib1”

Note the use of “level:i” here to specify that the level should be treated as an integer !  

Using grib_ls to inspect the two files confirms that the contents are correct.  For example:

Code Block
 % grib_ls -p centre,shortName,typeOfLevel,level,dataDate t_ordered.grib1  
  t_ordered.grib1
  centre       shortName    typeOfLevel  level        dataDate     
  ecmf         t            isobaricInhPa  300          20190201    
  ecmf         t            isobaricInhPa  400          20190201    
  ecmf         t            isobaricInhPa  500          20190201    
  ecmf         t            isobaricInhPa  700          20190201    
  ecmf         t            isobaricInhPa  850          20190201    
  ecmf         t            isobaricInhPa  1000         20190201
  6 of 6 grib messages in t_ordered.grib1
 
  6 of 6 total grib messages in 1 files

2. Use grib_set to change the date and time to 12UTC on 4 February 2019 for all messages in file1.grib1

Expand
titleSolution...

To change the date and time for the GRIB messages in file.grib1 use:

Code Block
% grib_set -s dataDate=20190204,dataTime=12 file.grib1 file_fixed.grib1


Using grib_ls confirms that this has been done correctly:


Code Block
% grib_ls -p centre,shortName,typeOfLevel,level,dataDate,dataTime file_fixed.grib1
   file_fixed.grib1
   centre       shortName    typeOfLevel  level        dataDate     dataTime     
   ecmf         z            isobaricInhPa  1000         20190204     12        
   ecmf         t            isobaricInhPa  1000         20190204     12        
   ecmf         z            isobaricInhPa  850          20190204     12        
   ecmf         t            isobaricInhPa  850          20190204     12        
   ecmf         z            isobaricInhPa  700          20190204     12        
   ecmf         t            isobaricInhPa  700          20190204     12        
   ecmf         z            isobaricInhPa  500          20190204     12        
   ecmf         t            isobaricInhPa  500          20190204     12        
   ecmf         z            isobaricInhPa  400          20190204     12        
   ecmf         t            isobaricInhPa  400          20190204     12        
   ecmf         z            isobaricInhPa  300          20190204     12        
   ecmf         t            isobaricInhPa  300          20190204     12        
   12 of 12 grib messages in file_fixed.grib1
 
   12 of 12 total grib messages in 1 files
  • Repeat but change the date and time for T at 500hPa only
Expand
titleSolution...

To make the same change of date and time for the temperature at 500hPa only use the -w option with grib_set to make sure that the correct message is changed:

Code Block
% grib_set -w shortName=t,level=500 -s dataDate=20190204,dataTime=12 file.grib1 t500_fixed.grib1

Again, grib_ls can be used to confirm the output is correct:

Code Block
 % grib_ls -p centre,shortName,typeOfLevel,level,dataDate,dataTime t500_fixed.grib1  t500_fixed.grib1
  centre       shortName    typeOfLevel  level        dataDate     dataTime     
  ecmf         z            isobaricInhPa  1000         20190201     0        
  ecmf         t            isobaricInhPa  1000         20190201     0        
  ecmf         z            isobaricInhPa  850          20190201     0        
  ecmf         t            isobaricInhPa  850          20190201     0        
  ecmf         z            isobaricInhPa  700          20190201     0        
  ecmf         t            isobaricInhPa  700          20190201     0        
  ecmf         z            isobaricInhPa  500          20190201     0        
  ecmf         t            isobaricInhPa  500          20190204     1200           
  ecmf         z            isobaricInhPa  400          20190201     0        
  ecmf         t            isobaricInhPa  400          20190201     0        
  ecmf         z            isobaricInhPa  300          20190201     0        
  ecmf         t            isobaricInhPa  300          20190201     0        
  12 of 12 grib messages in t500_fixed.grib1
 
  12 of 12 total grib messages in 1 files
  • Repeat so that T at 500hPa only is written to the output file
Expand
titleSolution...

If grib_set is used with the -S option, only the changed GRIB message is copied to the output file:

Code Block
 % grib_set -S -w shortName=t,level=500 -s dataDate=20190204,dataTime=12 file.grib1 t500_only.grib1


Again, grib_ls can be used to confirm the output is correct:

Code Block
% grib_ls -p centre,shortName,typeOfLevel,level,dataDate,dataTime t500_only.grib1  t500_only.grib1
 
 centre       shortName    typeOfLevel  level        dataDate     dataTime                                                         
  ecmf         t            isobaricInhPa  500          20190204     12           
  1 of 1 grib messages in t500_only.grib1
 
  1 of 1 total grib messages in 1 files

3. Use grib_to_netcdf to convert the GRIB messages in file2.grib1 to NetCDF.

  • Try with both the default data type (NC_SHORT) and NC_FLOAT.
  • Check the data values in each case with ncdump.
Expand
titleSolution...

To convert the GRIB messages in file.grib1 to netCDF with data type NC_SHORT use:

Code Block
% grib_to_netcdf -D NC_SHORT -o out2.nc file2.grib1
grib_to_netcdf: Version 2.10.0
grib_to_netcdf: Processing input file 'file2.grib1'.
grib_to_netcdf: Found 4 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: 4.4.1 of May  8 2017 15:47:29 $
grib_to_netcdf: Creating large (64 bit) file format.
grib_to_netcdf: Defining variable 't2m'.
grib_to_netcdf: Done.

Using ncdump to print the values for the variable t2m shows:

Code Block
% ncdump -v t2m out2.nc
  ...
  data:

  t2m =
   6307, -14374,
   -8825, -32766,
   32767, 10489,
   18439, 1565,
   16883, 652,
   3044, -2386,
   7017, 146,
   564, -1652 ;
 }

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 attributes.

Repeating but setting the netCDF data format to NC_FLOAT gives:

Code Block
% grib_to_netcdf -D NC_FLOAT -o out2a.nc file2.grib1
grib_to_netcdf: Version 2.10.0
grib_to_netcdf: Processing input file 'file2.grib1'.
grib_to_netcdf: Found 4 GRIB fields in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out2a.nc'
grib_to_netcdf: NetCDF library version: 4.4.1 of May  8 2017 15:47:29 $
grib_to_netcdf: Creating large (64 bit) file format.
grib_to_netcdf: Defining variable 't2m'.
grib_to_netcdf: Done.


% ncdump -v out2a.nc
  ...
  data:

  t2m =
   273.2383, 269.6726,
   270.6294, 266.5017,
   277.8003, 273.9594,
   275.33, 272.4207,
   275.0618, 272.2634,
   272.6757, 271.7396,
   273.3607, 272.1761,
   272.2482, 271.8661 ;
 }

In this case, the data values do not need further processing.

  • Repeat but set the Reference date to 6 February 2019 and compare with previous results
Expand
titleSolution...

To set the reference date to 1 January 2019, use the -R option

Code Block
% grib_to_netcdf -R 20190206 -o out2b.nc file2.grib1
grib_to_netcdf: Version 2.10.0
grib_to_netcdf: Processing input file 'file2.grib1'.
grib_to_netcdf: Found 4 GRIB fields in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out2b.nc'
grib_to_netcdf: NetCDF library version: 4.4.1 of May  8 2017 15:47:29 $
grib_to_netcdf: Creating large (64 bit) file format.
grib_to_netcdf: Defining variable 't2m'.
grib_to_netcdf: Done.

Compare the time variable settings from out2.nc and out2b.nc:

Code Block
% ncdump -v time out2.nc
  ...
    variables:
       ...
       int time(time) ;
               time:units = "hours since 1900-01-01 00:00:00.0" ;
               time:long_name = "time" ;
               time:calendar = "gregorian" ;

    data:

         time = 1043886, 1043892, 1043898, 1043904 ;
    ...

    % ncdump -v time out2b.nc
    ...
        variables:
        ...
        int time(time) ;
                time:units = "hours since 2019-02-06 00:00:00.0" ;
                time:long_name = "time" ;
                time:calendar = "gregorian" ;

    data:

         time = -114, -108, -102, -96 ;
    ...

The time units are different and the time values are negative in out2b.nc because the date of the GRIB data is 1 February  2019  (so before the new reference date).

4. Use grib_to_netcdf to convert the GRIB messages in file3.grib1 to NetCDF.

  • What happens … and why ?
Expand
titleSolution...

Converting file3.grib1 to netCDF gives the following:

Code Block
% grib_to_netcdf -o out4.nc file3.grib1
grib_to_netcdf: Version 2.10.0
grib_to_netcdf: Processing input file 'file3.grib1'.
grib_to_netcdf: Found 1 GRIB field in 1 file.
grib_to_netcdf: Ignoring key(s): method, type, stream, refdate, hdate
grib_to_netcdf: Creating netCDF file 'out4.nc'
grib_to_netcdf: NetCDF library version: 4.4.1 of May  8 2017 15:47:29 $
grib_to_netcdf: Creating large (64 bit) file format.
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 is possible only for GRIB data with gridType=regular_ll or gridType=regular_gg.