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Using the grib_api tools

 

You will find some example data in $SCRATCH/ectrain/trx/openifs or you can use output from one of your own experiments.  Let's play !

Getting help with the tools

Running any of the GRIB API tools without options or input files will produce a UNIX 'man page' style output describing the usage.  Try:

getting help
grib_ls

Listing the contents of GRIB files

An inventory of the contents of GRIB files can be obtained with the grib_ls command.

  1. For a single file, try:

    grib_ls ICMGGepc8+000000

  2. Multiple files:

    grib_ls ICMGG*

  3. Counting the number of messages in a file

    grib_count ICMGGepc8+000000

Customising the output of grib_ls

  1. Specifying the information to display:

    grib_ls -p dataDate,dataTime,stepRange,shortName,typeOfLevel,level ICMGGepc8+000000

  2. Specifying the type of key (s: string, i: integer).  To output the centre as a string (default):

    grib_ls -p centre:s ICMGGepc8+000000

    or as an integer:

    grib_ls -p centre:i ICMGGepc8+000000

  3. Listing specific namespace information.  Try:

    grib_ls -n parameter ICMGGepc8+000000

    and

    grib_ls -n statistics ICMGGepc8+000000

    Other namespaces to try are time, vertical and geography.

  4. Order the output.  Try:

    grib_ls -B "level desc" ICMSHepc8+000000

    Compare and contrast with:

    grib_ls -B "level:i desc" ICMSHepc8+000000

  5. The where option:  IS

    grib_ls -w shortName=stl1 ICMGGepc8+000000

    IS NOT

    grib_ls -w shortName!=stl1 ICMGGepc8+000000

    OR

    grib_ls -w shortName=stl1/z ICMGGepc8+000000

    AND

    grib_ls -w shortName=q,level=10 ICMGGepc8+000000

  6. Specifying the step unit.  Compare the output from:

    grib_ls -n time ICMGGepc8+000000

    and

    grib_ls -s stepUnits=m -n time ICMGGepc8+000012

    The stepUnits can be set to s,m,h,3h,6h,12h,D,M,Y,10Y,30Y or C.  The result must be expressible as an integer (try setting stepUnits=D above). This is a temporary change for display purposes only !

grib_ls is a tool meant mainly for interactive use.  If you want to use the tools in a shell script then take a look at grib_get instead. It works in much the same way as grib_ls but is less verbose and also fails if an error occurs.

Obtaining detailed information about a GRIB message with grib_dump

  1. To print all of the keys available for a particular GRIB message:

    grib_dump -w shortName=stl1 ICMGGepc8+000012

  2. To obtain a listing which is more like the WMO tables:

    grib_dump -O -w shortName=stl1 ICMGGepc8+000012

  3. To obtain even more detail:

    grib_dump -OatH -w shortName=stl1 ICMGGepc8+000012

Printing the data values

For grid point data, the grib_get_data tool prints a list of latitude, longitude and data values from one or more GRIB files. 

  1. Try

    grib_get_data -w shortName=z  ICMGGepc8+000000

  2. The format of the data values can be controlled with a C-style format statement.  Try:

    grib_get_data -w shortName=z -F"%.4f" ICMGGepc8+000000

  3. Other information can be printed along with the lat-lon-values triplet:

    grib_get_data -p dataDate,dataTime,stepRange -w shortName=stl1 -F"%.4f" ICMGGepc8+000000

If you need the values in a program then do not be tempted to use grib_get_data to output the information to a file and then read the file in your program.  It is more efficient to use the appropriate Fortran 90, C or Python interface to GRIB API to access the data values directly.

Using the smart copy

The structure of GRIB allows files to be simply copied and concatenated with the UNIX 'cat' command.  However, it's sometimes desirable to do something more sophisticated.  The grib_copy tool provides a way to copy only selected content from GRIB files.

 

Resources and further reading

  1. The WMO FM 92 GRIB Manuals can be obtained from www.wmo.int/pages/prog/www/WMOCodes.html
  2. The ECMWF GRIB API manual is available at https://software.ecmwf.int/wiki/display/GRIB/Home
  3. ECMWF parameter database: http://old.ecmwf.int/publications/manuals/d/gribapi/param
  4. GRIB 1 keys http://old.ecmwf.int/publications/manuals/d/gribapi/fm92/grib1/
  5. GRIB 2 keys http://old.ecmwf.int/publications/manuals/d/gribapi/fm92/grib2/
  6. Edition independent keys http://old.ecmwf.int/publications/manuals/d/gribapi/keys/ 
  7. The GRIB API Tools are documented at GRIB tools
  8. Details of the GRIB API Fortran 90 interface:Fortran package grib_api
  9. Example Python scripts and Fortran 90 and C programs can be found at GRIB API examples
  10. ECMWF GRIB API training course material: GRIB API: Library and Tools



 

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