Difference between revisions of "How to read GRIB files!"

From mn/geo/geoit
Jump to: navigation, search
(Created page with "Back to ECMWF overview[http://muspelheim.nilu.no/w/index.php/ECMWF] ''Updated by nik: 12 March 2013'' GRIB decoding ''GRIB = GRIdded Binary is a compressed format for meter...")
 
 
(20 intermediate revisions by 3 users not shown)
Line 1: Line 1:
−
Back to ECMWF overview[http://muspelheim.nilu.no/w/index.php/ECMWF]
+
Back to [[ECMWF]] overview
  
 +
''Updated by jfb: 15 March 2014''
  
−
''Updated by nik: 12 March 2013''
+
<br/>'''GRIB'''
  
 +
'''GRI'''dded '''B'''inary is a compressed format for meterological data used since 1985.
  
−
GRIB decoding
+
Requires software for encoding and decoding.
  
  
−
''GRIB = GRIdded Binary is a compressed format for meterological data used since 1985.''
 
  
 +
== GRIB1 ==
  
−
'''GRIB1:'''
+
Currently used for ECMWF operational ''surface'' and ''pressure level'' data.
  
 +
GRIB1 data is stored in 5 ''sections'' where section 4 includes the compressed data and the other sections are data specifications.
  
 +
One GRIB1 file may include many messages (blocks of sections).
  
−
'''GRIB2:'''
+
 
−
was introduced for ECMWF operational model level data on 18 May 2011.  
+
 
 +
 
 +
 
 +
== GRIB2 ==
 +
 
 +
was introduced for ECMWF operational ''model level'' data on '''18 May 2011'''.
  
 
Migration from GRIB1 to GRIB2 was necessary because GRIB1 data could only hold 126 model levels.
 
Migration from GRIB1 to GRIB2 was necessary because GRIB1 data could only hold 126 model levels.
Line 22: Line 31:
 
Number of model levels in the high resolution forecast will be increased from 91 to 137 in June 2013.
 
Number of model levels in the high resolution forecast will be increased from 91 to 137 in June 2013.
  
−
Pressure levels will still have GRIB1 but eventually this will also be changed.  
+
Pressure levels will still have GRIB1 but eventually this will also be changed.
  
 
"Old" model data will still have GRIB1 format in the archive.
 
"Old" model data will still have GRIB1 format in the archive.
  
−
GRIB2 also allows more flexibility and is more complex than GRIB1.  
+
GRIB2 also allows more flexibility and is more complex than GRIB1.
  
−
For example.
+
For example higher precision of variables.
  
 
  GRIB1: Lon/lat is in milli-degree
 
  GRIB1: Lon/lat is in milli-degree
 
  GRIB2: Lon/lat is in micro-degree
 
  GRIB2: Lon/lat is in micro-degree
  
 +
<br/>GRIB2 data are stored in 8 ''sections'' where section 7 includes the compressed data and the other sections are data specifications.
 +
 +
Thus, the structure of GRIB1 and GRIB2 messages is different, the order of the sections are different and there are additional sections for GRIB2.
 +
 +
== GRIB-decoding ==
 +
 +
To decode GRIB data there are software available from ECMWF.
 +
 +
This software is the '''GRIB_API (Application Programming Interface)''' library.
 +
 +
This software hides the binary layer of the message and uses a key/value approach to access the information in a GRIB message.
 +
 +
GRIB1 and GRIB2 uses the '''same''' function call.
 +
 +
'''Many keys to identify a parameter are edition independent, use these where possible!'''
 +
 +
<br/>'''GRIB_API''' can be used either through
 +
 +
Command line tools
 +
Fortran90/C/Python interfaces
 +
 +
 +
 +
== COMMAND-LINE GRIB TOOLS ==
 +
 +
General syntax for use of the grib tools
 +
 +
'''grib_&lt;tool&gt; [options] grib_file1 grib_file2... [output file]'''
 +
 +
<br/>Manuals for each tool are available by running the tool without any options or input file.
 +
 +
<br/>'''Tools to inspect the content of and compare GRIB messages'''
 +
 +
'''grib_ls'''            summary of the content of a GRIB file. (Do not use in scripts - only interactive mode!)
 +
                   
 +
                    A default list of keys in printed without using options:
 +
                    edition, centre, typeOfLevel, level, dataDate, stepRange, dataType, shortName, packingType, gridType
 +
                   
 +
                    Useful options
 +
                                      -p        keys to print (other than the default list)
 +
                                      -P        additional keys to print (additional to the default list)
 +
                                      -m        print MARS keys. Very useful to see the keys as they would be specified in a MARS request!
 +
                                                E.g. domain, levtype, levelist, date, time, step, param, class, type, stream, expver
 +
                                      -w        "where", prints only the content for specified key=value. (Can be used with all grib tools).
 +
                                      -l        Find the lat/long value in the model close to a given lat/long
 +
                    Example:
 +
                    '''grib_ls -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030100'''
 +
                    prints the information about the temperature field at level 1:
 +
                    edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType
 +
                    2            ecmf        20130301    an          regular_ll  0            hybrid      1            t            grid_jpeg
 +
                   
 +
                    '''grib_ls -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030103'''
 +
                    prints the information about the temperature field at level 1:
 +
                    edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType
 +
                    2            ecmf        20130301    fc          regular_ll  3            hybrid      1            t            grid_jpeg
 +
                   
 +
                    You can see that this is GRIB edition 2, and data for midnight (00) is analysis data while 03 is forecast data 3 hours from midnight (stepRange).
 +
                    The data is on a regular lat-long grid.
 +
 +
 +
 +
 +
 +
'''grib_dump'''          provides a more detailed view and listing of the file.
 +
                   
 +
                    Useful options
 +
                                      -O to show the content in WMO style (easier to read)
 +
                                      -w        "where", prints only the content for specified key=value
 +
 +
                    Example:
 +
                    '''grib_dump -O -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030103'''
 +
 +
 +
 +
'''grib_get'''
 +
 +
'''grib_get_data'''
 +
 +
<br/>'''Tools for counting and copying some messages'''
 +
 +
'''grib_copy'''
 +
 +
'''grib_count'''
  
 +
<br/>'''Tools for making changes to the content of a GRIB message'''
  
−
'''To decode GRIB data there are software available from ECMWF. '''
+
'''grib_set'''
  
−
This software is is the GRIB_API (Application Programming Interface) library.
+
'''grib_filter'''
  
−
This software hides the binary layer and uses a key/value approach to access the information.
+
== FORTRAN GRIB TOOLS ==
  
−
GRIB1 and GRIB2 uses the same function call.
+
Grib API
  
−
There are both command line tools, and Fortran90/C/Python interfaces.
+
[[Category:Tools]][[Category:Models]][[Category:ECMWF]]

Latest revision as of 17:23, 30 January 2015

Back to ECMWF overview

Updated by jfb: 15 March 2014


GRIB

GRIdded Binary is a compressed format for meterological data used since 1985.

Requires software for encoding and decoding.


GRIB1

Currently used for ECMWF operational surface and pressure level data.

GRIB1 data is stored in 5 sections where section 4 includes the compressed data and the other sections are data specifications.

One GRIB1 file may include many messages (blocks of sections).



GRIB2

was introduced for ECMWF operational model level data on 18 May 2011.

Migration from GRIB1 to GRIB2 was necessary because GRIB1 data could only hold 126 model levels.

Number of model levels in the high resolution forecast will be increased from 91 to 137 in June 2013.

Pressure levels will still have GRIB1 but eventually this will also be changed.

"Old" model data will still have GRIB1 format in the archive.

GRIB2 also allows more flexibility and is more complex than GRIB1.

For example higher precision of variables. 

GRIB1: Lon/lat is in milli-degree
GRIB2: Lon/lat is in micro-degree


GRIB2 data are stored in 8 sections where section 7 includes the compressed data and the other sections are data specifications.

Thus, the structure of GRIB1 and GRIB2 messages is different, the order of the sections are different and there are additional sections for GRIB2.

GRIB-decoding

To decode GRIB data there are software available from ECMWF.

This software is the GRIB_API (Application Programming Interface) library.

This software hides the binary layer of the message and uses a key/value approach to access the information in a GRIB message.

GRIB1 and GRIB2 uses the same function call.

Many keys to identify a parameter are edition independent, use these where possible!


GRIB_API can be used either through 

Command line tools
Fortran90/C/Python interfaces


COMMAND-LINE GRIB TOOLS

General syntax for use of the grib tools

grib_<tool> [options] grib_file1 grib_file2... [output file]


Manuals for each tool are available by running the tool without any options or input file.


Tools to inspect the content of and compare GRIB messages 

grib_ls             summary of the content of a GRIB file. (Do not use in scripts - only interactive mode!)
                   
                   A default list of keys in printed without using options: 
                   edition, centre, typeOfLevel, level, dataDate, stepRange, dataType, shortName, packingType, gridType
                   
                   Useful options
                                     -p        keys to print (other than the default list)
                                     -P        additional keys to print (additional to the default list)
                                     -m        print MARS keys. Very useful to see the keys as they would be specified in a MARS request!
                                               E.g. domain, levtype, levelist, date, time, step, param, class, type, stream, expver
                                     -w        "where", prints only the content for specified key=value. (Can be used with all grib tools).
                                     -l        Find the lat/long value in the model close to a given lat/long
                   Example:
                   grib_ls -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030100
                   prints the information about the temperature field at level 1: 
                   edition      centre       date         dataType     gridType     stepRange    typeOfLevel  level        shortName    packingType
                   2            ecmf         20130301     an           regular_ll   0            hybrid       1            t            grid_jpeg
                   
                   grib_ls -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030103
                   prints the information about the temperature field at level 1: 
                   edition      centre       date         dataType     gridType     stepRange    typeOfLevel  level        shortName    packingType
                   2            ecmf         20130301     fc           regular_ll   3            hybrid       1            t            grid_jpeg
                   
                   You can see that this is GRIB edition 2, and data for midnight (00) is analysis data while 03 is forecast data 3 hours from midnight (stepRange).
                   The data is on a regular lat-long grid.




grib_dump           provides a more detailed view and listing of the file. 
                   
                   Useful options
                                     -O to show the content in WMO style (easier to read)
                                     -w        "where", prints only the content for specified key=value

                    Example:
                   grib_dump -O -w shortName=t,level=1 /flex_wrk/WIND_FIELDS/ECMWF/GLOBAL/OPER_fields/2013/03/EN13030103



grib_get

grib_get_data


Tools for counting and copying some messages 

grib_copy

grib_count


Tools for making changes to the content of a GRIB message 

grib_set

grib_filter

FORTRAN GRIB TOOLS

Grib API

Retrieved from "https://wiki.uio.no/mn/geo/geoit/index.php?title=How_to_read_GRIB_files!&oldid=1083"