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

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m (COMMAND-LINE GRIB TOOLS)
m (COMMAND-LINE GRIB TOOLS)
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                                       -l        Find the lat/long value in the model close to a given lat/long
 
                                       -l        Find the lat/long value in the model close to a given lat/long
 
                     Example:
 
                     Example:
                     grib_ls -w shortName=t,level=1 EN13030100
+
                     '''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:  
 
                     prints the information about the temperature field at level 1:  
 
                     edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType
 
                     edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType
 
                     2            ecmf        20130301    an          regular_ll  0            hybrid      1            t            grid_jpeg
 
                     2            ecmf        20130301    an          regular_ll  0            hybrid      1            t            grid_jpeg
 
                      
 
                      
                     grib_ls -w shortName=t,level=1 EN13030103
+
                     '''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:  
 
                     prints the information about the temperature field at level 1:  
 
                     edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType
 
                     edition      centre      date        dataType    gridType    stepRange    typeOfLevel  level        shortName    packingType

Revision as of 13:51, 13 March 2013

Back to ECMWF overview[1]


Updated by nik: 13 March 2013


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
                                      -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 midnight is analysis data while 03 is forecast data.



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_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