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NCAR Research Data Archive Blog

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What is GRIB?

Posted: 06 Jul 2015 11:37 AM PDT
http://feedproxy.google.com/~r/NcarResearchDataArchiveBlog/~3/44wxV3wJ0A8/what-is-grib.html?utm_source=feedburner&utm_medium=email

"What is GRIB?" is a nontrivial question. There isn't even agreement on  
whether it stands for GRIdded Binary or General Regularly-distributed  
Information in Binary form.   This is an idiosyncratic and non-official  
take on GRIB by an autodidact of met data.

GRIB and BUFR are World Meteorological Organization (WMO) standards for  
data exchange.  WMO calls them data exchange formats, but many refer to  
them simply as data formats.  The loss of that one middle word is  
significant.

Met data is continually being exchanged around the world between nations  
and the emphasis is on timely data that can be consumed or ingested by data  
processing systems at the national weather centers.  Transmission  
efficiency and robustness (to network outages) takes priority over  
extensive metadata.

Binary data is more compact that human-readable ASCII, but is often not  
portable between machines.  GRIB is designed to be a portable and  
endian-independent binary format.

A full-fledged data format should ideally contain all the information  
required to understand the data in the file in a self-contained manner.   
However, GRIB files need an independent Parameter Table to map GRIB 'codes'  
correctly to the physical parameters they represent.  Apply the wrong  
parameter table and you could produce garbage instead of insightful  
analysis.

[I personally agree with (though it is not official RDA policy) John  
Caron's plea that GRIB and BUFR are not suitable data formats for long-term  
archival purposes on their own.  However, we archive data as close to  
their 'raw' form as possible, but try to provide information and metadata  
to help future users read and interpret the data correctly.]

World-wide, more weather data is exchanged in GRIB than any other format.   
Whether you like GRIB or not; if you want to work with gridded weather  
data, you need to learn to work with GRIB.  You may even need to learn how  
to work with more than one flavor of GRIB.  (Yes, there is more than one  
flavor of GRIB.)

GRIB works very well as a data exchange or data transmission format for use  
in operational weather centers, who are not concerned with long-term data  
preservation.  Data archivists and curators need to work a bit harder to  
ensure that the correct metadata (e.g. GRIB parameter tables) are kept with  
the corresponding data files.  We work with the national centers to keep  
abreast of the changes and document them for RDA users.

For instance, go to ds084.1 NCEP GFS 0.25 Degree Global Forecast Grids  
Historical Archive and read its GRIB2 parameter table in HTML.  Try that  
with any of our GRIB data holdings.

Note, these codes exist in the parameter table files, but not in the GRIB  
files themselves.  You have to apply the correct table so that the  
appropriate codes are applied.  Fortunately, many modern GRIB readers and  
viewers are smart enough to infer the correct table to apply based upon the  
header information.

The gridded fields of values within GRIB files are sometimes referred to as  
a 'message', a nod to its heritage as a transmission format.  Each message  
is 'atomic' in the sense that it is the smallest unit of data that makes  
any sense.  They contain a starting point, an end point (in bytes), GRIB  
codes describing the message contents, and the data values themselves.

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