GRIB Data Format used for the COSMO-Model System

Last updated: February 2013

GRIB Software: DWD GRIB1 Library

To code / decode the meteorological data to GRIB, special software is needed. While a DWD-written software, the GRIB1-library is used to work with GRIB 1 data, the new application programmers interface grib_api from ECMWF is used for dealing with GRIB 2. But grib_api can also deal with GRIB 1 data. The approach, how data is coded to / decoded from GRIB messages is rather different in these software packages. While the GRIB1-library provides interfaces to code / decode the full GRIB message in one step, the grib_api uses the so-called key/value approach, where the single meta data could be set. Both approaches will be discussed shortly:

DWD GRIB library libgrib1.a

In the DWD GRIB1 library, full GRIB messages are coded / decoded in one step. The main interface routines are:

• copen, cclose
  to open and close GRIB files.
• cuegin, cuegex
  to read and write GRIB messages from / into files.
• grbex1
  to code GRIB meta data and the values of a variable to GRIB messages.

  The meta data for the different sections have to be written to INTEGER arrays of a certain length:

  • ipds(321): values for the product definition section
  • igds(626): values for the grid definition section
  • ibms(  3): values for the bit-map section
  • ibds( 11): values for the binary data section
  And the values of the variable to be coded are given in a REAL array
  • field(:): values of the variable
  The routine grbex1 takes all these arrays as input and returns the coded GRIB message in an INTEGER array iblock, where the octets of the message are written into the single bytes.
• grbin1
  to decode GRIB meta data and the values of a variable from GRIB messages.

  Input to the routine grbin1 is an INTEGER array iblock, which contains the octets of the message in its bytes. grbin1 then returns the meta data for the different sections in INTEGER arrays (see above) and the values of the variable in a REAL array.

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