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Exploring climate model data

In which format are climate data usually saved?

All the input and output fields of climate models and preprocessor programs (e.g. the ones providing interpolated initial and boundary conditions) are stored in GRIB or NetCDF format.

GRIB format

GRIB is a data format, commonly used in meteorology, to store historical and forecast weather data. GRIB means "GRIdded Binary" and is designed for the international exchange of processed data, in the form of grid-point values expressed in binary form. The GRIB-code is part of the FM-system of binary codes of the World Meteorological Organization (WMO). A GRIB file is made up of a series of records. Each elementary unit (record) contains the following sections:

  • 0 Indicator sec.: Length of record
  • 1 Product definition sec.: Identifier of the forecast field
  • 2 Grid description sec.: Information about the grid
  • 3 Bit-map sec.: (optional) Undefined grid values
  • 4 Binary data sec.: Data values
  • 5 End sec.: Proper Identifier

Currently, three versions of GRIB are available. Version 0 was used to a limited extent by projects and is no longer in operational use. The first edition is used operationally worldwide by most meteorological centres, for Numerical Weather Prediction output (NWP). A newer generation has been introduced, known as GRIB second edition, and data is slowly changing over to this format. Some of the second-generation GRIB are used for derived product distributed in Eumetcast of Meteosat Second Generation. For further information see the WMO Guide about GRIB format.

NetCDF format

NetCDF means Network Common Data Form. It is a set of software libraries and self-describing, machine-independent data formats that support the creation, access, and sharing of array-oriented scientific data. University Corporation for Atmospheric Research (UCAR) maintains the NetCDF dataformat.

The data format is "self-describing". This means that there is a header which describes the layout of the rest of the file, in particular the data arrays, as well as arbitrary file metadata in the form of name/value attributes. The data arrays are rectangular, and stored in a simple and regular fashion that allows efficient subsetting. NetCDF is scalable, as a small subset of a large dataset may be accessed efficiently. Moreover, data may be appended to a properly structured file without copying the dataset or redefining its structure.

NetCDF is commonly used in climatology, meteorology and oceanography applications. A wide range of application software has been written, which makes use of NetCDF files. For further information see the Unidata NetCDF guide.

To promote easy processing and sharing of NetCDF data the CF conventions have been adopted by a number of projects and groups as a primary standard. The conventions define metadata that provide a definitive description of what the data in each variable of the NetCDF file represents, and the spatial and temporal properties of the data.

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The IS-ENES project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration.

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