GDT netCDF conventions for climate data

This standard defines a set of conventions adopted in order to promote the interchange and sharing of files created with the netCDF Application Programmer  Interface (API). The standard is based upon version 2.4 of netCDF. Documentation of the netCDF API may be found in the "NetCDF Users’ Guide", Version 2.4, February 1996, available from https://www.unidata.ucar.edu/software/netcdf/. The standard is named “GDT” from the names of the authors.

This standard is intended for use with climate data, and was designed with data generated by GCMs particularly in mind. We recognise that there are limits to what a standard can practically cover; we restrict ourselves to issues which we believe to be of common and frequent concern in the design of climate metadata.  Although this is specifically a netCDF standard, we feel that most of the ideas are of wider application. Our main purpose is to propose a clear, adequate and flexible definition of the metadata needed for climate data. The metadata objects could be contained in file formats other than netCDF. Interconversion of the  metadata between files of different formats will be facilitated if they are based on similar ideas.

This standard is mostly additional to the conventions sponsored by COARDS (https://www.unidata.ucar.edu/software/netcdf/conventions.html). In addition, all Unidata recommendations are supported here unless noted to the contrary. Comments indicate the places where there are differences between the standards. Comments given in emphasised type and CDL examples given in slanted typewriter type are not part of the standard. Note that examples typically show only the  details relevant to the point under discussion, and hence may be incomplete with respect to the provisions of the complete standard.

Successful transmission of data depends upon the receiver of the file having software that will correctly interpret it. For this reason, a strategy that is as  conservative as possible in the use of attributes and encoding techniques will best promote portability of data.

This standard also refers to the udunits standard supported by Unidata. The udunits package is available via https://www.unidata.ucar.edu/software/udunits/. See section 11 for details of how the package is used by this convention to define units for physical quantities.

Useful comments and suggestions from Karl Taylor, John Sheldon, Jan Polcher, Bryant McAvaney, Harvey Davies, John Caron, Steve Hankin and contributors to the netCDF newsgroup have influenced the development of this standard. We have made some changes to gain greater compatibility with the NCAR CSM netCDF standard.