A fundamental requirement for exchange of scientific data is the
ability to describe precisely the physical quantities being represented.
To some extent this is the role of the long_name
attribute as defined in the NUG. However, usage of
long_name is completely ad-hoc. For some applications
it would be desirable to have a more definitive description of the
quantity, which would allow users of data from different sources to
determine whether quantities were in fact comparable. For this reason an
optional mechanism for uniquely associating each variable with a standard
name is provided.
A standard name is associated with a variable via the attribute
standard_name which takes a string value comprised of a
standard name optionally followed by one or more blanks and a standard
name modifier (a string value from Appendix C, Standard Name Modifiers).
The set of permissible standard names is contained in the standard name table. The table entry for each standard name contains the following:
The name used to identify the physical quantity. A standard name contains no whitespace and is case sensitive.
Representative units of the physical quantity. Unless it is
dimensionless, a variable with a standard_name
attribute must have units which are physically equivalent (not
necessarily identical) to the canonical units, possibly modified by
an operation specified by either the standard name modifier (see
below and Appendix C, Standard Name Modifiers) or by the
cell_methods attribute (see Section 7.3, “Cell Methods” and Appendix E, Cell Methods), or both
(with the standard_name modifier operating
first).
The description is meant to clarify the qualifiers of the fundamental quantities such as which surface a quantity is defined on or what the flux sign conventions are. We don"t attempt to provide precise definitions of fundumental physical quantities (e.g., temperature) which may be found in the literature.
When appropriate, the table entry also contains the corresponding GRIB parameter code(s) (from ECMWF and NCEP) and AMIP identifiers.
The standard name table is located at http://cf-pcmdi.llnl.gov/documents/cf-standard-names/standard-name-table/current/cf-standard-name-table.xml
, written in compliance with the XML format, as described in Appendix B, Standard Name Table Format. Knowledge of the XML format is
only necessary for application writers who plan to directly access the
table. A formatted text version of the table is provided at http://cf-pcmdi.llnl.gov/documents/cf-standard-names/standard-name-table/current/standard-name-table
, and this table may be consulted in order to find the standard name that
should be assigned to a variable. Some standard names (e.g.
region and area_type) are used to
indicate quantities which are permitted to take only certain standard
values. This is indicated in the definition of the quantity in the
standard name table, accompanied by a list or a link to a list of the
permitted values.
Standard names by themselves are not always sufficient to describe a
quantity. For example, a variable may contain data to which spatial or
temporal operations have been applied. Or the data may represent an
uncertainty in the measurement of a quantity. These quantity attributes
are expressed as modifiers of the standard name. Modifications due to
common statistical operations are expressed via the
cell_methods attribute (see Section 7.3, “Cell Methods” and Appendix E, Cell Methods).
Other types of quantity modifiers are expressed using the optional
modifier part of the standard_name attribute. The
permissible values of these modifiers are given in Appendix C, Standard Name Modifiers.
Example 3.1. Use of standard_name
float psl(lat,lon) ;
psl:long_name = "mean sea level pressure" ;
psl:units = "hPa" ;
psl:standard_name = "air_pressure_at_sea_level" ;
The description in the standard name table entry for
air_pressure_at_sea_level clarifies that "sea level"
refers to the mean sea level, which is close to the geoid in sea
areas.
Here are lists of equivalences between the CF standard names and the standard names from the ECMWF GRIB tables, the NCEP GRIB tables, and the PCMDI tables.