Dear all,
at the CF day in Paris there was a discussion on a Controlled Vocabulary (CV) system to specify variables in the CF header semantically. Please, find below a first draft of a proposal for this.
On the behalf of the World Data Center CLimate (WDCC)
Heinke Hoeck and Frank Toussaint
Proposal for a CF Controlled Vocabulary as additional CF Header Entry
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(WDCC, Hamburg, July 10th, 2007)
In earth sciences, the NetCDF-CF standard gets more and more used. Not only in the field of climate and forecast but also in adjacent scientific branches of Earth system research.
However, information that provides a definitive description of what the data in each variable represents is often spread over various attributes in the NetCDF-CF file: one standard name can well stand for different parameters if it is accompanied by modifiers like "variance" or by a CF scalar coordinate. Other CF standard_names, of course, are "standalone" in the sense that they represent a completely defined parameter.
On the other hand, climatologists and other scientists have named their quantities by identifiers that are convenient to use, e.g., in climate models but are often not intrinsic systematic.
To allow for common semantic designators that define file contents in the granularity needed, we propose a CF "Controlled Vocabulary" (CV). Each of these CV keywords should reflect a well defined set of attribute/value pairs of the NetCDF-CF standard including a CF standard_name. The CV entry should be included in the variable's section of the NetCDF-CF header in addition to the existing CF standard attributes.
To keep consistency between the CV keywords of a standard Controlled Vocabulary and the associated attribute/value set a mapping between both should be maintained on a central server by e.g. the CF standard names committee. A reference to the CV mapping should be included in the file header information.
By this approach the consistency of the existing CF attributes is not affected, nor is the CF conformance of the file or the concept of CF standard_names. Nevertheless, the CV will enable the CF community to join frequently used attribute/value sets into one semantic entity, making handling of complex variables easier and more community adapted. E.g., climatological standard designators like ice day or tropical night, which are difficult to integrate into the existent CF standard_name scheme can be addressed by their defining set of CF attributes wherever CV keywords are used (e.g., portals, catalogues). The consistency between CV keyword and CF standard_names plus attributes should be ensured by the CF checker.
Additionally, we want below to add a few concrete examples of candidates for a CV controlled vocabulary. These can more or less easily be expressed by existing CF standard names plus attributes. With some others we still are in discussions.
Examples of CV candidates (xml notation)
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<cfControlledVocabulary cvName="CERAtopics">
<!-- example 1: topic is alias for standard_name: radiation ========== -->
<item controlledName="toa incoming shortwave flux">
<variable typeName="float tisf">
<standardName>toa_incoming_shortwave_flux</standardName>
<units>W m-2</units>
</variable>
</item>
<!-- example 2: cell methods: min ==================================== -->
<item controlledName="minimum surface temperature">
<variable typeName="float tmin">
<longName>air temperature minima during coming 6 hours</longName>
<standardName>air_temperature</standardName>
<units>K</units>
<cellMethods>time: minimum (interval: 6 hours)</cellMethods>
</variable>
</item>
<!-- example 3: scalar coordinate: 2m level ========================== -->
<item controlledName="air_temperature-at2m">
<variable typeName="float t2m">
<longName>air temperature at 2 metres height</longName>
<standardName>air_temperature</standardName>
<coordinates>lev2m</coordinates>
</variable>
<variable typeName="float lev2m">
<longName>level of 2 metres height</longName>
<units>meter</units>
</variable>
<data variable="lev2m">2.</data>
</item>
<!-- example 4: cell methods + scalar coord: daily temp max at 2m ==== -->
<item controlledName="maximum 2-meter temperature">
<variable typeName="float tmax_2m">
<longName>daily air 2m temperature maxima</longName>
<standardName>air_temperature</standardName>
<units>K</units>
<cellMethods>time: maximum (interval: 1 day)</cellMethods>
<coordinates>lev2m</coordinates>
</variable>
<variable typeName="float lev2m">
<longName>level of 2 metres height</longName>
<units>meter</units>
</variable>
<data variable="lev2m">2.</data>
</item>
</cfControlledVocabulary>
--
/** Dr. Frank Toussaint Max-Planck-Institut f?r Meteorologie
* Pfitznerstr. 69 M&D / World Data Center - Climate
* 22761 Hamburg Bundesstr.53 - 20146 Hamburg
* priv.Tel.: 040-3861 9285 office phone: +49-40-41173-175
* www.Leuchtturm-Atlas.de e-mail: Frank.Toussaint at zmaw.de */
Received on Thu Jul 12 2007 - 04:59:36 BST
