Coordinate Type: Ensemble
|Reported by:||markh||Owned by:||cf-conventions@…|
Description (last modified by markh)
The description of the dimension of a data variable which describes an ensemble of forecasts may involve a number of elements of metadata.
Coordinate variables and auxiliary coordinates describing an ensemble need to be able to be labelled as such.
It is useful to encapsulate information about the nature of the ensemble with the coordinate. Current use cases for metadata standardisation are: the original size of the ensemble; the single-model or multi-model nature of the ensemble, the presence of an explicit control forecast as realization=0.
4. Technical Proposal
4. Coordinate Types
... The attribute axis may be attached to a coordinate variable or auxiliary coordinate variable and given one of the values X, Y, Z,
orT . These labels whichstand for a longitude, latitude, vertical, ortime or ensemble axis respectively. ...
4.6 Ensemble Coordinate
Variables representing an ensemble or collection of realizations shall have an attribute axis with a value E. These variables are discrete, as described in section 4.5; they do not represent continuous quantities.
Ensemble variables have a number of optional standardised attributes available for use.
A data variable representing an ensemble will commonly have an ensemble coordinate with the standard name realization; this is not mandatory. The realization standard name is used to provide a unique identifying number to each ensemble member within the ensemble.
An ensemble coordinate providing a string label for each ensemble member within the ensemble may also be included, using the standard name ensemble_member_label. An ensemble_member_label coordinate shall have unique values and missing values are not allowed.
An ensemble coordinate with the standard name realization or ensemble_member_label may include an attribute named ensemble_control_member.
This value provides a definition that one member of the ensemble is the control member and identifies this member. This control member shall have the identified value within the ensemble coordinate's data.
The absence of this attribute shall be interpreted as a negative statement, explicitly stating that there is no control member identified within the ensemble: all members have equal status, no control member existed in the ensemble.
An ensemble coordinate may be identified as being from one model or multiple models by providing further variables identified as coordinates or auxiliary coordinates by the data variable. All of these coordinates shall have an attribute axis with a value E.
The standard name's source and institution are used to identify the multiplicity of models which the ensemble is taken from, one other or both may be present.
Such coordinates may be scalar coordinate variables or they may be attached to the same dimension(s) as other ensemble coordinates referenced by a data variable. Scalar coordinates are commonly used to define a single model ensemble; in this case, this is informationally equivalent to auxiliary coordinates with identical values.
The absence of such source and institution coordinates shall not be interpreted as a positive or negative statement. No inference on the multiplicity of models which the ensemble is taken from shall be inferred from the absence of such coordinates.
| ensemble_control_member | D | C | ref:<<ensemble-coordinate>> | The identifier of the member of the ensemble which is the control member. |
4 Coordinate Types
- The axis attribute may
onlybe attached to a coordinate variable or auxiliary coordinate variable only.
- The only legal values of axis are X, Y, Z,
andT and E (case insensitive).
- The axis attribute must be consistent with the coordinate type deduced from units and positive.
The axis attribute is not allowed for auxiliary coordinate variables. A data variable must not have more than one coordinate variable with a particular value of the axis attribute.
4.6.2 Ensemble Control Member ====
- A variable may have an attribute ensemble_control_member only if it has an axis attribute of E and a standard_name of either realization or ensemble_member_label.
Information regarding the nature of an ensemble is encoded in an ensemble coordinate, analogous to temporal coordinates.
Encoding of the presence of a control member and the single or multiple model nature of the ensemble is standardised.
Future standardisation of ensemble characteristics has a model to follow.
6. Status Quo
At present there is no standardised way of capturing information or characteristics about an ensemble, all information is encoded ad hoc by data producers.
For example, the size of the ensemble can only be inferred from the length of a realization dimension. If the ensemble is sliced, to leave only one member, the size of the ensemble is lost in the resulting data variable. This example is particularly problematic for conversion from CF to other data formats.