Opened 6 years ago

Last modified 5 years ago

#142 new enhancement

Coordinate Type: Ensemble — at Initial Version

Reported by: markh Owned by: cf-conventions@…
Priority: medium Milestone:
Component: cf-conventions Version:
Keywords: Cc:

Description

1. Title

Ensemble Coordinates

2. Moderator

TBC

3. Requirement

The description of the dimension of a data variable which describes an ensemble of forecasts may involve a number of elements of metadata.

Such a coordinate is characterised by the standard_name realization.

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

4.6 Ensemble Coordinate

Variables representing an ensemble or collection of realizations must have a standard_name of realization and a units of '1'. These variables are discrete, as described in section 4.5. They do not represent continuous quantities.

Optionally the ensemble coordinate may have an attribute axis with a value E.

Ensemble variables have a number of optional standardised attributes available for use. Further bespoke attributes to describe the ensemble in project specific ways are allowed.

4.6.1 Ensemble Size

An ensemble coordinate may include an attribute named ensemble_size representing the original size of the ensemble.

This attribute shall have a value which is a positive integer.

This value provides a context for the realization, which is preserved though sub-setting, slicing and statistical processing. it is expected to remain unchanged through such operations on the data variable, which will alter the values of the realization coordinate.

4.6.2 Single Model Ensemble

An ensemble coordinate may include an attribute named single_model_ensemble representing the assertion that the ensemble of members all originate from the same numerical model.

This is a boolean field and may only contain the values true or false.

The absence of this attribute shall not be interpreted as a positive or negative statement. No inference on the models providing ensemble members shall be inferred from the absence of this attribute.

4.6.3 Ensemble Control Member

An ensemble coordinate may include an attribute named ensemble_control_member_0 representing the interpretation of the realization value of 0. This is a boolean field and may only contain the values true or false.

This value provides a definition that one member of the ensemble is the control member. This control member shall have the identifying value 0 within the realization dimension.

The absence of this attribute shall not be interpreted as a positive or negative statement. No inference on the interpretation of realization zero within the ensemble shall be inferred from the absence of this attribute.

5. Benefits

Information regarding the nature of an ensemble is encoded in an ensemble coordinate, analogous to temporal coordinates.

Encoding of the ensemble size, 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.

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