Hello Jonathan
This topic is not related to CF#108 (which I need to get back to and consider in detail).
This is an explicit piece of metadata which is provided by some systems and models and is applied to the ensemble collection, rather than statistics derived from that collection.
In most cases there will be no CellMethod but there will be a realization dimension and coordinate.
This piece of metadata could be considered to be:
'The expected length of the realization coordinate and dimension, if the whole ensemble collection is present.'
This is preserved during subset operations, so that I may be looking one ensemble member, with a realization coordinate value of 4, but I know that this is from an ensemble of 17 members.
This is useful metadata which is available from some models and we would like to preserve it in a standardised way in our CF NetCDF data sets.
I hope this clarifies the question
all the best
mark
________________________________________
From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] on behalf of Jonathan Gregory [j.m.gregory at reading.ac.uk]
Sent: 06 October 2014 18:15
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] realization | x of n
Dear Mark
Is this related to ticket 108? In that ticket, you and I have very similar
proposals, while Karl has made a more general proposal. In our simple
proposal, the number of realizations would the size of the collapsed axis,
and you could have a cell_methods entry for the collapsed axis such as
realization: mean (dimension: members)
where "realization" is allowed because it's a standard_name, and "members"
is the name of a dimension which contains the original size of the ensemble
before collapse. This would work in my version of the proposal because it
does not require any coordinate vars to exist with this dimension (though they
could if you want to supply them too).
Best wishes
Jonathan
----- Forwarded message from "Hedley, Mark" <mark.hedley at metoffice.gov.uk> -----
> From: "Hedley, Mark" <mark.hedley at metoffice.gov.uk>
> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> Date: Fri, 3 Oct 2014 10:52:00 +0000
> Subject: [CF-metadata] realization | x of n
>
> Hello CF
>
> The standard name
> realization
> is available for use:
> Realization is used to label a dimension that can be thought of asa statistical sample, e.g., labelling members of a model ensemble.
>
> It is common practice in forecasting and some forecasting formats to explicitly state the number of members an ensemble contained.
>
> This would be generally be expected to be a single value across an ensemble collection. As such I am not sure that a standard name and a coordinate would be the most useful way to encode this piece of information.
>
> Thus, we could look to provide a new data variable attribute, something like
> number_of_realizations
> to store this metadata.
>
> However it is not impossible for this to vary across some coherent data set so perhaps a standard_name of
> number_of_realizations
> would be a sensible approach, even if in most cases this would be a scalar coordinate
>
> Please may I ask for some opinions on this matter?
>
> thank you
> mark
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----- End forwarded message -----
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Received on Tue Oct 07 2014 - 07:56:57 BST
