Hello Nan,
one of the aims of using 'realization' was to make this term less tied
to models, though it looks as though this isn't working. So a review is
a good idea. I'm still not sure about tying this to the term 'ensemble'
though.
I think there are a set of problems around representing errors that are
closely related, and so treating them in a similar way in CF **may**
make CF meta-data easier to use for both human users (fewer concepts to
remember and possible concept transfer from one context to another) and
applications (through reuse of code in slightly different contexts).
The underlying class of problem is something like: some things have
errors in them, and these errors have complex distributions (not
represented by an analytic function, possibly with covariances between
variables), how do we represent these complex error distributions in CF?
So this could include model based forecasts, forecasts based on
statistical fits to empirical data, and also observations of quantities
(though I'm less clear about the subtleties of obs - and we didn't think
about these in the original proposal). If the error characteristics are
sufficiently complex that simple summary statistics (like standard
deviations) are not enough then a one way of representing these errors
is through a set of sample points, and their weights - from these you
can generate moments of the error distribution, or cumulative
distribution functions, or probability distribution functions.
The term 'realization' was introduced to try and capture the dimension
of the 'sample points'. In the context of a model based forecast each
ensemble member provides a 'sample point'. The original proposal we
considered using the term 'sample' rather than 'realization', but
thought this was too overloaded a term e.g. grab sample. We also
proposed the standard name 'realization_weight' to represent the other
part of the error characterisation, but I'm not sure it ever made it
through the process.
Though I guess things aren't quite as clear cut as I've tried to make
them - especially in the context of observations. In some cases the
sample dimension is actually just one of the other dimensions - like
time or space (or both). I don't have enough experience of observations
to say how widely used other ways of generate error characteristics are
(e.g. through computer simulation of an instrument), and if there are
how you would represent the errors and the terms you'd use to talk about
them.
Another possible complication is that not all uses of ensembles result
directly in a quantitative error estimate of some quantity - so loading
the 'ensemble' dimension towards this use may not be helpful.
So I think it comes down to something like:
1. what is the main use of ensembles?
2. is the main use of ensembles a special case of a broader problem? if
yes...
3. does CF gain by accommodating the broader problem? if yes...
4. what are the most useful terms for talking about the broader
problem.
5. if the initial answers to any of the above are wrong, does CF have
the mechanisms to recover - how well would users and applications cope
with 'aliased' dimensions?
Sorry, there are no concrete proposals in this mail, but I figured a bit
of context behind why we didn't use a standard name based on the term
'ensemble' might be useful.
Jamie
> -----Original Message-----
> From: cf-metadata-bounces at cgd.ucar.edu
> [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Nan Galbraith
> Sent: 06 April 2010 21:30
> To: John Caron
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] ensemble dimension
>
> Hi All -
>
> The term realization is, apparently, *perfectly* clear to you
> modelers, but it conveys no information at all to me.
>
> Since it looks like it's going to be adopted, I hope you'll
> provide a really clear definition in the standard - something
> that even an oceanographer will understand.
>
> Thanks!
>
> - Nan
>
>
> John Caron wrote:
> > Jonathan Gregory wrote:
> >> Dear all
> >>
> >> "realization" is fine as a standard name. I had forgotten we had
> >> introduced it.
> >> I withdraw my suggestion of ensemble_member_identifier.
> >>
> >> Thus, the standard name (of realization) can be used to
> identify an
> >> ensemble axis. I think that providing an axis attribute as
> well could
> >> be
> >> helpful: with
> >> spatiotemporal axes we have both methods of
> identification, and it is
> >> possible there might be ensemble axes in which realization
> was a not
> >> a good choice of standard name.
>
> --
> *******************************************************
> * Nan Galbraith (508) 289-2444 *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 *
> *******************************************************
>
>
>
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Received on Wed Apr 07 2010 - 05:14:06 BST
