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: [email protected] > [mailto:[email protected]] On Behalf Of Nan Galbraith > Sent: 06 April 2010 21:30 > To: John Caron > Cc: [email protected] > 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 * > ******************************************************* > > > > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
