Two quick comments:
1. "We could perhaps ... introduce a new standard name such as
ensemble_member_id (a string) or ..."
* Have you hit on a void in CF that needs to be filled? Unless
I've overlooked it CF has not yet standardized the manner in
which ensemble axes are to be identified -- a standard_name for
the ensemble axis, units, etc.. (The string "ensemble" does not
appear in the CF 1.6 document.)
2. I'd like to cast a strong vote *against* the idea of overloading the
cell_methods attribute (trac #108) for the purpose of describing
collapsed axes. What may appear as an appealing short cut now,
seems to me destined come back to bite us later. Cell_methods, as
the name suggests, is the place to document the transformations that
have occurred within grid cells -- beneath the resolution of the grid.
The concept that is under discussion here is how to document an
algorithm that was applied across the entire grid domain when
collapsing an axis. The concept is the same that is needed when
documenting an axis collapsed by, say, a long term time average or
a vertical integration. Isn't this the type of content that
normally gets (partially) documented in the standard_name modifier
-- "integral of ..." "change over time of ..."? Should a starting
point for this discussion be a addition of "ensemble average of ..."
to the list of standard_name modifiers? (The cons of packing
transformation descriptions into standard_name deserve debate as
well as the pros, though for reasons that lie outside of the current
discussion.)
What's needed is a consistent mechanism for documenting the details
of how an axis was collapsed. We should consider how (say) the time
limits of an axis that has been collapsed by performing a long term
time average should get documented. The answer to this question
should guide the manner of documenting the collapse of an ensemble
axis. It is unlikely that cell_methods would be part of this answer.
- Steve
========================================
On 11/9/2013 1:07 PM, Jonathan Gregory wrote:
> Dear Mark
>
> Thanks for the clarifications in your last email. If I have understood this
> correctly now, there are two needs, which might be distinct.
>
> (1) Indicate that a collapsed (size-one) axis is an ensemble axis, although
> it doesn't have a coord var or any aux coord vars. I can see that it might
> well not have these vars because there might not be anything useful you could
> put in them. In the absence of these vars, the dimension name in the cell
> methods isn't informative. We could perhaps solve this by introducing a new
> standard name such as ensemble_member_id (a string) or ensemble_member_number
> (a number) or perhaps both. These could anyway be useful. These standard names
> could appear in your cell_methods, following section 7.3.4, instead of the name
> of the size-one dimension, indicating that the statistic applied to all the
> available members of an ensemble, without needing any coord information. I
> don't think this change would require an alteration to the convention.
>
> (2) Point to the coordinate information which applied to the axis before the
> collapse. This could be useful for any sort of collapsed axis, and in fact I
> think we have discussed it before at some point in the last 15 years! I agree
> with your suggestion that it would be logical to record this in cell_methods
> as a standardised comment. An alternative would be to add an attribute to the
> collapsed coord var, but you don't have those (as you say), and also the
> collapse may apply to a combination of axes. You suggest listing all the coord
> or aux coord vars in the cell_methods comment. Would it not be sufficient, and
> more economical, to give the name of the uncollapsed dimension(s)?
>
> Best wishes
>
> Jonathan
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