[CF-metadata] Standardizing how ensemble (realization) axes are encoded

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 19 Nov 2013 11:31:19 +0000

Dear Steve

My argument is this: the standard name is the name of a quantity contained
in the variable. Time and longitude are quantities. An ensemble is not a
quantity, however. In fact, an ensemble_member is not a quantity either,
perhaps, but it would be understood to mean ensemble_member "identifier" in
some undefined sense, so it's more like a usual standard name.

However I agree that, in cell_methods, for an omitted ensemble axis, it would
be more natural to read "ensemble: mean" than "ensemble_member: mean". We
could specifically allow "ensemble" as a keyword with this sense (i.e. to
indicate a statistic calculated over the members of an ensemble, when there
is no dimension or coordinate for the ensemble), like we
allow "area" as a special keyword in cell_methods to stand for the combination
of horizontal axes (whatever they are). To do that would require a change to
the convention, because it's not just a standard_name.

As I said, permitting axis='E' would also be a convention change.

If anyone would like to propose these additions in a trac ticket, I'd
support it.

Best wishes

Jonathan


On Fri, Nov 15, 2013 at 11:27:59AM -0800, Steve Hankin wrote:
> On second though it is all thoroughly ambiguous, whether CF names
> its axis standard_names for the collective or for the individual
> member. "ensemble" seems immediately clear and short, compared to
> "ensemble_member", which seems a bit labored. But I understand your
> concern over consistency with "realization". Either choice is fine
> with me.
>
> - Steve
>
> ============================================
>
> On 11/15/2013 11:15 AM, Steve Hankin wrote:
> >
> >On 11/15/2013 10:30 AM, Jonathan Gregory wrote:
> >>Dear Steve et al.
> >>
> >>>I support the idea that the term "ensemble" be allowed (by whatever
> >>>machinery) as an alias for "realization".
> >>It'd be fine to have an standard_name alias, I agree, but I
> >>think it should be
> >>"ensemble_member", not "ensemble". The ensemble is the
> >>collection of members
> >>(aka realizations).
> >
> >It feels a bit backwards, doesn't it? By this logic shouldn't the
> >standard_name "time" be "snapshot" instead -- named for its
> >individual members, rather than for its collective?
> >
> >"time" "depth", "longitude" and "latitude" axes all represent the
> >collection of many individual points. By analogy an "ensemble"
> >axis would be the collective of many individual members. It's
> >true that, the choice of "realization" as a standard_name took the
> >opposite outlook. hmmm ... Are we better off to maintain
> >consistency with CF's well known geo-spatial axis standard_names?
> >or consistency with "realization"?
> >
> > - Steve
> >>
> >>>axis="E" also seems like an appropriate step to maintain consistency
> >>>with other well known axis types, given the high likelihood that
> >>>ensemble axes will become commonplace in the future.
> >>That would require a change to the convention to be proposed on
> >>a trac ticket
> >>by someone.
> >>
> >>Best wishes
> >>
> >>Jonathan
Received on Tue Nov 19 2013 - 04:31:19 GMT