#152: Time mean over area fractions which vary with time
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Reporter: martin.juckes | Owner: cf-conventions@…
Type: enhancement | Status: new
Priority: medium | Milestone:
Component: cf-conventions | Version:
Resolution: | Keywords:
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Comment (by martin.juckes):
Dear Karl, Jonathan,
Karl's last comment is correct: there is an inconsistency between the 3rd
example and the text. I think the best resolution is to make the example
consistent with the text be replacing "over sea" with "over
all_area_types". The example as it stands is a possible construction, but
I think the mean over the whole grid cell, as stated in the text, is the
more useful example to present.
For the 2nd comment: the statement that we cannot encode a the weighted
mean of a velocity, with the current convention, in such a way that the
time mean mass transport is correctly represented. The approach followed
by SIMIP to deal with this in CMIP6 is to request mass transport variables
(for which standard names exist, e.g. sea_ice_x_transport).
I agree that we should omit "horizontal area of the" from the sentence at
the start of 7.3.3: "... is assumed to have been evaluated over the entire
horizontal area of the cell." We may have, for instance, an array which
represents sea ice properties at a certain point in an emsemble of
simulations and as a fuction of time. In this case we would still be using
an area_type, but using it to identify portions of an ensemble rather than
portions of a horizontal area. The shorter text " .... is assumed to have
been evaluated over the entire cell" conveys all the necessary
information. I agree with Jonathan, however, that we do not need to move
away from the restriction to the existing area_type usage at present. A
more general ` ... where condition_xxx` is conceivable, but I'm not sure
we would want to fit it into the existing area_type framework.
Reviewing the ticket again I have, however, seen an additional problem
with the first example: `area: mean where sea_ice time: mean` .. the
problem is the `area: mean where sea_ice` is undefined where there is no
sea ice, so how do we take the time mean? There are 3 options: (1) fill
with zero where there is no sea ice, (2) report the time mean as undefined
or (3) take the time average over those cells where there is sea ice. The
last should be encoded `area: mean where sea_ice time: mean where
sea_ice`. As far as the convention is concerned, we should try to avoid
usage which is ambiguous. I think that (2) is the best interpretation of
the first example as far as the convention goes, but this probably means
it is not a very useful formulation from a scientific perspective and we
should perhaps advise against its use. If people want a time mean over the
whole time period they should use a formulation which gives a completely
unambiguous definition at each time. Thus, I would change the wording to
say that the first example is ambiguous for time varying area_type and
should, in general be avoided in this case.
regards,
Martin
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Ticket URL: <http://cf-trac.llnl.gov/trac/ticket/152#comment:13>
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