#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 apamment):
Hi Martin, Jonathan and Karl,
On the mailing list I expressed support for Martin's suggestion to extend
the use of 'where' in
cell_methods to accommodate time means and I support the changes proposed
in this ticket. I agree that we
should confine ourselves to addressing Martin's current use cases and
leave further extensions until they
are needed.
The inclusion of the additional examples and detailed explanation is very
useful - without them
it would be difficult to understand the proposed changes. I can see that a
lot of work has already gone
into making the text as clear as possible. I am struck by the fact that
this section of the conventions is getting steadily longer and more
complicated so to help people navigate
through the text it would be useful to signpost even more clearly which
parts describe the use
cases for constant grid cell portions and time varying portions,
respectively.
I suggest re-ordering and extending the first proposed paragraph so that
it serves as an introduction and
overview for the whole of section 7.3.3:[[BR]]
'By default, the statistical method indicated by cell_methods is assumed
to have been evaluated over the
entire horizontal area of the cell. Sometimes, however, it is useful to
limit consideration to only a
portion of a cell (e.g. a mean over the sea-ice area). Grid cell
“portions” that can be considered are
only those permitted to be associated with the standard_name of area_type.
There are two options for
indicating when a quantity represents a portion of a cell and these are
explained in the following two
paragraphs. The portion of the grid cell to which the statistical method
is applied may remain constant
in time, for example, the portion occupied by land or sea; examples 7.6
and 7.7 illustrate this use
case. The remainder of the section following example 7.7 describes
instances when the grid cell portion
varies in time, for example, when the area occupied by sea-ice changes
during the sampling period.'
The first sentence of the second paragraph would then need to be reworded
slightly:
'The first method for indicating that a statistic applies to only a
portion of a cell can be used for
the common case that the cell_method applies to a single area-type.'
The penultimate sentence in the paragraph following the proposed example
7.8 currently reads:
'The partial mean provides the contribution to the mean over the entire
grid from a specified
area type.' Shouldn't this say 'entire grid cell' rather than 'entire
grid'?
Best wishes,
Alison
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Ticket URL: <http://cf-trac.llnl.gov/trac/ticket/152#comment:20>
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