#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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Old description:
> Following a discussion on the mailing list, I'd like to propose adding a
> new example to the CF Convention document to illustrate the use of
> cell_methods to specify different mean quantities when using a mask which
> is time varying (e.g. sea_ice). The qualifier `where` has been introduced
> into the `cell_methods` to specify masked spatial operations, e.g. `area:
> mean where sea_ice` to represent a spatial mean over sea ice. The current
> convention does not explicitly comment on whether the `where` construct
> can be used with other dimensions. For the CMIP6 data request there is a
> requirement to specify the temporal mean of quantities averaged over sea
> ice, and the spatial extent of the sea ice is generally varying in time.
>
> The proposal is to make it clear that use of `where` for non-spatial
> dimensions is allowed by adding examples in section 7. It is also
> necessary to provide these examples to clarify the subtle differences
> implied by different formulations of the `cell_methods` statement.
>
> The following should be added as a new example in section 7.3.3:
>
> Example 7.8: Time mean over area fractions which vary with time
>
> {{{
> float simple_mean(lat,lon):
> simple_mean:cell_methods: area: mean where sea_ice time: mean
>
> float weighted_mean(lat,lon):
> weighted_mean:cell_methods: area: time: mean where sea_ice
>
> float partial_mean(lat,lon):
> partial_mean:cell_methods: area: mean where sea_ice over sea time:mean
> }}}
>
> When the area fraction is varying with time, there are several different
> ways in which a time mean can be formulated. Three of these are
> illustrated in this example. Suppose, for instance, we are averaging over
> three time steps and the data at one grid point is -10, -6, -2 with area
> fractions .75, .50, .25. The values of the simple_mean, weighted_mean and
> partial mean are, respectively, (-10 -6 -2)/3 = -6, (-10*.75 - 6*.5
> -2*.25)/(.75+.5+.25) = -7.33 , and (-10*.75 - 6*.5 -2*.25)/3 = -3.667.
> The partial mean provides the contribution to the mean over the entire
> grid from a specified area type. The simple mean is weighting each time
> period equally, while the weighted mean provides equal weighting to each
> unit area of `sea_ice`.
New description:
Following a discussion on the mailing list, I'd like to propose adding a
new example to the CF Convention document to illustrate the use of
cell_methods to specify different mean quantities when using a mask which
is time varying (e.g. sea_ice). The qualifier `where` has been introduced
into the `cell_methods` to specify masked spatial operations, e.g. `area:
mean where sea_ice` to represent a spatial mean over sea ice. The current
convention does not explicitly comment on whether the `where` construct
can be used with other dimensions. For the CMIP6 data request there is a
requirement to specify the temporal mean of quantities averaged over sea
ice, and the spatial extent of the sea ice is generally varying in time.
The proposal is to make it clear that use of `where` for non-spatial
dimensions is allowed by adding examples in section 7. It is also
necessary to provide these examples to clarify the subtle differences
implied by different formulations of the `cell_methods` statement.
== New example for time-varying area fractions ==
''The following new example and explanatry text should be added in section
7.3.3:''
Example 7.8: Time mean over area fractions which vary with time
{{{
float simple_mean(lat,lon):
simple_mean:cell_methods: area: mean where sea_ice time: mean
float weighted_mean(lat,lon):
weighted_mean:cell_methods: area: time: mean where sea_ice
float partial_mean(lat,lon):
partial_mean:cell_methods: area: mean where sea_ice over sea time: mean
}}}
When the area fraction is varying with time, there are several different
ways in which a time mean can be formulated. Three of these are
illustrated in this example. Suppose, for instance, we are averaging over
three time steps and the data at one grid point is -10, -6, -2 with area
fractions .75, .50, .25. The values of the simple_mean, weighted_mean and
partial mean are, respectively, (-10 -6 -2)/3 = -6, (-10*.75 - 6*.5
-2*.25)/(.75+.5+.25) = -7.33 , and (-10*.75 - 6*.5 -2*.25)/3 = -3.667. The
partial mean provides the contribution to the mean over the entire grid
from a specified area type. The simple mean is weighting each time period
equally, while the weighted mean provides equal weighting to each unit
area of `sea_ice`.
In example 7.8, `time` could be replaced by any other coordinate over
which an average is taken, such as an ensemble index.
--
Comment (by martin.juckes):
Hello Jonathan,
The answer to the first question is yes, and I've modified the layout to
make it a bit clearer.
On the 2nd point, I'm not sure. What I'm proposing is clarifying the usage
with time-varying area fractions, or, in principle, area fractions which
vary with any other coordinate dimension (e.g. an ensemble index). At the
moment the construct is still dependent on the standard name
`area_fraction`, so it can only apply to horizonal areas.
I've added a sentence to the proposed insert to clarify that `time` can be
replaced by another dimension.
regards,
Martin
--
Ticket URL: <http://cf-trac.llnl.gov/trac/ticket/152#comment:2>
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