#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:
Keywords: |
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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`.
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Ticket URL: <http://cf-trac.llnl.gov/trac/ticket/152>
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