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152 Time mean over area fractions which vary with time martin.juckes cf-conventions@… "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.
" enhancement new medium cf-conventions