Hi Charlie (Carlos),
I'm pretty sure the cell_methods should only be an attribute of a
non-coordinate variable (*not* an attribute of "time").
Your 1-step and 2-step methods will yield the same answer (if you keep
track of how long each season is when you do the averaging, and you
weight the samples appropriately). CF doesn't try to record in the
cell_methods attribute the details of your algorithm. So I think you
have two options for specifying the cell_methods (for time):
If you have a "bounds" attribute for time, then "time: mean".
If you have a "climatology" attribute for time, then "time: mean within
years time: mean over years"
I'd go with the first option I think.
You can place additional information in parentheses as allowed for in
section 7.3.2 of the convention.
Best regards,
Karl
On 6/25/15 1:48 PM, Charlie Zender wrote:
> Dear CFers,
>
> What should the cell_methods of a non-coordinate
> variable (e.g., temperature) in a climatology?
>
> We have climate model output archived as monthly means
> in each gridcell, so the cell_methods attribute of these
> monthly data begin as "time: mean" (i.e., monthly mean).
> We then create a climatology by a sequence of one or two
> more temporal-averaging steps. The one-step method puts
> all the months in the hopper and averages those.
> The two-step method first averages the months into four
> climatological seasons, and then averages those four
> seasons into the climatological annual mean.
>
> My understanding is that the resulting time coordinate
> cell_methods should be
>
> "time: mean within years time: mean over years"
>
> and that the time coordinate should supplant the time
> bounds attribute/variable with a climatology bounds
> attribute variable with appropriate endpoints.
>
> My question is what should be the cell_methods of the
> non-coordinate variables? I can think of five plausible
> answers:
>
> 1. "time: mean" (it's just a time-mean, after all)
> 2. "time: mean time: mean" (one-step climatology)
> 3. "time: mean time: mean time: mean" (two-step climo)
> 4. "time: mean within years time: mean over years" (same as time)
> 5. "time: mean time: mean within years time: mean over years"
> 6. ???
>
> I favor 1 or 4 because 2, 3, and 5 seem redundant (though
> they do convey information about the arithmetic method).
> What is the CF-recommendation on this?
>
> Muchas gracias,
> Carlos
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