Hi Charlie,
I agree with Karl - if the end result is basically a mean over the whole period (even though it might have been computed in two steps) then it seems simpler and clearer to just specify "time: mean".
Obviously in other circumstances it is essential to record the fact that it's a two-stage calculation e.g. we generate monthly values of mean daily maximum temperature for which the cell method would be:
"time: maximum within days time: mean over days"
One slight downside (in my opinion) is that closely related variables are then treated slightly differently e.g. for monthly values:
mean daily maximum temperature - "climatology" attribute with cell method of "time: maximum within days time: mean over days"
mean daily minimum temperature - "climatology" attribute with cell method of "time: minimum within days time: mean over days"
mean temperature (computed by averaging the daily max and min values) - "bounds" attribute with cell method of "time: mean"
Can anyone out there give a reason for there being two separate attributes, "climatology" and "bounds". Our monthly stats are for individual calendar months, not 30-year averages, yet the convention seems to require us to specify a "climatology" attribute due to the two-stage calculation method. It seems to me that the nature of the variable is clear from the cell method. Is the use of the "climatology" attribute simply to alert the user, or does it affect the way the cell methods are interpreted (in a way that I haven't grasped)? Put another way, is it non-compliant to combine a "bounds" attribute with a cell method of "time: maximum within days time: mean over days"?
Regards,
Dan
________________________________
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Karl Taylor
Sent: 25 June 2015 23:14
To: Charlie Zender; CF Metadata Mail List
Subject: Re: [CF-metadata] cell_methods for climatology?
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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