Dear John,
there is a distinct difference between cell_methods and the kind of
operators I am talking about (or I misunderstand the cell_methods description):
cell_methods describe operations that are done with respect to variable
dimensions (averaging over time and/or lon or lat, etc.). From the CF1.5
document:
"Each "name: method" pair indicates that for an axis identified by name, the
cell values representing the field have been determined or derived by the
specified method."
The "modifier" or "operator" describes operations that are done to the variable
values themselves without changing the dimensions. Let me clarify:
Take a global model output of temperature, for example. How do you describe
temperature differences between this model and another one? The resulting
quantity is still an "air_temperature" (OK, actually
"air_temperature_difference") with units of "K". Yet, it would be nice to know
that this field is a result of differencing two models. "Difference" could be
accomodated relatively easily with the standard_name modifier (but how do you
describe what has been differenced?). More complicated operations, such as
normalized mean bias (X-Y/(X+Y)) will at some point be impossible to maintain
through standard_name modifiers, I believe.
Reading through the CF1.5 description of cell_methods again, I see that
this is probably the way to go, but one would need to define a way of
expressing such methods that are not associated with a dimension. For example,
this could be done with "<standard_name>:difference", but this might be rather
clumsy (think of
"atmosphere_absorption_optical_thickness_due_to_particulate_organic_matter_ambient_aerosol:difference").
"self:difference" could be another option, with additional information in
paranthesis (e.g. "self:difference (ERA-interim, CRU)"). Writing only
"difference" is a third option - however this complicates the syntax again,
because you parse for colon in some cases but not always.
Cheers,
Martin
> -----Original Message-----
> From: Jonathan Gregory [mailto:j.m.greg...@reading.ac.uk]
> Sent: Thursday, February 24, 2011 7:08 PM
> To: Schultz, Martin
> Cc: cf-metadata@cgd.ucar.edu
> Subject: Re: [CF-metadata] standard_name modifiers
>
> Dear Martin
>
> The idea of the modifiers was to provide standard names for
> ancillary data, such as count of obs, standard error, and so
> on. The other kinds of thing you mention, such as means over
> periods and other statistics, can often be described by
> cell_methods, which is more flexible because it refers to the
> dimensions of the data.
>
> Best wishes
>
> Jonathan
>
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