[CF-metadata] standard_name modifiers

From: Schultz, Martin <m.schultz>
Date: Fri, 25 Feb 2011 10:40:07 +0100

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.gregory at reading.ac.uk]
> Sent: Thursday, February 24, 2011 7:08 PM
> To: Schultz, Martin
> Cc: cf-metadata at 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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Received on Fri Feb 25 2011 - 02:40:07 GMT