[CF-metadata] Composite area types in CMIP6 data

Jonathan Gregory j.m.gregory at reading.ac.uk
Fri Jan 6 09:59:34 MST 2017

Dear Martin

It's legal to have more than one axis of a given standard_name but it is
possibly inconvenient, because analysis software could not use the standard_
name of those two coordinate variables to distinguish them; instead, it would
have to use the long_name or the variable name, which is not a CF-like method.

Another possibility would be to introduce new standard_names, which are
more precise, such as area_type_of_land_use and area_type_of_vegetation in this
case, while also keeping area_type as the union. That would require a
rearrangement of the area_type table.

On the whole, I think in this case it would be best to introduce a new
combined area_type of pastures_of_c4_plant_functional_types i.e. flatten it.
I guess this will also be more convenient for analysts than having to search
the combination of two dimensions. You are right of course that doing this
could turn the area_type table into an N^2 or even an N^n problem, which would
be unmanageable, but if this is the *only* use-case, or even if we had a dozen
such use-cases, it is not a problem. Thus, I would say we cross this bridge
when we come to it, rather than anticipating a problem which hasn't arisen

Best wishes


On Fri, Jan 06, 2017 at 11:43:57AM +0000, martin.juckes at stfc.ac.uk wrote:
> Date: Fri, 6 Jan 2017 11:43:57 +0000
> From: martin.juckes at stfc.ac.uk
> To: j.m.gregory at reading.ac.uk, taylor13 at llnl.gov, alison.pamment at stfc.ac.uk
> CC: chris.d.jones at metoffice.gov.uk, cf-metadata at cgd.ucar.edu
> Subject: Composite area types in CMIP6 data
> Hello,
> In CMIP6 we have a request for some data on pasture land with C4 functional type. We already have CF area types for pasture and c4 plants in general. We could construct the required information as follows:
>     float pastureFracC4(time, lat, lon) ;
>         pastureFracC4:coordinates = "type ftpye" ;
>         pastureFracC4: standard_name = "area_fraction";
>         ......
>     char type(strlen) ;
>         type:long_name = "Pasture Land" ;
>         type:standard_name = "area_type" ;
>     char ftype(strlen) ;
>         ftype:long_name = "Plant Functional Type" ;
>         ftype:standard_name = "area_type" ;
> data:
>  type = "patsures" ;
>  ftype = "c4_plant_functional_types";
> This would provide the information that the variable pastureFracC4 refers to areas that are both "pastures" and "c4_plant_functional_types". An altenative would be to add a new area type, but I have the feeling that the area_type list will become unmanagable if we keep adding new terms for combinations of existing terms.  Can anyone see a problem with using two area_type coordinates?
> regards,
> Martin

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