[CF-metadata] cell metrics

[Christiane Textor]

Dear Jonathan et al.,

Can I conclude from your discussion that standard names describing the 
grid are to be accepted as "data variables in their own right"? This was 
demanded by the HTAP community?

grid_cell_area
grid_cell_height

(For the latter I prefer height to thickness, because it indicates a 
vertical direction.

Best regards,
Christiane




Jonathan Gregory a écrit :
> Dear Simon, Christiane, et al.
> 
> 
>>>>>* grid_cell_area and _height. These are metrics for the grid, rather than
>>>>>quantities which need standard names. Grid cell area should be specified 
>>>>>by a
>>>>>cell_measures variable of area (CF 7.2). Grid cell height can be deduced 
>>>>>as the
>>>>>difference between the lower and upper boundary in the vertical 
>>>>>coordinate. If
>>>>>it has to be stored separately we could add a cell_measures for it.
>>>>>
>>>>
>>>>Yes, I know that the cell_measures exist, but it is convenient to have 
>>>>the grid information stored in variables. It would be nice if these 
>>>>names could be added.
>>>
>>>Perhaps we could add standard names of area (m2) and thickness (m)?
>>
>>Are you suggesting this as an alternative to the use of cell_measures or 
>>are you just proposing standard names for the measure variables?
> 
> 
> It is not an alternative to cell measures. The proposal, from Christiane, was
> for a standard name for such quantities as data variables in their own right.
> Such data variables could also be pointed to by cell_measures.
> 
> 
>>I'm curious as to the reason for 
>>introducing an alternative method for specifying cell metrics in 
>>addition to the existing cell bounds and cell_measures?  Doesn't this 
>>just complicate things, leading to possible confusion / ambiguity?  Does 
>>this make the use of cell_measures optional? (...obsolete?)
> 
> 
> I also asked whether it was necessary (in the >>>> text at the top). I think
> cell_measures would in any case remain useful as a pointer to locate the
> metrics easily. Sometimes cell_measures is not necessary because you could
> find the metric variable by searching the file for a data variable with the
> appropriate standard name (if we defined these) and the same grid as the data
> variable, but this is more laborious, and as your example shows it could be
> insufficient if the lat-lon coordinates are not defined. We probably had this
> discussion when we introduced cell_measures. It would be interested to know if
> anyone else has views.
> 
> 
>>a) a standard name for 'cell_volume'? (since volume is already a 
>>standard cell_measure name), and
> 
> Yes, we could do.
> 
> 
>>b) a new cell_measure name of 'thickness' (or height), which would refer 
>>to a measure variable with standard_name = 'cell_thickness'?
> 
> Yes, possibly.
> 
> Neither of these has been requested and, as usual, we would not add them unless
> they were specifically asked for.
> 
> Best wishes
> 
> Jonathan
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-- 
Christiane Textor
GMES France Atmosphère - GEMS France
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Email: christiane.textor at aero.jussieu.fr