[CF-metadata] WRF staggered grids and vertical coordinates

[John Caron]

Brian Eaton wrote:

>Hi John,
>
>I agree with Jonathan and Karl that the current conventions are sufficient
>to describe the WRF grids.
>
>I don't see that the proposed stagger attribute adds any new functionality.
>
>  
>
>>The stagger attribute associates 2 coordinate axes, indicating that they
>>represent the same coordinates, but are offset from each other. A
>>    
>>
>
>Why not use standard names to make these associations?  For example, the x
>and x_stag axes could each have the standard name projection_x_coordinate.
>Then determining the stagger is a simple matter of inspection of the
>coordinate values.
>
hmm, that would be ok with me.

so the meaning of two different variables with "projection_x_coordinate" 
attribute. is that they are different samplings of the same coordinate?  

so that would solve the problem of actually associating the coordinate 
variables. you still have the problem of requiring that extra lat, lon 
variables must be present in the file. what do you think about allowing 
those to be derived by the client?

>
>  
>
>>and 6 new 4D vertical coordinates (possibly only 3 if they dont need to
>>keep P and PB seperate)  (in principle you might need as many as 16  4D
>>variables).
>>    
>>
>
>You mentioned in an earlier post (27 Oct 2003) that the pressures are
>defined on the unstaggered grid.  In that case the pressures on the
>staggered grids are obtained by interpolation and it wouldn't be necessary
>to include them in the file.  
>
so if i understand you, its ok to derive the z coordinates from 
staggered x and y coordinates, in the case that the z coordinates are 
dependent on x and y.

>Also, as you point out it's probably only
>necessary to include the total pressure, and not the base and perturbation
>parts.
>
i dont know the answer to that question from a science point of view, 
but ill ask the WRF group.

>
>Your previous use of formula_terms to indicate that the base and
>perturbation parts should be added to obtain the total pressure was not
>consistent with the intent of that attribute.  It is designed to provide a
>formula to calculate a dimensional quantity like height or pressure from
>the dimensionless coordinate values and other data like surface height or
>surface pressure.
>
yes, i realize it's using it in a somewhat more general way, but for the 
same purpose: getting to a standard coordinate from a more specialized 
set of data. i also realize that you are enumerating the allowed 
calculations in appendix D.

i will see if WRF is willing to output the combined fields, to see if 
this is still an issue.

thanks for joining the discussion