[CF-metadata] What do models assume for the shape of the Earth?

[Bryan Lawrence]

Hi John et al ...

> Well, Mr Toyoda has shown that the difference between geodetic
> (ellipsoidal earth) and geocentric (spherical earth) latitude is around
> 20 km at mid-latitude.  My understanding of the conversation is that we
> should assume spherical earth for global models, unless otherwise
> stated. Now when I send that info to a GIS package that uses an
> ellipsoidal earth, presumably they will correct the coordinates to take
> this into account.
>
> Now for models that do have accuracy in which 20 km matters, they will
> need to record information as to earth shape in order to get correct
> geolocation. Perhaps there are no such models yet (??) but someday there
> may be.

My point is that one should worry about this error when it matters. Rich's 
situation is one where it matters.  But for a global model, a 20km grid box 
is equivalent to an 80km true resolution in the output fields. Doing a 
correction on the coordinates on output back to an ellipsoid is something you 
can do, but I would argue is counter to the underlying scale analysis that 
went into the equations in the model ... Karl is right about how orographic 
coordinates are used in the UM ... and I'd be surprised if it was different 
in any other global model.

Having said all that, I agree with Jonathan, the data from such models can be 
projected in whatever manner is convenient ... 


> If CF doesnt want to cope with this, it would be helpful if CF stated
> that the scope of its mission is to deal with model data with resolution
> greater than such and such.

Don't get me wrong (I keep only telling half the story), CF needs a way to 
support real projections ... and a way to distinguish between them. I'll try 
and reply to Rich this weekend ...

(Which is what I was trying to say about nowcasting models in my original 
email, I think they'll have this problem too).

> Perhaps these are issues to be discussed at the GO-ESSP conference?

Yes, but it wont hurt to thrash it out more here ... ideally we should go to 
the GO-ESSP meeting with a good understanding of the issues, if we go with an 
intention to discuss them all in detail, we'll not make a lot of progress.

>In any case, I really appreciate everyone's help in responding to this 
>question, and in everyone's thoughts about what CF should do.

I think most of us appreciate the detailed analysis of the real problems on 
the frontiers of CF that you do ... speaking personally, I wish I had the 
time to be as thorough as you ... 

Cheers
Bryan