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

From: John Caron <caron>
Date: Thu, 07 Apr 2005 11:07:02 -0600

Bryan Lawrence wrote:

>On Wednesday 06 April 2005 20:19, Jonathan Gregory wrote:
>
>
>>The data provider probably doesn't care very much. This information is not
>>seen as critical for interpretation or even formulation of the model. Hence
>>you can probably legitimately serve it using any reasonable assumption.
>>
>>
>
>I would go even further than this. While it is possible that the coordinate
>transformation error becomes more significant at very high resolution global
>models and mesoscale model scales, the actual significance of the model
>results at those scales is poor (typically, for a grid point model, one
>should be looking at an effective resolution about four times the grid
>resolution). What that means is the error in the grid projection
>transformation (if there is one) is vastly smaller than the effective grid
>registration (ie. how accurately the model coordinates reflect the real
>world). This is probably true even in a data assimilation model, where the
>model knows a bit more about the real world ...
>
>Nowcasting models at the very finest scales may be a different story, but in
>that case the grid formulation should probably be rather different than a
>global spherical formulation ... ("should" I say, don't know that they
>are :-)
>
>
>I don't think that CF can cope with everything ...
>
>
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.

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.

Another possibility would be to create an extension of CF for model data
with resolution less than such and such.

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

In any case, I really appreciate everyone's help in responding to this
question, and in everyone's thoughts about what CF should do.
Received on Thu Apr 07 2005 - 11:07:02 BST