[CF-metadata] geoid, sea surface, height, and standard names

Karl Taylor taylor13 at llnl.gov
Tue Mar 21 16:49:31 MDT 2017

On 3/21/17 9:20 AM, Jonathan Gregory wrote:
> Dear Karl
>> sea_surface_height_above_geoid
>> I'm not sure it's true that "In an ocean GCM the geoid is the
>> surface of zero depth".  Many ocean models have an ocean surface
>> that rises above the geoid in some areas and falls below in other
>> areas.  Moreover, under conditions of sea level change, the global
>> mean model surface of zero depth will vary and not necessarily
>> coincide to some fixed geoid.  Would it be better to omit the
>> sentence about ocean models?
> There is a surface z=0, with respect to which height and depth are measured
> in ocean models. The surface z=0 is usually the geoid, so it's not the
> same as mean sea level.  As we have discussed in other emails, in models which
> conserve volume rather than mass, the geoid can't change, but in the real
> world and in models which conserve mass, it can.  However, there's a choice to
> be made: you can stick with the original geoid (for the original volume of
> the ocean) or make it time-dependent. Either way, I think the statement is
> correct, but we could omit it if you think it's unhelpful in the definition.
O.K.  I understand what is meant now, but it leads to the rather 
confusing implication that the model's surface can be located at a 
*depth" that is not zero.  When I think of an ocean depth measurement, I 
usually think of it as measure of the distance to the the *surface*.   
If this isn't the usual understanding, then it's o.k. with me to leave 
in the sentence about the ocean GCM. Otherwise, I think it might confuse 
folks, and we should delete it.

thanks and best regards,
> Best wishes
> Jonathan
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