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,
Karl
Best wishes
Jonathan
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