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

Jonathan Gregory j.m.gregory at reading.ac.uk
Tue Mar 21 10:20:00 MDT 2017

Dear Karl

> sea_surface_height_above_geoid
> The geoid is a surface of constant geopotential with which mean sea
> level would coincide if the ocean were at rest. (The volume enclosed
> between the geoid and the sea floor equals the mean volume of water
> in the ocean.) In an ocean GCM the geoid is the surface of zero
> depth, or the rigid lid if the model uses that approximation. "Sea
> surface height" is a time-varying quantity. By definition of the
> geoid, the global average of the time-mean sea surface height (i.e.
> mean sea level) above the geoid must be zero.
> 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.

> depth_below_geoid
>       As above, we should consider omitting the sentence about ocean
> models.

As above.

> sea_surface_height_above_sea_level
> sea_level means mean sea level, which is close to the geoid in sea
> areas. "Sea surface height" is a time-varying quantity. The standard
> name for the height of the sea surface above the geoid is
> sea_surface_height_above_geoid.
> Is there a convention for what interval of time applies to the
> "mean"?  I presume the interval is longer than the tidal period and
> seasonal changes (due to seasonal circulation changes and
> temperature changes), but shorter than climate change time-scales.

Yes. It means long enough to average out all tidal periods, so more than
the 18.6-year nodal tide period. In practice it is a bit vague, though.

> Finally, what would be the appropriate standard name for a variable
> measuring global mean sea level (say relative to ca. 1850) or global
> mean sea level change?  I don't think any of the above would do.

We have standard names for


"Global mean sea level" (not change) doesn't really mean anything by itself,
I would say.

> 1.  geopotential_height includes an explanatory note: " Geopotential
> is the sum of the specific gravitational potential energy relative
> to the geoid and the specific centripetal potential energy.
> Geopotential height is the geopotential divided by the standard
> acceleration due to gravity. It is numerically similar to the
> altitude (or geometric height) and not to the quantity with standard
> name height, which is relative to the surface."

> For geopotential_height, "numerically similar to" could be better
> stated, I think, as "approximately the same as".


Best wishes


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