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

Karl Taylor taylor13 at llnl.gov
Fri Mar 17 13:17:55 MDT 2017


Hi all,

I've been looking at the standard names used to describe the vertical 
location of the sea surface and have some questions.

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?

depth_below_geoid

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

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.

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.

Also, I noticed that in the standard names table,

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."

2. altitude includes an explanatory note:  " Altitude is the (geometric) 
height above the geoid, which is the reference geopotential surface. The 
geoid is similar to mean sea level.

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

best regards,
Karl

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