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

Jim Biard jbiard at cicsnc.org
Fri Mar 17 14:17:45 MDT 2017

There's a lot of ugly all through this. Is there a geodesist 
(geodesyst?) in the house?

On 3/17/17 3:17 PM, Karl Taylor wrote:
> 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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