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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