[CF-metadata] Re: meaning of depth in the ocean

From: Rich Signell <rsignell>
Date: Tue, 26 Oct 2004 11:17:45 -0400

Jonathan Gregory wrote:

>This problem is broader than dimensionless coordinates. The basic problem is
>what "depth" means as a vertical coordinate in the ocean or the ocean
>model. At present we have a standard_name of depth, defined relative to the
>surface, which is further defined as mean sea level.
>
I just got off the phone with John Wilkin, another ocean modeler, and he
reminded me
that mean sea level should *not* be thought of as the local geopotential
for ocean models.
Consider the Gulf Stream region, where there is a jump in mean sea level
of order 1 meter!

>But...
>
>- it might be relative to the geoid or some other geopotential surface, instead
>of mean sea level, so perhaps we should have separate standard names for
>depth_below_geoid and depth_below_surface.
>
>
>
We could have differing standard names, or we could just say that depth
is defined
relative to "the datum" and have a "datum_offset" field that contains the
offset between "the datum" and the geoid.

>- what does it mean for an ocean model? So far (e.g. for the IPCC data
>collection) we have been assuming that in a rigid-lid model, "geoid" means
>the rigid lid z=0. What does z=0 mean in an ocean model with a free surface?
>
>
>
z=0 means the same thing whether free surface or rigid lid.
In free surface models, there is a variable (e.g. "zeta") that
represents the departure
of the free surface from the "datum" (z=0).

>>What do you think about adding to the CF convention the specification of
>>a 2D variable
>>(function of lat, lon) called "geoid_offset" (or equivalent) that would
>>specify the
>>offset between the "local datum" (z=0) and the geoid?
>>
>>
>
>If the local datum is a geopotential, a single number rather than a field ought
>to be enough to specify the offset, shouldn't it?
>
>
Hmmm... perhaps you are right. I was thinking before that we might
want to specify the
datum relative to mean sea level, which would technically require a 2D
offset field, but in light of
John Wilkin's point, I can't actually think of any cases where this
would make sense.

>
>
>>And while we are on the subject, are we being intentionally ambiguous by
>>referring
>>to "the geoid"? I'm thinking about NAD27, GRS80, WGS84, etc..
>>
>>
>
>Yes, we are being vague. In some cases e.g. ocean models for climate, this
>doesn't matter. But sometimes it clearly would matter. So we may need some
>further way to specify the geoid, as one of the reference ellipsoids.
>
>
Yes, why not list the reference ellipsoid. For the cases where it
will matter people will
be sure to specify the correct ellipsoid, and for all the other cases,
it just won't matter.

-Rich

-
Richard P. Signell rsignell at usgs.gov
U.S. Geological Survey Phone: (508) 457-2229
384 Woods Hole Road Fax: (508) 457-2310
Woods Hole, MA 02543-1598
Received on Tue Oct 26 2004 - 09:17:45 BST