[CF-metadata] meaning of depth in the ocean

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 26 Oct 2004 15:40:26 +0100

Deat Rich

These are tricky issues, which we've worried about a bit wrt sea level
quantities, but not sorted out thoroughly.

> Usually ocean modelere work with water depths relative
> to some local geopotential, which for places like the Great Lakes could
> be hundreds of meters above the geoid.

Yes, I suppose so. I had only thought of the connected global ocean, for
which "geoid" is appropriate.

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. 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.

- 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?

> 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?

> 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.

Comments on the above?

Cheers

Jonathan
Received on Tue Oct 26 2004 - 08:40:26 BST