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

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 26 Oct 2004 23:37:49 +0100

Dear Rich

> 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!

Exactly. Hence the geoid (or geopotential surface) and the "surface" (= mean
sea level) should not be confused in defining standard names.

> 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.

I tend to think it is better to make the surface/geoid distinction even more
blatant by putting it in the standard name.

> 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).

The difference is that a rigid-lid ocean has fixed volume so z=0 is always its
geoid in effect. However, unless zeta is constrained to have a zero global
average, in a free-surface model the volume is not fixed so it might be that
the geoid is a field with z = the global average of zeta, rather than z=0, if
you define the geoid to be the surface the ocean would assume at rest. What
do you think?

> Yes, why not list the reference ellipsoid.
if relevant, yes. If OGC has a standard naming convention for them, as John
Caron suggests, we could adopt it. Does anyone know where to find it?

Best wishes

Jonathan
Received on Tue Oct 26 2004 - 16:37:49 BST