Jim:
To provide you a point of reference the GOES-R cloud and ash cloud "height" products are anchored by NOAA National Weather Prediction model data which includes geopotential height data.
Responses to your questions ...
(1) Which mean sea level datum?
As you suggest, it is a purely theoretical surface where the acceleration of gravity is exactly 1G. It is my understanding that this is a commonly used (if not standard model) for this type of thing. It occurred to me there are other less prevalent theoretical models out there in use.
(2) What's the resolution of the measurement?
I don't understand your question.
(3) how do I connect this measure to some other measure that is relative to a different vertical datum?
Equations exist that define the relationship between the theoretical surface where the acceleration of gravity is exactly 1G and a vertical datum. I hope you don't mind if I leave that exercise to the reader. The point is that a specific equation is required.
very respectfully,
randy
----------------------------------------
From: "Jim Biard" <jbiard at cicsnc.org>
Sent: Wednesday, February 19, 2014 9:23 AM
To: CF-metadata at cgd.ucar.edu
Cc: "Randy Horne" <rhorne at excaliburlabs.com>
Subject: Re: [CF-metadata] vertical datums and new standard name: geopotential_height_at_cloud_top
Randy,
Thanks for re-iterating this. Of course, now I can't resist asking ... (I'm not doing this to pick on you, but as a way to point out the issues at hand with vertical measures.)
Which mean sea level datum? There are a number of them. Or is this a purely theoretical surface where the acceleration of gravity is exactly 1G? What's the resolution of the measurement? If you aren't measuring relative to a specific vertical datum, how do I connect this measure to some other measure that is relative to a different vertical datum?
Grace and peace,
Jim
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On Feb 18, 2014, at 3:22 PM, Randy Horne <rhorne at excaliburlabs.com> wrote:Jim:
Our two GOES-R products that are height quantities are geopotential heights - (1) the quantity for which the standard_name is being requested now (geopotential_height_at_cloud_top) and (2) the height of volcanic ash clouds for which we got a standard name added last year ( geopotential_height_at_volcanic_ash_cloud_top).
These heights are measured from mean sea level (represented with constant gravity).
very respectfully,
randy
On Feb 14, 2014, at 1:02 PM, Jim Biard <jbiard at cicsnc.org> wrote:Randy,
Is the geopotential height zero at the geoid? (You are unfortunate to propose this while antennae are twitching over verticality! Please forgive me.) I hadn't realized before this that we have some rather specific (and somewhat erroneous) definitions for height and altitude in the standard name table. I see that the verbiage you have in your definition is common to a number of other names, so I guess it's OK until we get some things straightened out (if we ever do!).
Grace and peace,
Jim
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On Feb 12, 2014, at 4:20 PM, rhorne at excaliburlabs.com wrote:Dear All:
I would like to propose a new standard name:
geopotential_height_at_cloud_top
definition:
Cloud_top refers to the top of the highest cloud. 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.
canonical units:m
The timing of this proposal, and its relationship to the currently on-going vertical datums discussion, is purely coincidental.
very respectfully,
randy _______________________________________________
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