Dear Jim,
It's a fair question, but I think there is a distinction between using "surface" as part of a standard name, and using it as an English word in the definitions which are meant to describe and explain the name in more detail.
When "surface" occurs in a name it always does mean the lower boundary of the atmosphere (except in one or two cases where, for historical reasons, it means "near the lower boundary of the atmosphere", e.g. sea_surface_temperature).
When we use the word "surface" in definitions it often does also mean the lower boundary of the atmosphere and I think that is always clear from the context. In a few cases, the word "surface" is used to describe some conceptual rather than actual physical surface, for example, in this thread I have suggested a new definition for air_pressure of 'Air pressure is the force per unit area which would be exerted when the moving gas molecules of which the air is composed strike a theoretical surface of any orientation' and we have agreed a definition of 'The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available.' I would hope the fact that we are saying "theoretical surface" and "estimated surface" would be enough to avoid confusion with any particular physical surface. We do also have existing names whose definitions use this more general interpretation of "surface". For example, angle_of_emer
gence is defined as 'The angle of emergence is that between the direction of a beam of radiation emerging from the surface of a medium and the normal to that surface' where again I think the context is clear.
If you find any examples of standard name definitions which say "surface" and the context isn't clear, please let me know and we can try to improve them. In general though, I think avoiding the use of the word "surface" entirely in definitions except to mean the lower boundary of the atmosphere would make life rather difficult. It would be a bit like trying to give directions to someone without being allowed to use the words "left" or "right" - an interesting exercise but one that could lead to some unnecessarily complicated instructions!
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jim Biard
Sent: 05 July 2017 15:30
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Standard names for CF trac ticket #143
Hi.
I have a question. Should we be using a word other than 'surface' in the definitions when speaking of anything other than 'the lower boundary of the atmosphere'? It's not majorly important, but it feels a bit confusing when we spend so much time defining the meaning of surface, then use it to mean something else. If folks are comfortable with mixing the uses of the word, then I'm not going to belabor it.
Grace and peace,
Jim
On 7/4/17 11:14 AM, Jonathan Gregory wrote:
Dear Alison
I think this is all fine. Thank you for your thoroughness. Yes, I agree, we
should also refer to ch 5 for the methods to provide a precise definition of
a reference ellipsoid.
Best wishes
Jonathan
----- Forwarded message from alison.pamment at stfc.ac.uk -----
Date: Tue, 4 Jul 2017 14:35:27 +0000
From: alison.pamment at stfc.ac.uk
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Standard names for CF trac ticket #143
Dear Jonathan, Roy, Nan and Karl,
Many thanks for the discussion of these names.
1. altitude_at_top_of_atmosphere_model (m)
'Altitude is the (geometric) height above the geoid, which is the reference geopotential surface. The geoid is similar to mean sea level. "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model.'
2. reference_air_pressure_for_atmosphere_vertical_coordinate (Pa)
'For models using a dimensionless vertical coordinate, for example, sigma, hybrid sigma-pressure or eta, the values of the vertical coordinate at the model levels are calculated relative to a reference level. "Reference air pressure" is the air pressure at the model reference level. It is a model-dependent constant.'
Jonathan has indicated he is happy with the suggested definitions for these names and no other comments have been received. Therefore, names (1) and (2) are accepted for inclusion in the standard name table and will be published in the July update.
3. height_above_sea_level (m)
As Nan and others have pointed out, this name would be better if it were height_above_mean_sea_level. Indeed in the recently concluded thread discussing standard names for mean sea level we have agreed to make a similar change in all existing names where mean_sea_level is the intended meaning (
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2017/059555.html). In that thread we also agreed that the definition of mean sea level should be: ' "Mean sea level" means the time mean of sea surface elevation at a given location over an arbitrary period sufficient to eliminate the tidal signals.' Therefore we now have:
height_above_mean_sea_level (m)
' "Height_above_X" means the vertical distance above the named surface X. "Mean sea level" means the time mean of sea surface elevation at a given location over an arbitrary period sufficient to eliminate the tidal signals.'
This name is accepted for publication in the standard name table and will be added in the next update at the end of July.
4. air_pressure_at_top_of_atmosphere_model (Pa)
' "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model.'
Jonathan has indicated he is happy with the suggested definition and no other comments have been received. We have just agreed a new definition for mean sea level and in the interests of adding at least brief definitions for hitherto undefined quantities in the standard name table, I think that air_pressure is another term that would benefit from some attention. Indeed, the definition of the standard name air_pressure currently reads 'No help available'. I suggest the following text:
'Air pressure is the force per unit area which would be exerted when the moving gas molecules of which the air is composed strike a theoretical surface of any orientation'. I suggest adding this to the current proposal and to the 19 existing air_pressure names currently in the standard name table. Then we'd have:
air_pressure_at_top_of_atmosphere_model (Pa)
' "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model. Air pressure is the force per unit area which would be exerted when the moving gas molecules of which the air is composed strike a theoretical surface of any orientation.'
What do you think?
5. height_above_geopotential_datum_at_top_of_atmosphere_model (m)
6. height_above_geopotential_datum (m)
7. surface_height_above_geopotential_datum (m)
8. sea_surface_height_above_geopotential_datum (m)
9. sea_floor_depth_below_geopotential_datum (m)
Jonathan has suggested the definition text for 'geopotential_datum' should be:
'The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available.'
I think this looks fine and agree that the definition shouldn't refer to reference_ellipsoid. Thank you also for reminding me about trac ticket 118 where the term 'geopotential datum' was first discussed. In that ticket Jonathan says:
However, the definitions of standard_names which refer to the geoid or a geopotential datum could draw attention to the possibility of precisely specifying the reference surface
by using a grid_mapping attribute. If ticket 143 is agreed, I will make a standard_name proposal on the email list in which this point could be included.
We are having this discussion because ticket 143 has indeed been agreed, so I'd suggest that we also need to add the following text to all the defintions of names 5 - 9:
'To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
The names would then be as follows:
5. height_above_geopotential_datum_at_top_of_atmosphere_model (m)
' "Height_above_X" means the vertical distance above the named surface X. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention. "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model.'
6. height_above_geopotential_datum (m)
' "Height_above_X" means the vertical distance above the named surface X. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
7. surface_height_above_geopotential_datum (m)
' "Height_above_X" means the vertical distance above the named surface X. The surface called "surface" means the lower boundary of the atmosphere. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
8. sea_surface_height_above_geopotential_datum (m)
' "Height_above_X" means the vertical distance above the named surface X. "Sea surface height" is a time-varying quantity. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
9. sea_floor_depth_below_geopotential_datum (m)
' "Depth_below_X" means the vertical distance below the named surface X. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
Are these okay?
10. sea_floor_depth_below_reference_ellipsoid (m)
' "Depth_below_X" means the vertical distance below the named surface X. A reference ellipsoid is a regular mathematical figure that approximates the irregular shape of the geoid. A number of reference ellipsoids are defined for use in the field of geodesy.'
Presumably we should also add some text about using grid_mapping to specify which reference ellipsoid is being used to the definition of this name? So we'd have:
' "Depth_below_X" means the vertical distance below the named surface X. A reference ellipsoid is a regular mathematical figure that approximates the irregular shape of the geoid. A number of reference ellipsoids are defined for use in the field of geodesy. To specify which reference ellipsoid is being used, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention.'
Okay?
I assume we should also now update the definitions of all the existing ten geoid names and five reference_ellipsoid names to include the text about using grid_mapping. Do others agree?
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
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Received on Wed Jul 05 2017 - 10:06:43 BST
