Date: Tue, 4 Jul 2017 14:35:27 +0000
From: [email protected]
To: [email protected]
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: [email protected]
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
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