Dear Alison and Nan
Many thanks for doing this, Alison, and apologies that I didn't have time for
it sooner myself. I think this is all fine except for the phrase defining
geopotential datum, which appears in several of them. I would say
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.
The geoid is a geopotential (equipotential) surface, namely the one which
encloses (between itself and the solid Earth below) a volume equal to the
volume of the ocean, often informally described as the surface the ocean would
have if it were at rest. The ellipsoid isn't a geopotential surface. We intro-
duced the term "geopotential datum" in
https://cf-trac.llnl.gov/trac/ticket/118
Nan commented
Re: The term 'height_above_sea_level', defined as "Height_above_X" means the
vertical distance above the named surface X. "sea_level" means *mean* sea
level, which is close to the geoid in sea areas.' Someone recording observed
met data on a buoy, looking for the best term for the sensor heights, might
easily choose this instead of 'height', but the height in that case is above
actual sea level; varying over time. If this variable is meant to be 'height
above mean sea level', that should be its name.
There are about a dozen standard names using sea_level to mean mean_sea_level
consistently, and we don't currently use the phrase mean_sea_level at all. If
people agree, we could rename the existing standard names as Nan suggests for
this new one.
Best wishes
Jonathan
----- Forwarded message from alison.pamment at stfc.ac.uk -----
> Date: Thu, 18 May 2017 11:37:54 +0000
> From: alison.pamment at stfc.ac.uk
> To: cf-metadata at cgd.ucar.edu
> Subject: [CF-metadata] Standard names for CF trac ticket #143
>
> Dear Jonathan, All,
>
> CF trac ticket #143 (https://cf-trac.llnl.gov/trac/ticket/143) has been agreed and will be included in CF 1.7. A number of new standard names are needed to support the implementation of this ticket. They are names for constants used in the formula_terms attribute of parameterized vertical coordinates.
>
> The following names are proposed.
>
> air_pressure_at_top_of_atmosphere_model (Pa)
> ' "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model.'
>
> 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.'
>
> 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."
>
> 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 the geopotential reference level (level of zero geopotential) where this is not a specifically named level such as the geoid or a reference ellipsoid. "Top of atmosphere model" means the upper boundary of the top layer of an atmosphere model.'
>
> height_above_geopotential_datum (m)
> '"Height_above_X" means the vertical distance above the named surface X. The "geopotential datum" is the geopotential reference level (level of zero geopotential) where this is not a specifically named level such as the geoid or a reference ellipsoid.'
>
> 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 the geopotential reference level (level of zero geopotential) where this is not a specifically named level such as the geoid or a reference ellipsoid.'
>
> sea_surface_height_above_geopotential_datum (m)
> ' "Height_above_X" means the vertical distance above the named surface X. The "geopotential datum" is the geopotential reference level (level of zero geopotential) where this is not a specifically named level such as the geoid or a reference ellipsoid. "Sea surface height" is a time-varying quantity.'
>
> sea_floor_depth_below_geopotential_datum (m)
> ' "Depth_below_X" means the vertical distance below the named surface X. The "geopotential datum" is the geopotential reference level (level of zero geopotential) where this is not a specifically named level such as the geoid or a reference ellipsoid.'
>
> 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.'
>
> height_above_sea_level (m)
> ' "Height_above_X" means the vertical distance above the named surface X. "sea_level" means mean sea level, which is close to the geoid in sea areas.'
>
> I have based the definitions on my own reading of ticket #143 and on existing names. I'd welcome comments to improve them.
>
> 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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----- End forwarded message -----
Received on Thu May 25 2017 - 08:02:02 BST
