[CF-metadata] Standard names for CF trac ticket #143

Jonathan Gregory j.m.gregory at reading.ac.uk
Thu May 25 10:23:48 MDT 2017


Dear Karl

Yes, it is a bit vague. Mean sea level is a commonly used concept, but if you
wanted to be precise you would use standard names referring to geopotential
datum or geoid instead, and state which definition was being used in the
grid mapping. I think vagueness is OK in standard names if there are datasets
which need to use them.

Actually my "dozen" was an overestimate because not all of them are talking
about regional sea level. The ones affected are

air_pressure_at_sea_level:Pa                                   
sea_floor_depth_below_sea_level:m
sea_surface_height_above_sea_level:m
surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid:m s-1
surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid:m s-1
surface_geostrophic_sea_water_x_velocity_assuming_sea_level_for_geoid:m s-1
surface_geostrophic_sea_water_y_velocity_assuming_sea_level_for_geoid:m s-1

Best wishes

Jonathan

----- Forwarded message from Karl Taylor <taylor13 at llnl.gov> -----

> Date: Thu, 25 May 2017 09:05:05 -0700
> From: Karl Taylor <taylor13 at llnl.gov>
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Standard names for CF trac ticket #143
> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0)
> 	Gecko/20100101 Thunderbird/52.1.1
> 
> Dear all,
> 
> I kinda like the idea of changing "above sea level" to "above mean
> sea level", but it still remains somewhat vague, since the period
> over which the mean is computed isn't specified.  Or is there some
> accepted time?  In any case maybe it is o.k. to be vague????
> 
> best regards,
> Karl
> 
> 
> On 5/25/17 7:02 AM, Jonathan Gregory wrote:
> >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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