Dear Jonathan and Philip,
Thank you for your feedback.
Finally I think we don't need to insist on this story of average vs cell methods, I made a confusion. The process is actually easier.
>From sea surface heights NetCDF products ('sea_surface_height_above_sea_level' in CF) we are computing the tendency of these heights for each grid point. Which can basically be translated in CF to
"tendency_of_sea_surface_height_above_sea_level". Am I Right?
Olivier.
-----Message d'origine-----
De?: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] De la part de Cameron-smith, Philip
Envoy??: vendredi 6 juillet 2012 19:38
??: Jonathan Gregory; cf-metadata at cgd.ucar.edu
Objet?: Re: [CF-metadata] Standard names for sea level change
Hi Olivier, et al.,
I note that we have several different types of sea_level defined in CF. I have copied here the ones I found (there may be others). Does one of these meet your needs?
global_average_sea_level_change
Global average sea level change is due to change in volume of the water in the ocean, caused by mass and/or density change, or to change in the volume of the ocean basins, caused by tectonics etc. It is sometimes called "eustatic", which is a term that also has other definitions. It differs from the change in the global average sea surface height relative to the centre of the Earth by the global average vertical movement of the ocean floor. Zero sea level change is an arbitrary level.
global_average_steric_sea_level_change
Global average steric sea level change is caused by changes in sea water density due to changes in temperature (thermosteric) and salinity (halosteric). Zero sea level change is an arbitrary level.
global_average_thermosteric_sea_level_change
Global average thermosteric sea level change is the part caused by change in density due to change in temperature i.e. thermal expansion. Zero sea level change is an arbitrary level.
ocean_rigid_lid_pressure_expressed_as_sea_surface_height_above_geoid
"Ocean rigid lid pressure" means the pressure at the surface of an ocean model assuming that it is bounded above by a rigid lid.
sea_surface_height_above_geoid
alias: sea_surface_elevation
alias: sea_surface_elevation_anomaly
The geoid is a surface of constant geopotential with which mean sea level would coincide if the ocean were at rest. (The volume enclosed between the geoid and the sea floor equals the mean volume of water in the ocean.) In an ocean GCM the geoid is the surface of zero depth, or the rigid lid if the model uses that approximation. "Sea surface height" is a time-varying quantity. By definition of the geoid, the global average of the time-mean sea surface height (i.e. mean sea level) above the geoid must be zero. The standard name for the height of the sea surface above mean sea level is sea_surface_height_above_sea_level. The standard name for the height of the sea surface above the reference ellipsoid is sea_surface_height_above_reference_ellipsoid.
sea_surface_height_above_sea_level
alias: sea_surface_height
sea_surface_height_above_reference_ellipsoid
"Sea surface height" is a time-varying quantity. A reference ellipsoid is a mathematical figure that approximates the geoid. The geoid is a surface of constant geopotential with which mean sea level would coincide if the ocean were at rest. The ellipsoid is an approximation because the geoid is an irregular shape. A number of reference ellipsoids are defined for use in the field of geodesy. The standard name for the height of the sea surface above the geoid is sea_surface_height_above_geoid. The standard name for the height of the sea surface above mean sea level is sea_surface_height_above_sea_level.
Best wishes,
Philip
-----------------------------------------------------------------------
Dr Philip Cameron-Smith, pjc at llnl.gov, Lawrence Livermore National Lab.
-----------------------------------------------------------------------
> -----Original Message-----
> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf
> Of Jonathan Gregory
> Sent: Friday, July 06, 2012 8:01 AM
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Standard names for sea level change
>
> Dear Olivier
>
> > I appreciate the idea suggested by Philip, which is remove 'local' and simply
> use "tendency_of_average_sea_level_change". I think it's a fine solution.
>
> I'm sorry to say I still have concerns with this:
>
> * Why is "average" needed? As Philip said, we usually convey that idea in the
> cell_methods, which can distinguish if necessary between a local point
> measurement e.g. "lat: lon: point" and an area-average "lat: lon: mean"
> or "area: mean". Or does "average" refer to the time-mean?
>
> * "Sea level change" is not well-defined. Could you say more precisely what
> this quantity is, if it is not change in ocean thickness (that was my guess)?
>
> Best wishes and thanks
>
> Jonathan
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