Dear Jonathan,
Would the following changes (in bold) to the caveat make you more comfortable?
Global average sea level change is a calculated parameter and should NOT be understood as the global spatial mean of local changes in observed mean sea level!
I have no strong feelings about the position of the caveat in the definition.
Cheers, Roy.
Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to enquiries at bodc.ac.uk. Please also use this e-mail if your requirement is urgent.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jonathan Gregory <j.m.gregory at reading.ac.uk>
Sent: 22 June 2017 18:12
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Standard names for mean sea level change
Dear Alison
I agree with all you say below, except that I think the WARNING is a bit too
severe. :-) I would suggest putting it a bit later in the definition. Because
global average sea level change refers to the volume of the global ocean, it
does not *have* to be calculated as the mean of local sea level change. For
example, for the part of GMSLR which is due to adding water to the ocean from
glaciers and ice-sheets, etc. (I'm not proposing a standard name for this just
now, though we might need one) you do not need to calculate the effect that
would have on local sea level, and from that the global mean. In fact the local
MSL change is rather hard to calculate, because it involves the propagation of
salinity change within the ocean, changes in ocean circulation, and the effect
of the mass redistribution upon the geoid and the solid Earth. But none of that
makes a difference to the global mean - it's just a redistribution. For the
global mean, you just divide the volume of water added by the surface area of
the world ocean - easy! However, you *could* go through all the complexity, and
then average it out again. It would not be incorrect to do so.
Best wishes
Jonathan
----- Forwarded message from alison.pamment at stfc.ac.uk -----
> Date: Thu, 22 Jun 2017 11:49:35 +0000
> From: alison.pamment at stfc.ac.uk
> To: rkl at bodc.ac.uk, j.m.gregory at reading.ac.uk, cf-metadata at cgd.ucar.edu
> Subject: RE: [CF-metadata] Standard names for mean sea level change
>
> Dear Jonathan and Roy,
>
> Many thanks for your replies to this thread. Apologies, Roy, for missing your previous explanation about mean sea level in the thread about standard names for trac ticket 143. It seems that both discussions are converging on the same view that we should explicitly say 'mean_sea_level' in standard names where that is really the intention, rather than confining it to the definition. Therefore, I think this change is agreed for both existing and new names.
>
> In this thread we seem to have reached agreement that the general 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.' In the case of names for either local or global mean changes in mean_sea_level we would add the additional sentence 'Zero mean sea level change is an arbitrary level.'
>
> For Jonathan's three newly proposed names in this thread, I think this leaves us with:
>
> thermosteric_change_in_mean_sea_level (m)
> 'Thermosteric sea level change is the part caused by change in sea water density due to change in temperature i.e. thermal expansion. "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. Zero mean sea level change is an arbitrary level. The sum of the quantities with standard names thermosteric_change_in_mean_sea_level and halosteric_change_in_mean_sea_level has the standard name steric_change_in_mean_sea_level.'
>
> halosteric_change_in_mean_sea_level (m)
> 'Halosteric sea level change is the part caused by change in sea water density due to change in salinity. "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. Zero mean sea level change is an arbitrary level. The sum of the quantities with standard names thermosteric_change_in_mean_sea_level and halosteric_change_in_mean_sea_level has the standard name steric_change_in_mean_sea_level.'
>
> steric_change_in_mean_sea_level (m)
> 'Steric sea level change is caused by changes in sea water density due to changes in temperature (thermosteric) and salinity (halosteric). "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. Zero mean sea level change is an arbitrary level. The sum of the quantities with standard names thermosteric_change_in_mean_sea_level and halosteric_change_in_mean_sea_level has the standard name steric_change_in_mean_sea_level.'
>
> Okay?
>
> The change to using mean_sea_level, rather than simply sea_level means that the following aliases need to be created:
>
> sea_floor_depth_below_sea_level -> sea_floor_depth_below_mean_sea_level
>
> sea_surface_height_above_sea_level -> sea_surface_height_above_mean_sea_level
>
> surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid -> surface_geostrophic_eastward_sea_water_velocity_assuming_mean_sea_level_for_geoid
>
> surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid -> surface_geostrophic_northward_sea_water_velocity_assuming_mean_sea_level_for_geoid
>
> surface_geostrophic_sea_water_x_velocity_assuming_sea_level_for_geoid -> surface_geostrophic_sea_water_x_velocity_assuming_mean_sea_level_for_geoid
>
> surface_geostrophic_sea_water_y_velocity_assuming_sea_level_for_geoid -> surface_geostrophic_sea_water_y_velocity_assuming_mean_sea_level_for_geoid
>
> tendency_of_sea_surface_height_above_sea_level -> tendency_of_sea_surface_height_above_mean_sea_level
>
> air_pressure_at_sea_level -> air_pressure_at_mean_sea_level
>
> For all of these names the definitions would be unchanged except we would replace 'sea_level means mean sea level, which is close to the geoid in sea areas' with '"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.' Okay?
>
> Roy was concerned about the syntax used in the existing names:
> global_average_steric_sea_level_change
> global_average_sea_level_change
> global_average_thermosteric_sea_level_change
> phase_of_global_average_sea_level_change
> tendency_of_global_average_sea_level_change
> amplitude_of_global_average_sea_level_change
> Jonathan has pointed out that 'average' here refers to a global (i.e spatial) average/mean which is distinct from the time mean used to calculate local mean_sea_level. I'm leaning towards keeping 'global_average' in the names themselves. I think the use of different terminology helps to underline Jonathan's point that global_average_thermosteric_sea_level_change is not simply the global mean of thermosteric_change_in_mean_sea_level, which is very important to interpreting these names correctly. Certainly I think the order of the syntax is correct for these quantities (the mean/average definitely belongs with the 'global'). However, I do think there is room to improve the defintions. In fact, I would go so far as to say 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level!' should be the first sentence in all the definitions.
>
> Currently the definition of global_average_sea_level_change reads:
> '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.'
>
> With the addition of the health warning above, I think this text is a good basis for use in all the names. I suggest amended definitions as listed below, in which I have also tried to make the wording describing steric changes match as closely as possible that used in Jonathan's three new names.
>
> global_average_sea_level_change (m)
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! 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.'
>
> tendency_of_global_average_sea_level_change (m year-1)
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! 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. "tendency_of_X" means derivative of X with respect to time.'
>
> amplitude_of_global_average_sea_level_change
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! 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. Amplitude is the magnitude of a wave modelled by a sinusoidal function. A coordinate variable of harmonic_period should be used to specify the period of the sinusoidal wave.'
>
> phase_of_global_average_sea_level_change (degree)
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! 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. Phase is the initial angle of a wave modelled by a sinusoidal function. A coordinate variable of harmonic_period should be used to specify the period of the sinusoidal wave.'
>
> global_average_steric_sea_level_change (m)
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! Global average steric sea level change is caused by changes in sea water density due to changes in temperature (thermosteric) and salinity (halosteric). This changes the volume of water in the ocean. Zero sea level change is an arbitrary level.'
>
> global_average_thermosteric_sea_level_change (m)
> 'N.B. Global average sea level change should NOT be understood as the global spatial mean of local changes in mean sea level! Global average thermosteric sea level change is the part caused by change in sea water density due to change in temperature i.e. thermal expansion. This changes the volume of water in the ocean. Zero sea level change is an arbitrary level.'
>
> What do you think of these?
>
> I hope I've managed to cover all the points raised in this discussion :)
>
> 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 Lowry, Roy K.
> Sent: 14 June 2017 17:40
> To: Jonathan Gregory; cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Standard names for mean sea level change
>
> Dear Jonathan,
>
> I'm OK with losing the 'principal'. I know what I mean by that, but there are some (many) who might not!
>
> Cheers, Roy.
>
> Please note that I partially retired on 01/11/2015. I am now only working 7.5 hours a week and can only guarantee e-mail response on Wednesdays, my day in the office. All vocabulary queries should be sent to enquiries at bodc.ac.uk. Please also use this e-mail if your requirement is urgent.
>
> ________________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jonathan Gregory <j.m.gregory at reading.ac.uk>
> Sent: 14 June 2017 16:58
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Standard names for mean sea level change
>
> Dear Roy and Alison
>
> > To clarify, what I was getting at with the rationalisation of syntax was mixed use of 'average' and 'mean' for the same statistic and the fact that the names are opposite ways around. For example, instead of thermosteric_change_in_mean_sea_level and global_average_thermosteric_sea_level_change why not have average_thermosteric_sea_level_change and global_average_thermosteric_sea_level_change (possibly with all averages changed to means)?
>
> The use of "average" rather than "mean" (my choice, I admit) was made to avoid
> confusion with "mean sea level". However if we're introducing "mean" now anyway
> I wouldn't mind changing "average" to "mean", since "mean" is more common.
> However, there is some subtlety and potential for confusion still!
>
> global_mean_[thermosteric_]sea_level_change is not the [thermosteric] change in
> global mean sea level, because there is no such quantity as "global mean sea
> level" without "change". The "mean" here means a spatial average. Moreover,
> you don't necessarily calculate these global quantities as a global mean of
> local quantities, because they really refer to change in the volume of the
> world ocean, divided by world ocean surface area. If they were really spatial
> means, we could use cell_methods to describe them instead of distinct standard
> names.
>
> thermosteric_change_in_mean_sea_level is the thermosteric change in (local)
> mean sea level. The "mean" here means a temporal average.
>
> > Finally, thinking about it my concerns about these new names being abused could be alleviated by the following definition:
> >
> > "Mean sea level" means the time mean of sea surface elevation at a given location over an arbitrary period sufficient to eliminate the principal tidal signal. Zero mean sea level change is an arbitrary level.
>
> That is fine with me, especially if it alleviates Roy's concerns. I would
> slightly prefer "tidal signals" instead of "principal tidal signal". Roy is
> quite right that climate models don't usually have tides anyway. For precision
> in the real world it is essential to specify a particular geopotential datum,
> since MSL is vague. Nonetheless "above MSL" is a commonly used phrase.
>
> Best wishes
>
> Jonathan
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