Or better Global average sea level change quantifies the change in volume of the world ocean, and is not calculated necessarily by considering local changes in mean sea level.
J ----- Forwarded message from Jonathan Gregory <j.m.greg...@reading.ac.uk> ----- > Date: Fri, 23 Jun 2017 12:11:32 +0100 > From: Jonathan Gregory <j.m.greg...@reading.ac.uk> > To: cf-metadata@cgd.ucar.edu > Subject: Re: [CF-metadata] Standard names for mean sea level change > User-Agent: Mutt/1.5.21 (2010-09-15) > > Dear Roy > > Not quite, because it could be "understood" as that - but it's not necessary. > > Global average sea level change quantifies the volume of the world ocean, and > is not calculated necessarily by considering local changes in mean sea level. > > Best wishes > > Jonathan > > ----- Forwarded message from "Lowry, Roy K." <r...@bodc.ac.uk> ----- > > > Date: Fri, 23 Jun 2017 08:01:01 +0000 > > From: "Lowry, Roy K." <r...@bodc.ac.uk> > > To: Jonathan Gregory <j.m.greg...@reading.ac.uk>, "cf-metadata@cgd.ucar.edu" > > <cf-metadata@cgd.ucar.edu> > > Subject: Re: [CF-metadata] Standard names for mean sea level change > > > > 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 > > enquir...@bodc.ac.uk. Please also use this e-mail if your requirement is > > urgent. > > > > > > ________________________________ > > From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of Jonathan > > Gregory <j.m.greg...@reading.ac.uk> > > Sent: 22 June 2017 18:12 > > To: cf-metadata@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.pamm...@stfc.ac.uk ----- > > > > > Date: Thu, 22 Jun 2017 11:49:35 +0000 > > > From: alison.pamm...@stfc.ac.uk > > > To: r...@bodc.ac.uk, j.m.greg...@reading.ac.uk, cf-metadata@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.pamm...@stfc.ac.uk > > > STFC Rutherford Appleton Laboratory > > > R25, 2.22 > > > Harwell Campus, Didcot, OX11 0QX, U.K. > > > > > > > > > From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf Of > > > Lowry, Roy K. > > > Sent: 14 June 2017 17:40 > > > To: Jonathan Gregory; cf-metadata@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 > > > enquir...@bodc.ac.uk. Please also use this e-mail if your requirement is > > > urgent. > > > > > > ________________________________________ > > > From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of > > > Jonathan Gregory <j.m.greg...@reading.ac.uk> > > > Sent: 14 June 2017 16:58 > > > To: cf-metadata@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 > > > _______________________________________________ > > > CF-metadata mailing list > > > CF-metadata@cgd.ucar.edu > > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > CF-metadata Info Page - mailman.cgd.ucar.edu Mailing Lists > > > mailman.cgd.ucar.edu > > > This is an unmoderated list for discussions about interpretation, > > > clarification, and proposals for extensions or change to the CF > > > conventions. > > > > > > ________________________________________ > > > This message (and any attachments) is for the recipient only. NERC is > > > subject to the Freedom of Information Act 2000 and the contents of this > > > email and any reply you make may be disclosed by NERC unless it is exempt > > > from release under the Act. 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