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. Any material supplied to NERC may be stored in > > an electronic records management system. > > ________________________________________ > > ----- End forwarded message ----- > _______________________________________________ > CF-metadata mailing list > CF-metadata@cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > ________________________________ > 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. Any material supplied to NERC may be stored in an electronic > records management system. > ________________________________ ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
