[CF-metadata] Standard names for mean sea level change

Lowry, Roy K. rkl at bodc.ac.uk
Wed Jun 14 07:55:02 MDT 2017


Dear Jonathan and Alison,

A couple more things prompted by Jonathan's reply.

I totally agree with creating the aliases and like Jonathan's definition, although I wouldn't specify 19 years anywhere. Even the standards like ODN are averaged over less than 10 years. One year seems to be commonly used in data sets like Permanent Service for Mean Sea Level.

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)?

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.

With that in place nobody should use it for the types of observational data where I feel the loose usage of 'mean sea level' is inappropriate. I'll leave the issue of appropriate Standard names for this use case for a later date!!!

Cheers, Roy.

-----Original Message-----
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Gregory
Sent: 14 June 2017 13:47
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] Standard names for mean sea level change

Dear Alison

Thanks for thinking about this.

> > I put [mean] in brackets because I'm not sure whether we've decided
> > to include "mean" in MSL names (that's a different discussion). The
> > above quantities are not global average; we already have global average names.
> >
> The usual practice has been not to include 'mean' in the names but to include ' "Sea level" means mean sea level' in the definitions. There are only fourteen existing sea level names so it wouldn't require a huge number of aliases if we did decide to change them. Personally, I think it would be useful to create those aliases because it further helps to avoid potential confusion between sea_surface_height and mean_sea_level if someone is just looking through the list of names without delving too far into the definitions. Do you agree?

Yes, I do, and actually I think the number of names affected is smaller than that. I would suggest only these ones need changing:

sea_floor_depth_below_sea_level
sea_surface_height_above_sea_level
surface_geostrophic_eastward_sea_water_velocity_assuming_sea_level_for_geoid
surface_geostrophic_northward_sea_water_velocity_assuming_sea_level_for_geoid
surface_geostrophic_sea_water_x_velocity_assuming_sea_level_for_geoid
surface_geostrophic_sea_water_y_velocity_assuming_sea_level_for_geoid
tendency_of_sea_surface_height_above_sea_level

since the others are for global average sea level change.

> The next question is what do we actually mean when we say 'mean_sea_level'. I had to go back to 2012 in the mailing list archives to find any discussion of this. There was a discussion about standard names for sea level change, originally proposed by Olivier Lauret. In this discussion we established that 'mean sea level' refers to a time mean  although the actual time period is not defined (see http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2012/055733.html). At the same time as creating the aliases I think it would be useful to clarify our existing definitions to say: ' "Sea level" means the time mean of sea level at a given location.' Do you agree? Would it be useful to tie down the definition any more precisely than that, for example, would 'mean sea level' generally be regarded as something representative of a year, a decade, a century, or longer, or do we prefer to leave that completely unspecified?

The definition in principle is that it's a time-mean which eliminates all the tidal variations (about 19 years). We could say that. The result depends on the the meaning period also because of unforced variability (and forced climate change).

> I assume that the three proposed names are 2D fields which describe the variation in (time) mean sea level at each grid point compared to some previous value.
Yes.

I suggest the following definitions:
> thermosteric_change_in_mean_sea_level (m) 'Thermosteric sea level
> change is the part caused by change in density due to change in temperature i.e. thermal expansion. "Sea level" means the time mean of sea level at a given location. Zero sea level change is an arbitrary level.'
> halosteric_change_in_mean_sea_level (m) 'Halosteric sea level change
> is the part caused by change in density due to change in salinity. "Sea level" means the time mean of sea level at a given location. Zero sea level change is an arbitrary 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). Zero sea level change is an arbitrary level.'
>
> Here I have adopted the looser wording for steric definitions as currently used in the global average sea level change names. I assume that is more appropriate for these quantities than the stricter definition in the sea surface height names. Is this okay?

Yes, they are correct and clear. The definitions in this case should not refer to standard conditions.

Best wishes and thanks

Jonathan
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