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

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Wed Jun 14 05:19:46 MDT 2017


Dear Jonathan, All,

In the NEMO discussion thread we have just agreed three new sea surface height change names. At the end of that discussion Jonathan proposed that we should also add three similar names for changes in mean sea level:

> Although I haven't an immediate use-case, I would say it's very likely that the
> corresponding stdnames for mean sea level change will be wanted sometime.
> They
> are quantities which I have often calculated and plotted, for example, but not
> yet archived in CF-netCDF files! If we add those quantities to the standard
> name table now as well, it might avoid people using the SSH names when really
> they mean MSL. That is, I'd propose we also add
> 
> thermosteric_change_in_[mean_]sea_level
> halosteric_change_in_[mean_]sea_level
> steric_change_in_[mean_]sea_level
> 
> 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?

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?

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. 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?

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.






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