[CF-metadata] Practical Salinity units

From: alison.pamment at stfc.ac.uk <alison.pamment>
Date: Fri, 5 Jun 2015 13:35:59 +0000

Dear Nan,

Thanks for checking through the definition. We've been focusing on sea_water_practical_salinity but, for the record, I should state that any changes we agree here will also be applied to change_over_time_in_sea_water_practical_salinity.

The sentence about remote sensing calibration was added as part of the TEOS-10 discussions. Certainly satellite retrievals that are not practical salinity should use the name sea_surface_salinity. I don't have any objection to removing the sentence as it doesn't directly affect the interpretation of the data values and one would hope that, if additional metadata were needed, people would add it anyway.

I'm happy with your suggestion to remove the sentence about PSS/psu and amend the following sentence.

I notice you removed the sentence about conversion of practical salinity to other salinity measures. I had put it in because Rich suggested it earlier in this thread - don't you think it might be helpful to users? In the definitions of Cox and Knudsen salinities we do provide some text about how to convert between the two, and also convert to practical salinity, so it wouldn't be unprecedented to include something like that.

Regarding the short names later on in the definition, I appreciate that they make it a bit less readable but I'd rather keep them in because they are actually the terms used in the TEOS-10 documentation to refer to those quantities. In common with the discussion we have just been having in the flux thread, we can use the salinity definitions to show what the various terms are called by other communities. Also, the definitions of cox, knudsen, absolute, preformed and reference salinity all cross-reference one another in a similar way to the practical salinity definition and if I change one then, for consistency, I should probably change them all.

So, as a compromise, can we agree on the following (with or without the conversion sentence)?

Practical Salinity is defined on the Practical Salinity Scale of 1978 (PSS-78) and is calculated from the electrical conductivity of sea water (as well as temperature and pressure). Practical Salinity is not a direct measure of salt concentration, rather, it is a relative scale calibrated against a standard potassium chloride solution containing 32.4356 g kg-1 of KCl whose conductivity is measured at 15 degrees Celsius. [Conversion of Practical Salinity to other salinity measures should be performed by application of the appropriate formula, for example, using the GSW Oceanographic Toolbox.] This name should not be used to describe salinity observations made before 1978, or ones not based on conductivity measurements. There are also standard names for the precisely defined salinity quantities sea_water_knudsen_salinity (S_K), used for salinity observations between 1901 and 1966, sea_water_cox_salinity (S_C), used for salinity observations between 1967 and 1977, sea_water_absolute_salinity (S_A), sea_water_
preformed_salinity (S_*), and sea_water_reference_salinity. Salinity quantities that do not match any of the precise definitions should be given the more general standard name of sea_water_salinity. Reference:www.teos-10.org; Lewis, 1980 doi:10.1109/JOE.1980.1145448

Best wishes,
Alison

------
Alison Pamment Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.


> -----Original Message-----
> From: Nan Galbraith [mailto:ngalbraith at whoi.edu]
> Sent: 04 June 2015 20:17
> To: Pamment, Alison (STFC,RAL,RALSP); cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Practical Salinity units
>
> Thanks, Alison.
>
> Should remote sensing data be called practical salinity, or the
> less-specific sea surface
> salinity? The definition pretty clearly states that practical salinity
> is derived from
> conductivity, so I wonder if we need to include anything at all about
> remote sensing
> data in this definition. Since I work with in situ data, this doesn't
> really matter to
> me, except that it does make the definition ... long and slightly confusing.
>
> Also, I'm a little concerned that the description of the alternate units
> - the ones we
> don't want people to use - might add confusion. Instead of:
>
> 'Officially S_P is dimensionless so that, while convenient, and while it
> is common
> practice, it is not officially sanctioned to say S_P = 35 psu. Often
> authors use PSS-78, as in
> S_P = 35 PSS-78'
>
> could we just say:
>
> 'Practical Salinity is not a direct measure of salt concentration, rather,
> it is a relative scale calibrated against a standard potassium chloride
> solution, and is therefore non-dimensional (canonical units='1')' ?
>
> Last, I think all the interspersing of short names is unneeded, so could
> we shorten
>
> 'There are also standard names for the precisely defined salinity
> quantities sea_water_knudsen_salinity,
> S_K (used for salinity observations between 1901 and 1966),
> sea_water_cox_salinity, S_C (used
> for salinity observations between 1967 and 1977),
> sea_water_absolute_salinity, S_A,
> sea_water_preformed_salinity, S_*, and sea_water_reference_salinity'
>
> to:
>
> 'There are also standard names for the precisely defined salinity quantities
> sea_water_knudsen_salinity, used for salinity observations between 1901
> and 1966,
> sea_water_cox_salinity, used for salinity observations between 1967 and
> 1977,
> sea_water_absolute_salinity, sea_water_preformed_salinity, and
> sea_water_reference_salinity. '
>
> Here's what I came up with using (mostly just) those changes
>
> Practical Salinity is defined on the Practical Salinity Scale of 1978
> (PSS-78)
> and is calculated from the electrical conductivity of sea water (as well
> as temperature
> and pressure). Practical Salinity is not a direct measure of salt
> concentration,
> rather, it is a relative scale calibrated against a standard potassium
> chloride
> solution containing 32.4356 g kg-1 of KCl whose conductivity is measured
> at 15
> degrees Celsius. If salinity is measured using remote sensing techniques
> and not conductivity,
> then it is recommended that additional metadata (calibration/validation
> information)
> be described in the variable comment attribute. This name should not be
> used to describe
> salinity observations made before 1978, or ones not based on
> conductivity measurements.
>
> There are also standard names for the precisely defined salinity quantities
> sea_water_knudsen_salinity,used for salinity observations between 1901
> and 1966
> sea_water_cox_salinity, used for salinity observations between 1967 and
> 1977,
> sea_water_absolute_salinity, sea_water_preformed_salinity, and
> sea_water_reference_salinity.
> Salinity quantities that do not match any of the precise definitions
> should be
> given the more general standard name of sea_water_salinity. Reference:
> www.teos-10.org; Lewis, 1980 doi:10.1109/JOE.1980.1145448
>
> Thanks again!
> Nan
>
>
>
> On 6/4/15 1:52 PM, alison.pamment at stfc.ac.uk wrote:
> > Dear Nan, All,
> >
> > I think we have reached agreement to change the canonical units of
> Practical Salinity from 1e-3 to 1. I've now had some time to work on the
> definition.
> >
> > Currently in the standard name table we have:
> > sea_water_practical_salinity (canonical units: 1e-3)
> > Practical Salinity, S_P, is defined on the Practical Salinity Scale of 1978
> (PSS-78) and is calculated from the electrical conductivity of sea water (as
> well as temperature and pressure). Officially S_P is dimensionless so that,
> while convenient, and while it is common practice, it is not officially
> sanctioned to say S_P = 35 psu. Often authors use PSS-78, as in S_P = 35 PSS-
> 78. If salinity was measured using remote sensing techniques and not
> conductivity, then it is recommended that additional metadata
> (calibration/validation information) be described in the variable comment
> attribute. This name should be used to describe salinity observations made
> from 1978 onwards (Practical Salinity is the salinity quantity stored by
> national data centres for post-1978 observations). The only exception to this
> is where the observed salinities are definitely known not to be recorded on
> the Practical Salinity Scale. There are also standard names for the precisely
> defined salinity quantities sea_water_knudsen_salinity, S_K (used for
> salinity observations between 1901 and 1966), sea_water_cox_salinity, S_C
> (used for salinity observations between 1967 and 1977),
> sea_water_absolute_salinity, S_A, sea_water_preformed_salinity, S_*, and
> sea_water_reference_salinity. Salinity quantities that do not match any of
> the precise definitions should be given the more general standard name of
> sea_water_salinity. Reference: www.teos-10.org; Lewis, 1980
> doi:10.1109/JOE.1980.1145448
> >
> > I suggest we change this to the following (note the addition of the 4th -
> 6th sentences):
> > sea_water_practical_salinity (canonical units: 1)
> > Practical Salinity, S_P, is defined on the Practical Salinity Scale of 1978
> (PSS-78) and is calculated from the electrical conductivity of sea water (as
> well as temperature and pressure). Officially S_P is dimensionless so that,
> while convenient, and while it is common practice, it is not officially
> sanctioned to say S_P = 35 psu. Often authors use PSS-78, as in S_P = 35 PSS-
> 78. Practical Salinity is not a direct measure of salt concentration and the
> units should not be interpreted as describing concentration in g kg-1 or parts
> per thousand as is the case with some other measures of salinity. Rather,
> Practical Salinity is a relative scale calibrated against a standard potassium
> chloride solution containing 32.4356 g kg-1 of KCl whose conductivity is
> measured at 15 degrees Celsius. Conversion of Practical Salinity to other
> salinity measures should be performed by application of the appropriate
> formula, for example, using the GSW Oceanographic Toolbox. If Practical
> Salinity was measured using remote sensing techniques and not
> conductivity, then it is recommended that additional metadata
> (calibration/validation information) be described in the variable comment
> attribute. This name should be used to describe salinity observations made
> from 1978 onwards (Practical Salinity is the salinity quantity stored by
> national data centres for post-1978 observations). The only exception to this
> is where the observed salinities are definitely known not to be recorded on
> the Practical Salinity Scale. There are also standard names for the precisely
> defined salinity quantities sea_water_knudsen_salinity, S_K (used for
> salinity observations between 1901 and 1966), sea_water_cox_salinity, S_C
> (used for salinity observations between 1967 and 1977),
> sea_water_absolute_salinity, S_A, sea_water_preformed_salinity, S_*, and
> sea_water_reference_salinity. Salinity quantities that do not match any of
> the precise definitions should be given the more general standard name of
> sea_water_salinity. Reference: www.teos-10.org; Lewis, 1980
> doi:10.1109/JOE.1980.1145448
> >
> > This definition is turning into something of an essay, but I think that is
> preferable to continued confusion about the meaning of the quantity or the
> interpretation of its units. Does it fit the bill? (The scale_factor attribute,
> mentioned in an earlier post, is defined solely for the purpose of data
> packing and should not be used to indicate the scaling of units, e.g.
> interpreting 0.001 as meaning 1.) Also, is the GSW Toolbox a software
> library? Is there a reference to it?
> >
> > Any names that mention absolute_salinity, preformed_salinity or
> reference_salinity will definitely keep their units of g kg-1. All these names
> were added as the result of a long discussion of TEOS-10 quantities and it
> was very carefully specified what their units should be.
> >
> > Roy's enquiry to the TEOS-10 group also indicates that the quantities
> sea_water_cox_salinity and sea_water_knudsen_salinity should keep their
> units of 0.001. Indeed, their definitions already specify that they should be
> expressed as 'unitless as a mass fraction per mil (0/00) or "parts per
> thousand" ' so it is very clear how to interpret them.
> >
> > Rich, Craig and Roy discussed sea_surface_salinity. At the moment this is a
> generic name and not tied to any precise definition of salinity. (The
> standard name definition makes a statement to that effect). The question
> was raised as to whether sea_surface_salinity should be regarded as
> Practical Salinity: the answer appears to be definitely "no". There is
> therefore no proposal to change the units of this quantity at present so it
> will remain as 0.001. The definition will also remain unchanged.
> >
> > Other generic salinity names (nine in total) including sea_ice_salinity and
> sea_water_salinity also currently have units of 0.001. As with
> sea_surface_salinity I don't think there is actually a specific proposal to
> change them so we should leave the units alone. As Jonathan has pointed
> out, these generic quantities are particularly useful for model data and I
> don't think that we should be trying to tie them down any more precisely.
> >
> > So what this now boils down to is that we are only discussing changes to
> the canonical units of sea_water_practical_salinity and the accompanying
> definition. If we can agree the definition text then I think the change can be
> accepted for publication in the standard name table.
> >
> > Best wishes,
> > Alison
> >
> > ------
> > Alison Pamment Tel: +44 1235 778065
> > NCAS/Centre for Environmental Data Archival Email:
> alison.pamment at stfc.ac.uk
> > STFC Rutherford Appleton Laboratory
> > R25, 2.22
> > Harwell Oxford, Didcot, OX11 0QX, U.K.
> >
> >
> >
>
>
> --
> *******************************************************
> * Nan Galbraith Information Systems Specialist *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 (508) 289-2444 *
> *******************************************************
>
>
Received on Fri Jun 05 2015 - 07:35:59 BST