Dear Rich S and Rich P,
Thanks for advising on the definition.
Rich Pawlowicz does seem to be suggesting that TEOS-10 should be the only source of conversion formulae. For absolute, preformed and reference salinity that makes sense as those quantities are defined by TEOS-10, aren?t they? Does it also provide conversion to/from Cox and Knudsen salinity?
For the less specific quantities, such as sea_water_salinity and sea_surface_salinity, the discussion in this thread suggests that these are likely to be either model output or satellite retrievals. I assume that people probably shouldn?t try converting them to one of the more precisely defined salinities as it wouldn?t be clear how to do it.
Combining these points with Nan?s suggestions and my most recent iteration of the definition leads us to:
?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 precisely defined salinity measures should be performed by application of the appropriate formulas specified by TEOS-10. 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_s
alinity (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.?
OK?
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk<mailto:J.A.Pamment at rl.ac.uk>
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
From: Signell, Richard [mailto:rsignell at usgs.gov]
Sent: 07 June 2015 11:55
To: Pamment, Alison (STFC,RAL,RALSP); Nan Galbraith
Subject: Fwd: [CF-metadata] Practical Salinity units
---------- Forwarded message ----------
From: Rich Pawlowicz <rpawlowicz at eos.ubc.ca<mailto:rpawlowicz at eos.ubc.ca>>
Date: Sunday, June 7, 2015
Subject: Re: [CF-metadata] Practical Salinity units
To: "Signell, Richard" <rsignell at usgs.gov<mailto:rsignell at usgs.gov>>
Rich - I think that is basically OK:
A very esoteric quibble is that ?conductance? should be
used in place of ?conductivity? in this sentence:
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.
because ?conductance? is actually the measured quantity, whereas
?conductivity? is the physical quantity we wish to measure, but
this raises other definitional issues that are just not worth going into
for this purpose - so leaving it as conductivity is least problematic.
Let?s see?.maybe change this sentence:
Conversion of
Practical Salinity to other salinity measures should be performed by
application of the appropriate formula, for example, using the GSW
Oceanographic Toolbox. I
to:
Conversion of Practical Salinity to other salinity measures
should be performed by application of the appropriate formulas
specified by TEOS-10.
(since the GSW toolbox merely implements these)
On Jun 4, 2015, at 12:16 PM, Signell, Richard <rsignell at usgs.gov<javascript:_e(%7B%7D,'cvml','rsignell at usgs.gov');>> wrote:
Rich,
Does this sound good?
-Rich
---------- Forwarded message ----------
From: <alison.pamment at stfc.ac.uk<javascript:_e(%7B%7D,'cvml','alison.pamment at stfc.ac.uk');>>
Date: Thu, Jun 4, 2015 at 1:52 PM
Subject: Re: [CF-metadata] Practical Salinity units
To: ngalbraith at whoi.edu<javascript:_e(%7B%7D,'cvml','ngalbraith at whoi.edu');>, cf-metadata at cgd.ucar.edu<javascript:_e(%7B%7D,'cvml','cf-metadata at cgd.ucar.edu');>
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_wat
er_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<
http://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 measure
d 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_salinit
y. Reference: www.teos-10.org<
http://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<javascript:_e(%7B%7D,'cvml','alison.pamment at stfc.ac.uk');>
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
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--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
--
Rich Pawlowicz, Associate Professor, Ocean Dynamics Laboratory
Dept. of Earth, Ocean, and Atmospheric Sciences, University of British Columbia
6339 Stores Rd., Vancouver BC CANADA V6T 1Z4
web: http://www.eos.ubc.ca/~rich tel: 604-822-1356
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
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