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, [email protected] 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_w
ater_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 measu
red 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_salin
ity. 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: [email protected]
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
--
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* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
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