I think "sea_surface_salinity" is practical salinity at the surface - and
should be dimensionless, or have units of '1'. There are probably many
other versions of PSAL in the table, I'm just a little pressed for time
right
now and can't find them all.
So I think we need to find all the salinities that are not identified as
absolute salinity and give them canonical units of '1'. The definition
of preformed salinity includes ' Preformed Salinity is Absolute Salinity'
so I guess it keeps its units.
Thanks - Nan
. On 6/2/15 12:27 PM, Signell, Richard wrote:
Nan, Alison, and Co:
I agree also. We should use "1" for "practical_salinity", and in the
comments say that to estimate other salinity variables such as
absolute_salinity, a formula must be used (perhaps such as those
provided in the GSW toolbox).
And one more thing: I see also that in CF Standard Name table 27 we
have "g kg-1" for "absolute_salinity" and "preformed_salinity", but
"0.001" for "sea_surface_salinity" and many other salinities. While
numerically they are the same, with "0.001" it's not clear whether
this is a percentage by mass or volume. Since nobody measures
salinity as "ml liter-1", we should specify "g kg-1" for all
salinities other than "practical_salinity", right?
This would be consistent with: http://www.teos-10.org/pubs/TEOS-10_Manual.pdf.
Screenshots here (and attached):
http://screencast.com/t/I3COnk3a
http://screencast.com/t/Zbk6uLJP643
-Rich
On Tue, Jun 2, 2015 at 12:22 PM, <alison.pamm...@stfc.ac.uk> wrote:
Dear Nan, All,
Reading through what you and Rich saying i.e. that the values are (and probably always
have been) in the range 0-40, then I agree that changing the canonical unit is not likely
to cause problems with existing data, which was my main concern. In fact, I think using
"1" does make more sense than 1 e-3 if practical salinity is a number on a
relative scale rather than having any direct relation to concentration. So I now support
this change. I will need to put some more thought into the definitions - I'm a little
pushed for time today but will have another think about this tomorrow.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/Centre for Environmental Data Archival Email: alison.pamm...@stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
-----Original Message-----
From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf
Of Nan Galbraith
Sent: 02 June 2015 15:47
To: cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] Salinity units
Hi All -
Can we move on this question?
I think the real problems with the unit .001, as mentioned by various
people
elsewhere in this thread, are that it invites users to treat it as a
scale factor,
or to mistake it for g/kg.
Would it be possible to change canonical units to '1' and to add to the
definitions
a statement that the 'units' field can't be used to convey a scale
factor, that
scaling of data has to be done in another field (is it scale_factor?)
for dimensionless
variables? I think this would protect existing datasets from
misinterpretation -
basically making '.001' equivalent to '1' for these data variable.
It seems unlikely that there are existing CF datasets that were written
using '1'
as a practical salinity unit, intending it to convey that data should be
divided by
1000 to return to its non-dimensional state. If there are such datasets
out there,
they presumably have the standard name table version included
somewhere in
their metadata - but it would be very surprising if the unit has been
used this way.
I don't see any down side to making this change, since it moves CF closer to
the widely-accepted view on PS units, without endangering any existing
data sets.
OceanSITES is champing at the bit to have this resolved, so I'd hate to
see us
drop the discussion at this point.
Cheers - and thanks -
Nan
On 5/27/15 1:56 PM, Signell, Richard wrote:
For all these salinity datasets, the actual data values for salinity
are in the range of 0-40, not 0.0-0.040. And I don't think people
want that to change. So the problem is users understanding the
difference between values of 0-40 that *are not* supposed to be used
as "g/kg" and values of 0-40 that *are* supposed to be used as "g/kg".
On Wed, May 27, 2015 at 1:11 PM,<alison.pamm...@stfc.ac.uk> wrote:
Dear Nan, All,
Certainly this topic has come up several times and we never seem to
quite
get to a solution that suits everybody.
I don’t know why 1e-3 was originally chosen for use in the standard
name
table, but even if you go back to version 1 it is in there, which means it
was agreed prior to 2006 when the CF website at PCMDI was set up.
The last time the question of salinity units was aired in detail was
during
the TEOS-10 discussions in 2011. Unfortunately, the mailing list archive
seems to be unavailable at the moment, but I can vouch for the fact
that the
current definitions of the salinity names came from the very detailed
discussions that we had at that time. In particular, we added the
following
wording to the definition of sea_water_practical_salinity: ‘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.’
Once upon a time (back in 2009) there was a discussion about allowing
CF to
use ‘psu’ as a unit in its own right, but I think the TEOS-10 discussion
made it clear that ‘psu’ is not really a unit at all, so that idea was
dropped and we continued to use 1e-3.
We should remember that the canonical unit of ‘1e-3’ doesn’t prevent
anyone
using ‘1’ in their files if they prefer it, and vice versa. As Jim has
already pointed out, UDunits can certainly cope with that. So in one
sense,
it doesn’t really matter to CF which we choose as the canonical unit as
long
as we can agree and, most importantly, make the definition really
really
clear so that consumers of the files know how to interpret the data.
In previous discussions there has never been unanimous agreement
about
whether it is better to use ‘1e-3’ or ‘1’. My niggling concern about
changing the unit after all these years is whether it will lead to
misinterpretation of existing data files. Is that going to be a problem?
We
have in the past changed the canonical units of standard names, but
only to
correct outright errors, rather than to change the interpretation of a
name.
How big a problem is it for the oceanographic community if we don’t
change
the unit?
If we do decide to go with ‘1’ as the canonical unit, is there a reference,
such as TEOS-10, which we can use to support our decision? It would be
useful to include it in the definition and hopefully reduce the need to
keep
revisiting this same question.
Either way, I think we can improve further on the definition to help
people
better understand the data.
Best wishes,
Alison
From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On
Behalf Of Nan
Galbraith
Sent: 27 May 2015 15:45
To: Rich Pawlowicz
Cc:cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] Salinity units
Hi all -
The '.001' units for P.S. doesn't mean that stored values of practical
salinity differs from A.S. by 'a factor of around a 1000', as far as I
know. If that's the logical inference, then this unit is really a problem,
and maybe we should do something about it.
I wish my CF email archive went back a little further, because there's
nothing (since 2004) that I can find that explains the rationale for
this unit. It certainly *looks* like a compromise between a unit for a
non-dimensional variable and PPT ... When this was originally under
discussion, way back when, I'll bet someone argued that it would
eventually
be a big problem. I'd really love to see that email thread!
Cheers -
Nan
On 5/26/15 11:52 AM, Rich Pawlowicz wrote:
I’m not sure what the best answer is either, but I think the “correct”
way
is
to have people deal with Practical Salinity in some special fashion in
their workflow, because it *is* defined in a weird way that is generally
incompatible with the general idea of ‘quantities with units’) - getting
a salinity definition that is aligned with the way all other quantities
in the world are defined was one of the motivating factors behind
TEOS-10!
So, essentially people would have to make their own choice about
what to do with ‘practical salinity’ for whatever they are doing.
I will point out, though, that having two kinds of data that differ
numerically by a factor of around a 1000 is a good way of getting
them to realize that they really are not exactly compatible - you
wouldn’t *want* Practical Salinity and Absolute salinity on the same
plot (“look - salinity increased by 0.16 g/kg everywhere in 2010!”)
But I understand that one might want to make this as painless as
possible.
On May 26, 2015, at 8:48 AM, Signell, Richard<rsign...@usgs.gov>
wrote:
Rich,
Thanks for this. Yes, I guess my concern is that folks will do a
catalog search for *salinity* variables, and with a few spot checks,
see that they are have data values in the range of 29-36 or so, and
then go ahead and run a workflow that converts all units using the
units attribute. And if "practical salinity" has units of "1" and
"absolute salinity" has units of "g/kg" = "0.001", then the data might
not appear on that fixed y-axis plot with [29 36]. But I don't
have a good alternative. I guess we have to rely that people will
realize from the standard_names that for comparison, you need to
estimate absolute salinity from practical salinity using tools like
GSW toolbox.
-Rich
On Fri, May 22, 2015 at 3:59 PM, Rich
Pawlowicz<rpawlow...@eos.ubc.ca>
wrote:
Ummm…I’m not entirely what you are asking, but
a) PSS-78 Practical Salinity is a dimensionless number. It was defined
such that "the numerical values of practical salinity would be similar to
the
numerical values of previous salinity data, when expressed in ‰”, but
it isn’t in fact ppt or anything, and you shouldn’t be multiplying it up or
down by factors of 1000.
b) "Previous salinity data”, (Cox or Knudsen salinity) which
was obtained from titrations, does in fact represent a
mass fraction of something (because you are titrating
with a mass of silver). This was denoted by the ppt ‘unit'.
c) TEOS-10 Absolute Salinity is also a mass fraction (of dissolved solute
on the Reference Composition Salinity Scale). However, nowadays the
SI brochure suggests that different quantities should be distinguished
by their symbols, not their units. So, there isn’t actually a
recommended
unit for Absolute Salinity. You can write
S_A = 35 g/kg = 0.035 kg/kg = 35000 mg/kg
or, again using SI rules and treating the units as a ‘thing’:
S_A/(g/kg) = 35
and any of these are valid - the same way lengths can be in
meters or km or mm or whatever is handy (this is also
true for preformed salinity).
‘ppt’ is discouraged as a unit of mass fraction because (for example) it
could be confused with ‘part per trillion’
Now, the gsw toolbox assumes ‘g/kg’ for its TEOS-10 salinity inputs
and outputs, but YOU don’t have to do that if you don’t want to.
I admit it is a little magic how we can ESTIMATE Absolute Salinity (with
units) from Practical Salinity (without units), but keep in mind that this
is only ONE possible way of estimating Absolute Salinity, and in fact it
is
a method that is metrologically somewhat suspect because of the
definition of PSS-78. S_A could also be obtained from density
measurements, for example - and then there is some other
conversion factor involving different units.
Rich.
On May 22, 2015, at 1:01 PM, Signell, Richard<rsign...@usgs.gov>
wrote:
Roy,
For sure dimensionless. But "1.0", "0.001" or "g/kg"?
The latest version (27) of the CF Standard Name list
(http://cfconventions.org/Data/cf-standard-names/27/build/cf-
standard-name-table.html)
states:
sea_water_salinity: "0.001"
sea_water_absolute_salinity: "g kg-1"
sea_water_practical_salinity: "0.001"
sea_water_preformed_salinity: "g kg-1"
sea_water_cox_salinity: "0.001"
and units packages, of course, would treat "g kg-1" the same as "0.001".
Yet in the IOC manual on equation of seawater:
http://www.teos-10.org/pubs/TEOS-10_Manual.pdf
it states (PDF page 176, printed page 166) that Practical Salinity
should have units of "1", while "Absolute Salinity" (the argument used
in the toolbox functions) and "Preformed Salinity" (used in numerical
ocean models) should have units "g kg-1".
So it appears that TEOS agrees with CF on units for Absolute Salinity
and Preformed Salinity, but not on Practical Salinity.
And OceanSites (as least here:
http://www.oceansites.org/docs/OS_PAP-3_201205_P_deepTS.txt)
is using "sea_water_practical_salinity" with units of "1", so they are
consistent with the TEOS publication, but not the current CF
convention (v27).
On the TEOS site, there is software to calculate Absolute Salinity
from Practical Salinity. So it would seem that the technically
correct thing to do would be to use the "gsw_SA_from_SP" routine to
convert OceanSites Practical Salinity (in units of "1") to Absolute
Salinity (in units of "g/kg") before comparing with the "Preformed
Salinity" output "g/kg" from ocean models.
I'm pretty confused though, so I'm cc'ing Rich Pawlowicz on this,
hoping for his input.
Thanks,
-Rich
On Fri, May 22, 2015 at 1:49 PM, Lowry, Roy K.<r...@bodc.ac.uk> wrote:
Dimensionless. Please????
This is the view of physical oceanographers for whom I have the
greatest
respect.
Cheers, Roy.
________________________________________
From: Reyna Jenkyns [re...@uvic.ca]
Sent: 22 May 2015 18:06
To:cf-metadata@cgd.ucar.edu; OceanSITES Data Management Team;
Nan Galbraith
Subject: Re: [CF-metadata] Salinity units
I'm interested in this topic since I didn't realize what had been
discussed
previously, and now I think we must be non-compliant as well. Is this
documented formally in the CF documentation?
Reyna Jenkyns | Data Stewardship Team Lead - Digital Infrastructure
Ocean Networks Canada | T 250 853 3908 | oceannetworks.ca
University of Victoria PO Box 1700 STN CSC 2300 McKenzie Avenue
Victoria, BC
V8W 2Y2
________________________________________
From: CF-metadata<cf-metadata-boun...@cgd.ucar.edu> on behalf of
Nan
Galbraith<ngalbra...@whoi.edu>
Sent: Friday, May 22, 2015 10:03 AM
To:cf-metadata@cgd.ucar.edu; OceanSITES Data Management Team
Subject: Re: [CF-metadata] Salinity units
Hello all -
It's been a long time, but is anyone interested in re-visiting the subject
of units for practical salinity in CF?
I was recently notified that my salinity data was likely to be
overlooked by
some users, because I'd used '1' as the units, not '.001'. Somehow, I'd
forgotten the (long-ago) discussion on the CF list about salinity units.
Some members of the OceanSITES project are interested in revising
our
format spec to encourage the use of '1' as an indication that salinity
does
not have units - but, of course, we'd mostly rather remain CF-
compliant.
Thanks for any feedback on this.
Cheers - Nan
On 6/17/09 2:48 AM, Lowry, Roy K wrote:
Dear All,
During an exercise with Alison mapping the CF Standard Names to a
units vocabulary in the BODC vocabulary server I noticed that the
units for salinity were '1.00E-03', i.e. parts per thousand. My
understanding in that since the introduction of the Practical
Salinity Scale that salinity is dimensionless with units of '1'. Is
there agreement for our changing the units in the Standard Name
table?
Cheers, Roy.
--
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