Hi.
I'm with Dan on this. I'm also feeling more and more confused by what
everyone thinks units of '0.001' is supposed to mean. A number of the
people writing on this topic appear to be asserting that this is some
sort of dimensional unit. Neither 1, 0.001, nor percent are dimensional
units, are they? They are all ways of referencing pure number
quantities, as far as I can tell. When playing with the UDUNITS2
software, they are all considered as simple variations in scale factor.
So, what are you intending for me to understand when you tell me that
'older' salinity has units of 0.001 and Practical Salinity has units of 1?
Grace and peace,
Jim
On 6/3/15 6:03 AM, Hollis, Dan wrote:
Hi all,
I'm not a user of salinity data, nor am I an expert on CF. However my
impression of this discussion is that the problem lies with the fact that the
canonical units appear to be used to represent two different properties i.e.
the dimensions and the units.
I would say that there are two classes of dimensionless quantity:
Those that genuinely have no units e.g. beaufort_wind_force
Those obtained by taking the ratio of other quantities that do have dimensions,
such that the dimensions cancel out in the result
My feeling is that in both cases the canonical unit should always be '1'
However for the second category the user still needs to know what units the
data are in (so that they can convert to other units, compare with other
datasets etc) i.e. the user needs to know if the values are in g/kg or kg/kg.
Surely this is the job of the units attribute, not the canonical units.
Note that this distinction applies to, and is more obvious for, dimensional
quantities. As I understand it, if the canonical units are m/s this does not
stop me from storing my data in km per hour or miles per second (or anything
else understood by udunits), I just need to indicate the actual units via the
units attribute.
So, I think we just need to extend this same thinking to dimensionless
quantities.
Apologies if I've misunderstood the issues...
Dan
Dan Hollis Climatologist
Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United
Kingdom
Tel: +44 (0)1392 886780 Fax: +44 (0)1392 885681
E-mail: dan.hol...@metoffice.gov.uk Website: http://www.metoffice.gov.uk
For UK climate and past weather information, visit
http://www.metoffice.gov.uk/climate
-----Original Message-----
From: CF-metadata [mailto:cf-metadata-boun...@cgd.ucar.edu] On Behalf Of John
Helly
Sent: 03 June 2015 08:51
To: Lowry, Roy K.; GODAE craig.don...@esa.int
Cc: cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] FW: Salinity units
Late to the party. TEOS-10 suggests that archival should be PSS and then
that is converted (using TEOS-10) to Absolute Salinity (Sa) for thermo
calculations.
J.
On 6/2/15 9:23 PM, Lowry, Roy K. wrote:
Hi Craig,
Supports having the units for sea surface salinity as 0.001. I certainly don't
want sea surface salinity to default to Practical Salinity.
Cheers, Roy.
-----Original Message-----
From: Craig Donlon [mailto:craig.don...@esa.int]
Sent: 03 June 2015 08:19
To: Lowry, Roy K.
Cc: cf-metadata@cgd.ucar.edu; rpawlow...@eos.ubc.ca
Subject: Re: [CF-metadata] FW: Salinity units
Roy
We have 3 satellites measuring sea surface salinity so we need to be a bit
careful here.
I believe guidance from CF for his variable will help standardise the approach
space agencies and projects are taking. Certainly not PSU!!
All the best
Craig
--
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Principal Scientist for Oceans and Ice
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On 3 Jun 2015, at 09:12, Lowry, Roy K. <r...@bodc.ac.uk> wrote:
Dear All,
As I suspected, the message below indicates a preference by the physical
oceanographers involved in TEOS-10 to use 0.001 for 'older style' salinities.
This works for me. All we need to do is to prevent semi-intelligent (dumb?)
data aggregation systems doing automatic units harmonisation on salinity and
creating a 3 order of magnitude error. Salinity interconversions for data
aggregation should be always be done using the relevant algorithm.
Sea surface salinity is a real fly in the ointment because it can either be a
model output field (units 0.001) or an observation, which could either be
'older' (pre-78) salinity (units 0.001) or Practical Salinity (units 1). Rich
Signell's suggestion of discontinuing usage of sea surface salinity is
something I argued for (but lost) a decade ago and it isn't an argument I wish
to resurrect because of the risk or re-opening the sea surface temperature can
of worms. As most sea surface salinity in CF is likely to be model output I
would suggest keeping 0.001 as the Canonical unit and recommending that if it's
Practical Salinity then label it as Practical Salinity.
Cheers, Roy.
-----Original Message-----
From: Rich Pawlowicz [mailto:rpawlow...@eos.ubc.ca]
Sent: 03 June 2015 00:26
To: Lowry, Roy K.
Subject: Re: [CF-metadata] Salinity units
Roy - I have to confess I am not entirely sure of the ramifications of making
changes or how units are modified (by powers of 10, for example) in the CF
system. I would agree with g/kg
for units on the TEOS-10 Reference Composition Salinity Scale. I am not sure
I would use
the same unit for old-style titration-salinities; keeping 0.001 might
be best for those
RIch.
On Jun 2, 2015, at 10:06 AM, Lowry, Roy K. <r...@bodc.ac.uk> wrote:
Dear Rich,
Do you have any comment on the CF community adopting g/kg as the canonical unit
for all forms of salinity other than Practical Salinity?
Regards,
Roy Lowry (oceanography domain CF Standards Names Committee member)
________________________________________
From: Signell, Richard [rsign...@usgs.gov]
Sent: 02 June 2015 17:27
To: Alison Pamment
Cc: CF metadata
Subject: Re: [CF-metadata] Salinity units
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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