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; 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; 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. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
