Dear All,
I have reviewed the whole of the TEOS-10 thread and the follow-up thread "SUGGESTED WAY FORWARD FOR CF-metadata:- lets finalize this discussion." This email summarizes the position reached to date in all the discussions. This is a lengthy posting so I have divided the summary into three sections: (1) the outcome of the original proposal; (2) the resulting consequences for existing names; (3) additional proposals for new names that have arisen as a result of the discussion. If there are any points that I have failed to address, please draw them to my attention.
(1) Outcome of the original proposal
The original proposal was for the following eight new standard names:
sea_water_conservative_temperature; K
change_over_time_in_sea_water_conservative_temperature; K s-1
sea_water_absolute_salinity; g kg-1
change_over_time_in_sea_water_absolute_salinity; g kg-1 s-1
sea_water_preformed_salinity; g kg-1
change_over_time_in_sea_water_preformed_salinity; g kg-1 s-1
sea_water_potential_enthalpy; J kg-1
change_over_time_in_sea_water_potential_enthalpy; J kg-1 s-1.
There has been considerable discussion of these names. There is agreement (in fact, unanimity) that we should introduce the names with the single modification that "enthalpy" should be replaced by "specific_enthalpy" to reflect the fact that the quantity is enthalpy per unit mass. This requires some reordering of the last two names, thus they will be introduced as:
specific_potential_enthalpy_of_sea_water; J kg s-1
change_over_time_in_specific_potential_enthalpy_of_sea_water; J kg-1 s-1.
These eight names are accepted for inclusion in the standard name table.
The units of the conservative temperature and potential enthalpy names can be added to the standard name table exactly as written above. The salinity quantities in g kg-1 are mass fractions and such quantities are usually given canonical units of '1' in the CF standard name table, i.e. they are dimensionless. Certainly this practice has been adopted for all the existing atmospheric chemistry names, e.g. mass_fraction_of_benzene_in_air. (Ocean biogeochemistry quantities have generally been expressed as mass concentrations (units of kg m-3) or mole concentrations (units of mol m -3)). We can prescribe canonical units of 1e-3, i.e. still dimensionless, but scaled. This has been done in the past for sea_water_salinity and related quantities. Unless anyone strongly disagrees, I propose to add the new salinity names to the standard name table with canonical units of 1e-3 and add a sentence in the explanations to say that the salinities are mass fractions usually expressed in g kg-1. The change_over_time names for
salinity would then have units of 1e-3 s-1. The full explanatory text for each name is given below.
The meaning of the names has been discussed at some length. Drawing on the original proposal, the discussion, the teos-10 documentation and the usual explanatory text for standard names I have constructed the following explanations. Please let me know if any of the text needs to be modified.
conservative_temperature: "Conservative Temperature" is defined as a term in the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. Conservative Temperature is specific potential enthalpy (which has the standard name specific_potential_enthalpy_of_sea_water) divided by a fixed value of the specific heat capacity of sea water, namely cp_0 = 3991.86795711963 J kg-1 K-1. Conservative Temperature is a more accurate measure of the "heat content" of sea water, by a factor of one hundred, than is potential temperature. Because of this, it can be regarded as being proportional to the heat content of sea water per unit mass. Enthalpy can be written either as (1) CpT, where Cp is heat capacity at constant pressure, T is absolute temperature, or (2) U+pV, where U is internal energy, p is pressure and V is volume.
change_over_time_in_sea_water_conservative_temperature: "change_over_time_in_X" means change in a quantity X over a time-interval, which should be defined by the bounds of the time coordinate. "Conservative Temperature" is defined as a term in the Thermodynamic Equation of SeaWater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. Conservative Temperature is specific potential enthalpy (which has the standard name specific_potential_enthalpy_of_sea_water) divided by a fixed value of the specific heat capacity of sea water, namely cp_0 = 3991.86795711963 J kg-1 K-1. Conservative Temperature is a more accurate measure of the "heat content" of sea water, by a factor of one hundred, than is potential temperature. Because of this, it can be regarded as being proportional to the heat content of sea water per unit mass. Enthalpy can be written either as (1) CpT, where Cp is heat capacity at constant pressure, T is absolute temperature, or (2) U+pV, where U is internal energy, p
is pressure and V is volume.
sea_water_absolute_salinity: "Absolute salinity", S_A, is defined as a term in the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. It is the mass fraction of dissolved material in sea water and TEOS-10 prescribes units of g kg-1. Absolute Salinity incorporates the spatial variations in the composition of seawater. This type of absolute salinity is also called "Density Salinity". In practice, Absolute Salinity is often calculated from Practical Salinity and a value of Absolute Salinity Anomaly which is found using computer software via a global look-up table. The version number of the software used to convert Practical Salinity to Absolute Salinity should be included by attaching a "comment" attribute to the data variable. Standard names exist for the related quantity, S_*, Preformed Salinity. Salinity observations made using measurements of the conductivity of sea water should be given the standard name of sea_water_practical_salinity.
change_over_time_in_sea_water_absolute_salinity: "change_over_time_in_X" means change in a quantity X over a time-interval, which should be defined by the bounds of the time coordinate. "Absolute salinity", S_A, is defined as a term in the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. It is the mass fraction of dissolved material in sea water and TEOS-10 prescribes units of g kg-1. Absolute Salinity incorporates the spatial variations in the composition of seawater. This type of absolute salinity is also called "Density Salinity". In practice, Absolute Salinity is often calculated from Practical Salinity and a value of Absolute Salinity Anomaly which is found using computer software via a global look-up table. The version number of the software used to convert Practical Salinity to Absolute Salinity should be included by attaching a "comment" attribute to the data variable. Standard names exist for the related quantity, S_*, Preforme
d Salinity. Salinity observations made using measurements of the conductivity of sea water should be given the standard name of sea_water_practical_salinity.
sea_water_preformed_salinity: "Preformed Salinity", S*, is defined as a term in the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. Preformed Salinity is a salinity variable that is designed to be as conservative as possible, by removing the estimated biogeochemical influences on the sea water composition. Preformed Salinity is Absolute Salinity, S_A (which has the standard name sea_water_absolute_salinity), minus all contributions to sea water composition from biogeochemical processes. Preformed Salinity incorporates the spatial variations in the composition of sea water because of its relationship to Absolute Salinity. It is a mass fraction of dissolved material in sea water and TEOS-10 prescribes units of g kg-1. Salinity observations made using measurements of the conductivity of sea water should be given the standard name of sea_water_practical_salinity.
change_over_time_in_sea_water_preformed_salinity: "change_over_time_in_X" means change in a quantity X over a time-interval, which should be defined by the bounds of the time coordinate. "Preformed Salinity", S*, is defined as a term in the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the international oceanographic community. Preformed Salinity is a salinity variable that is designed to be as conservative as possible, by removing the estimated biogeochemical influences on the sea water composition. Preformed Salinity is Absolute Salinity, S_A (which has the standard name sea_water_absolute_salinity), minus all contributions to sea water composition from biogeochemical processes. Preformed Salinity incorporates the spatial variations in the composition of sea water because of its relationship to Absolute Salinity. It is a mass fraction of dissolved material in sea water and TEOS-10 prescribes units of g kg-1. Salinity observations made using measurements of the conductivity
of sea water should be given the standard name of sea_water_practical_salinity.
specific_potential_enthalpy_of_sea_water: The potential enthalpy of a sea water parcel is the enthalpy after an adiabatic and isohaline change in pressure from its in situ pressure to the sea water pressure p = 0 dbar. "specific" means per unit mass. Enthalpy can be written either as (1) CpT, where Cp is heat capacity at constant pressure, T is absolute temperature, or (2) U+pV, where U is internal energy, p is pressure and V is volume.
change_over_time_in_specific_potential_enthalpy_of_sea_water: "change_over_time_in_X" means change in a quantity X over a time-interval, which should be defined by the bounds of the time coordinate. The potential enthalpy of a sea water parcel is the enthalpy after an adiabatic and isohaline change in pressure from its in situ pressure to the sea water pressure p = 0 dbar. "specific" means per unit mass. Enthalpy can be written either as (1) CpT, where Cp is heat capacity at constant pressure, T is absolute temperature, or (2) U+pV, where U is internal energy, p is pressure and V is volume.
(2) Consequences for existing names
(a) sea_water_salinity
Much of the discussion has centred around whether we should deprecate the use of the existing sea_water_salinity name by making it an alias of sea_water_practical_salinity. The conclusion reached is that we should not create the alias because: (i) there is in all likelihood much legacy data that has been written using the standard name sea_water_salinity and to change it to a narrower term risks altering the interpretation of those data, possibly incorrectly; (ii) some numerical models have formulations of the salinity variable that are definitely neither practical salinity nor any of the other precisely defined salinity terms.
The name sea_water_salinity will be retained as a separate entity in the standard name table. However, it was agreed during the discussion that its explanation should be modified to make the interpretation of the units more obvious and to henceforth deprecate the use of this name for salinity observations. I propose to modify the explanation to read as follows: 'The standard name sea_water_salinity is the salt content of sea water on the Practical Salinity Scale of 1978 (PSS-78), usually based on the electrical conductivity of sea water. There are standard names for the more precisely defined salinity quantities sea_water_practical_salinity, sea_water_absolute_salinity and sea_water_preformed_salinity. The more precise standard names should be used where appropriate for both modelled and observed salinities. In particular, the use of sea_water_salinity to describe salinity observations made from 1978 onwards is now deprecated in favour of the term sea_water_practical_salinity which is the salinity quantity s
tored by national data centres. The only exception to this is where the observed salinities are definitely known not to be recorded on the Practical Salinity Scale.'
(b) sea_water_temperature
There is agreement to retain the standard name sea_water_temperature as this is useful particularly for observations. It currently has no explanatory text. In response to the discussion I propose to add the following sentence: 'Sea temperature is the in situ (bulk) temperature of the sea water, not the surface or skin temperature.'
(c) units of sea_ice_salinity
While looking through the existing standard names for salinity (there are twelve in the table) I noticed that the canonical units of sea_ice_salinity are listed as 'psu'. I propose to change this to 1e-3 for consistency with all other salinity names and because psu is not really a unit but a dimensionless quantity. Psu is not part of UDunits. There have been past discussions on this mailing list of using 'psu' as a unit and I think the general conclusion, at least from the oceanography community, is that we should keep salinity units as dimensionless.
(d) Do we need to modify the explanations of all the existing salinity quantities?
As I mentioned, there are twelve salinity names already in the standard name table. We have decided to keep sea_water_salinity with an expanded explanation, but what about the other eleven? I assume we need to keep them, but do we also need to expand their explanations in line with sea_water_salinity? How is sea_ice_salinity calculated and/or observed? I think we need to be clear about what these names mean in the light of the new additions. Is there likely to be a future need to introduce tendencies, etc., for practical_salinity, absolute_salinity and preformed_salinity? The existing names are listed below for ease of reference and I would welcome comments on how to proceed:
change_over_time_in_sea_water_salinity; 1e-3
product_of_eastward_sea_water_velocity_and_salinity; m s-1
product_of_northward_sea_water_velocity_and_salinity; m s-1
sea_ice_salinity; psu (to be changed to 1e-3 as discussed above)
sea_surface_salinity; 1e-3
sea_water_salinity; 1e-3 (explanation to be modified as discussed above)
tendency_of_sea_water_salinity; 1e-3 s-1
tendency_of_sea_water_salinity_due_to_advection; 1e-3 s-1
tendency_of_sea_water_salinity_due_to_bolus_advection; 1e-3 s-1
tendency_of_sea_water_salinity_due_to_horizontal_mixing; 1e-3 s-1
tendency_of_sea_water_salinity_due_to_sea_ice_thermodynamics; 1e-3 s-1
tendency_of_sea_water_salinity_due_to_vertical_mixing; 1e-3 s-1
(3) Additional proposals for new names
(a) During the discussion it has emerged that there is also a need to introduce new names for practical salinity:
sea_water_practical_salinity
change_over_time_in_sea_water_practical_salinity
There is agreement on the need to introduce these names to act as a more precise description of salinity observations than we have previously had. They are accepted for inclusion in the standard name table. As with the absolute_salinity and preformed_salinity names I think these names should have the units of 1e-3 and 1e-3 s-1, respectively. The suggested explanations are given below.
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). This name should be used to describe salinity observations made from 1978 onwards; practical salinity is the salinity quantity stored by national data centres. 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_absolute_salinity and sea_water_preformed_salinity. Salinity quantities that do not match any of the precise definitions should be given the more general standard name of sea_water_salinity.
change_over_time_in_sea_water_practical_salinity: "change_over_time_in_X" means change in a quantity X over a time-interval, which should be defined by the bounds of the time coordinate. 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). This name should be used to describe salinity observations made from 1978 onwards; practical salinity is the salinity quantity stored by national data centres. 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_absolute_salinity and sea_water_preformed_salinity. Salinity quantities that do not match any of the precise definitions should be given the more general standard name of sea_water_salinity.
(b) sea_water_knudsen_salinity
There seems to be agreement on the need to introduce this name for salinity observations dating between 1901 and 1978. Am I correct in thinking its units should also be 1e-3? I am not really familiar with this quantity, but a small amount of reading around the subject seems to suggest that it should be regarded as a mass ratio and that it is determined from the chlorinity of a sea water sample using the relationships S=0.03+1.805Cl between 1901 and 1966 and S=1.80655Cl from 1967 onwards. Chlorinity seems to have been variously defined as the mass in grams of silver that reacts with 328.5233g of sea water and from 1967 onwards via a relationship with the conductivity of sea water. Suggestions on how to improve this explanation would be most welcome.
(c) sea_water_reference_salinity
There has been a suggestion that this quantity should also be introduced as a standard name. No comments have so far been received on this proposal.
I hope this is an accurate summary of everything that has been discussed so far. Provided we can agree on the final wording of the explanations, all the names that are now "accepted" can be added at the next update of the standard name table. I intend to make the update during the first half of December.
Best wishes,
Alison
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Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
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Received on Fri Nov 25 2011 - 13:03:58 GMT
