[CF-metadata] New standard names for NEMO ocean model output

From: alison.pamment at stfc.ac.uk <alison.pamment>
Date: Tue, 4 Apr 2017 10:05:48 +0000

Dear Kevin, Jonathan, Sebastien, Antonio, et al,

As promised, I have looked again at all the NEMO names and definitions. Thanks for all the helpful comments in the discussion. I think now we are on the points of being able to accept these names. Please could you have a look at the suggestions below and let me know if you are happy with them.

1. bottom_pressure_equivalent_height (m)

The discussion has established that the bottom pressure represents the mass of the water column at a given location (for unit area P = M * g = rho * g * h, where M is mass, g is acceleration due to gravity and h is thickness of water column). It then follows simply that h = M / rho.

Jonathan's suggestion of sea_water_mass_per_unit_area_expressed_as_thickness therefore seems like a better way to express this concept and I support this idea. For the definition I suggest
' "Thickness" means the vertical extent of a layer. The quantity with standard name sea_water_mass_per_unit_area_expressed_as_thickness is the thickness of the water column from sea floor to surface, minus any contribution to column thickness from steric changes. The sum of the quantities with standard names sea_water_mass_per_unit_area_expressed_as_thickness and ocean_steric_thickness is the total thickness of the sea water column. Sea_water_mass_per_unit_area is the mass per unit area of the sea water contained within each grid cell. The extent of an individual grid cell is defined by the horizontal coordinates and any associated coordinate bounds or by a string valued auxiliary coordinate variable with a standard name of "region".

Jonathan asked what density is used to convert mass to thickness, which would be useful information to include in the definition, and Sebastien has suggested the following wording:
'The sea water density used to do the conversion is assumed to be the density of water of standard temperature T=0?C and practical salinity S=35.0. Otherwise it should be provided as an auxiliary scalar using the standard name sea_water_density.'

I like this suggestion (sea_water_density is an existing standard name). If Kevin and the NEMO team are happy with Sebastien's text and mine I will include both in the final definition.

Because proposals (1) and (3) are closely linked I will comment next on the third one.

3. ocean_steric_thickness (m)

Jonathan has suggested that we might call this one ocean_steric_height_above_sea_level, but to be honest I prefer the thickness name. It is more consistent with the name we seem to be moving towards for proposal (1) and also I don't think mentioning sea level really adds anything to understanding the quantity. My understanding from one of Kevin's earlier posts is that the NEMO team are willing to accept a thickness name.

For the definition I'd suggest:
'"Thickness" means the vertical extent of a layer. The ocean halosteric thickness quantifies the thickness by which a column of water with standard temperature T=0?C and practical salinity S=35.0 would change if its temperature and salinity were changed to the observed values. The sum of the quantities with standard names sea_water_mass_per_unit_area_expressed_as_thickness and ocean_steric_thickness is the total thickness of the sea water column. The sum of the quantities with standard names ocean_halosteric_thickness and ocean_thermosteric_thickness is the total steric thickness of the water column, which has the standard name of ocean_steric_thickness.'

OK?

2. ocean_mixed_layer_thickness_defined_by_vertical_tracer_diffusivity_threshold (m)

This one was originally proposed as ocean_turbocline_depth, then ocean_turbocline_thickness, but Jonathan has posted his offline discussion with Eric Boisseson and they seem to have now agreed on the above version of the name.

The definition would then be:
' "Thickness" means the vertical extent of a layer. The ocean mixed layer is the upper part of the ocean, regarded as being well-mixed. Diffusivity is also sometimes known as the coefficient of diffusion. Diffusion occurs as a result of a gradient in the spatial distribution of mass concentration, temperature or momentum. The diffusivity may be very different in the vertical and horizontal directions. The diffusivity threshold should be specified by associating a coordinate variable or scalar coordinate variable with the data variable and giving the coordinate variable a standard name of ocean_vertical_tracer diffusivity.'

OK?

2a. During the discussion of this new name, Jonathan has also proposed that we turn the existing name ocean_mixed_layer_thickness_defined_by_vertical_tracer_diffusivity into an alias and replace it with ocean_mixed_layer_thickness_defined_by_vertical_tracer_diffusivity_deficit. The new version of the name would keep the original definition as follows:
'The ocean mixed layer is the upper part of the ocean, regarded as being well-mixed. The base of the mixed layer defined by temperature, sigma, sigma_theta, or vertical diffusivity is the level at which the quantity indicated differs from its surface value by a certain amount. The amount by which the quantity differs can be specified by a scalar coordinate variable.'

OK?

4. ocean_halosteric_thickness (m)

The definition for this name would be:
' "Thickness" means the vertical extent of a layer. The ocean halosteric thickness is the contribution of the salinity of the water column to the ocean steric thickness. It quantifies the thickness by which a column of water with standard practical salinity S=35.0 would change if its temperature and salinity were changed to the observed values. The sum of the quantities with standard names ocean_halosteric_thickness and ocean_thermosteric_thickness is the total steric thickness of the water column, which has the standard name of ocean_steric_thickness.'

OK?

5. ocean_thermosteric_thickness (m)

The definition for this name would be:
' "Thickness" means the vertical extent of a layer. The ocean thermosteric thickness is the contribution of the temperature of the water column to the ocean steric thickness. It quantifies the thickness by which a column of water with standard temperature T=0?C would change if its temperature were changed to the observed value. The sum of the quantities with standard names ocean_halosteric_thickness and ocean_thermosteric_thickness is the total steric thickness of the water column, which has the standard name of ocean_steric_thickness.'

OK?

6. ratio_of_sea_water_potential_temperature_anomaly_to_relaxation_timescale (K s-1)

This one was originally proposed as temperature_profile_anomaly_correction (K s-1) but Jonathan suggested the above version and this has been accepted by the NEMO team.

The definition of the above name would be:
'The quantity with standard name ratio_of_sea_water_potential_temperature_anomaly_to_relaxation_timescale is a correction term applied to modelled sea water potential temperature. The term is estimated as the deviation of model local sea water potential temperature from an observation-based climatology (e.g. World Ocean Database) weighted by a user-specified relaxation coefficient in s-1 (1/(relaxation timescale)). Potential temperature is the temperature a parcel of air or sea water would have if moved adiabatically to sea level pressure. The phrase "ratio_of_X_to_Y" means X/Y. The term "anomaly" means difference from climatology.'

The text about 'potential temperature', 'X/Y' and 'anomaly' is the same as used in the definitions of many existing CF standard names.

OK?

7. practical_salinity_profile_anomaly_correction (s-1)

This one hasn't received much comment, but I think we should apply the same reasoning as in proposal (6) and change it to
ratio_of_sea_water_practical_salinity_anomaly_to_relaxation_timescale (s-1). I see that Sebastien is suggesting the same approach.

The definition would be:
''The quantity with standard name ratio_of_sea_water_practical_salinity_anomaly_to_relaxation_timescale is a correction term applied to modelled sea water practical salinity. The term is estimated as the deviation of model local sea water practical salinity from an observation-based climatology (e.g. World Ocean Database) weighted by a user-specified relaxation coefficient in s-1 (1/(relaxation timescale)). The phrase "ratio_of_X_to_Y" means X/Y. The term "anomaly" means difference from climatology. Practical Salinity, S_P, is a determination of the salinity of sea water, based on its electrical conductance. The measured conductance, corrected for temperature and pressure, is compared to the conductance of a standard potassium chloride solution, producing a value on the Practical Salinity Scale of 1978 (PSS-78). This name should not be used to describe salinity observations made before 1978, or ones not based on conductance measurements. Conversion of Practical Salinity to other precisely defined salinity me
asures should use the appropriate formulas specified by TEOS-10. Other standard names for precisely defined salinity quantities are sea_water_absolute_salinity (S_A); sea_water_preformed_salinity (S_*), sea_water_reference_salinity (S_R); sea_water_cox_salinity (S_C), used for salinity observations between 1967 and 1977; and sea_water_knudsen_salinity (S_K), used for salinity observations between 1901 and 1966. Salinity quantities that do not match any of the precise definitions shoul d be given the more general standard name of sea_water_salinity. Reference: www.teos-10.org; Lewis, 1980 doi:10.1109/JOE.1980.1145448.'

The text about 'practical salinity', 'X/Y' and 'anomaly' is the same as used in the definitions of many existing CF standard names.

OK?

8. integral_of_sea_water_practical_salinity_wrt_depth (m)
9. integral_of_sea_water_practical_salinity_wrt_total_depth (m)

As previously, I'll take these two together because they are essentially the same quantity, just integrated over different thicknesses of the ocean. Having made that statement I can now immediately see that we should use just one name for both. At one stage I suggested using integral_of_sea_water_practical_salinity_wrt_depth_in_ocean_layer for (8) and ocean_integral_of_sea_water_practical_salinity_wrt_to_depth (9), but Antonio Cofi?o pointed out that this is actually inconsistent with existing names. I think he's right and I now disagree with my own earlier suggestion!

I now think the name we should be using for both is proposal (8):
integral_of_sea_water_practical_salinity_wrt_depth.

As Antonio points out, this is very similar in concept to the existing name integral_of_sea_water_potential_temperature_wrt_depth_expressed_as_heat_content.

The definition would be:
' "integral_of_Y_wrt_X" means int Y dX. The data variable should have an axis for X specifying the limits of the integral as bounds. "wrt" means with respect to. Depth is the vertical distance below the surface. Practical Salinity, S_P, is a determination of the salinity of sea water, based on its electrical conductance. The measured conductance, corrected for temperature and pressure, is compared to the conductance of a standard potassium chloride solution, producing a value on the Practical Salinity Scale of 1978 (PSS-78). This name should not be used to describe salinity observations made before 1978, or ones not based on conductance measurements. Conversion of Practical Salinity to other precisely defined salinity measures should use the appropriate formulas specified by TEOS-10. Other standard names for precisely defined salinity quantities are sea_water_absolute_salinity (S_A); sea_water_preformed_salinity (S_*), sea_water_reference_salinity (S_R); sea_water_cox_salinity (S_C), used for salinity obser
vations between 1967 and 1977; and sea_water_knudsen_salinity (S_K), used for salinity observations between 1901 and 1966. 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.'

The sentence 'The data variable should have an axis for X specifying the limits of the integral as bounds' is very important here because it explains how the depth over which the integration takes place should be specified in the metadata. As before, the text about practical salinity is for consistency with existing standard name definitions.

If we adopt this approach, proposal (9) can be dropped. This also means that we would not need to create an alias for the existing name integral_of_sea_water_potential_temperature_wrt_depth_expressed_as_heat_content.

OK?

Best wishes,
Alison

------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
Received on Tue Apr 04 2017 - 04:05:48 BST