[CF-metadata] New standard names for OMIP: physics

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Wed May 17 12:22:14 MDT 2017


Dear Stephen, Jonathan, Paul, All,

I have reviewed all the OMIP physics names that were proposed originally by Jonathan and Stephen (http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2015/057961.html). The proposals didn't receive any comments on the mailing list so I have looked at them all and my detailed comments are below.

Two of the names were agreed and published last year:

net_rate_of_absorption_of_shortwave_energy_in_ocean_layer (W m-2)
' "shortwave" means shortwave radiation. "Layer" means any layer with upper and lower boundaries that have constant values in some vertical coordinate. There must be a vertical coordinate variable indicating the extent of the layer(s). If the layers are model layers, the vertical coordinate can be model_level_number, but it is recommended to specify a physical coordinate (in a scalar or auxiliary coordinate variable) as well. Net absorbed radiation is the difference between absorbed and emitted radiation.'

sea_water_potential_temperature_at_sea_floor (K)
'Potential temperature is the temperature a parcel of air or sea water would have if moved adiabatically to sea level pressure. The potential temperature at the sea floor is that adjacent to the ocean bottom, which would be the deepest grid cell in an ocean model.'

I have accepted one more name (see 1b below). The rest of the names (36 in total) are currently under discussion. At this stage I have a lot of questions, but if we can work through them methodically we should be able to agree the names quite quickly. Most of the questions arise from how the quantities are defined.

1. Physical properties of sea water

a. square_of_brunt_vaisala_frequency_in_sea_water (s-2)
'Squared buoyancy frequency measuring ocean vertical stratification.'

The proposed name and units are consistent with the existing name square_of_brunt_vaisala_frequency_in_air so I think they are fine. To make the definition consistent with that of the existing name I suggest the following:
'The phrase "square_of_X" means X*X. Frequency is the number of oscillations of a wave per unit time. Brunt-Vaisala frequency is also sometimes called "buoyancy frequency" and is a measure of the vertical stratification of the medium.'

Okay?

For consistency, I think we should also add the last sentence to the definitions of the two existing names brunt_vaisala_frequency_in_air and square_of_brunt_vaisala_frequency_in_air as they don't currently contain any explanation of brunt-vaisala frequency.

b. specific_heat_capacity_of_sea_water (J kg-1 K-1)
'The specific heat capacity of sea water, Cp(ocean), is used in ocean models to convert between model prognostic temperature (potential or conservative temperature) and model heat content.'

The name, units and definition all look fine to me. This name is accepted for inclusion in the standard name table.

c. sea_water_salinity_at_sea_floor (1e-3)
'Salinity adjacent to the ocean bottom (at the deepest grid cell in a model).'

I assume this quantity is similar to the existing 'sea_surface_salinity' which is one of the few names where, for historical reasons, we use 'sea surface' to mean a near surface layer rather than the actual air-sea interface. By analogy it is reasonable to say that the proposed quantity is 'at_sea_floor' meaning a near floor layer. However, I'm not keen on extending this use of defined surfaces to describe layers (and we have just had a long conversation about this for the biogeochemistry names). If we take the same approach with this name as with the biogeochemistry names I think we could use the existing name sea_water_salinity and the depth (or height above sea floor) of the layer would be supplied using a scalar coordinate variable. Coordinate bounds should also be used to describe the thickness of the layer.

N. B. Although sea_water_potential_temperature_at_sea_floor has already been added to the table, we should  treat it consistently with the salinity name. If we choose to use simply 'sea_water_salinity', then we should also use 'sea_water_potential_temperature'. In that case sea_water_potential_temperature_at_sea_floor would be turned into an alias of sea_water_potential_temperature.

What do you think of this approach?

2. Integral quantities

a. integral_wrt_depth_of_product_of_sea_water_density_and_potential_temperature (kg C m-2)
'Product of grid cell thickness with density and potential temperature, summed over the depth of the ocean column. For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).'

For consistency with the syntax of existing names the 'wrt_depth' should come at the end, i.e. integral_of_product_of_sea_water_density_and_potential_temperature_wrt_depth. The canonical units of this quantity should be kg K m-2 (but it's fine to use Celsius in your files because the UDUnits software used in CF can convert Celsius to Kelvin). The wording of the definition also needs to be made consistent with existing names.

I'm not sure whether we need to include the sentence about Boussinesq models in the definition. Would the comment about using the reference density be generally true whenever a Boussinesq model is used? If so, the sentence should be included. If this way of doing the calculation is something that only applies to CMIP6 then I think it doesn't belong in the CF definition.

So, in summary, I suggest:
integral_of_product_of_sea_water_density_and_potential_temperature_wrt_depth (kg K m-2)
'The phrase "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. The phrase "product_of_X_and_Y" means X*Y. The phrase "wrt" means "with respect to". Depth is the vertical distance below the surface. Sea water density is the in-situ density (not the potential density). [For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).] Potential temperature is the temperature a parcel of air or sea water would have if moved adiabatically to sea level pressure.'

b. integral_wrt_depth_of_product_of_sea_water_density_and_conservative_temperature (kg C m-2)
'Product of grid cell thickness with density and conservative temperature, summed over the depth of the ocean column. For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).'

My comments regarding this name are the same as for proposal 2a. I suggest:
integral_of_product_of_sea_water_density_and_conservative_temperature_wrt_depth (kg K m-2)
'The phrase "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. The phrase "product_of_X_and_Y" means X*Y. The phrase "wrt" means "with respect to". Depth is the vertical distance below the surface. Sea water density is the in-situ density (not the potential density). [For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).] Conservative Temperature is defined as part of the Thermodynamic Equation of Seawater 2010 (TEOS-10) which was adopted in 2010 by the International Oceanographic Commission (IOC). Conservative Temperature is specific potential enthalpy (which has the standard name sea_water_specific_potential_enthalpy) 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. Reference: www.teos-10.org; McDougall, 2003 doi: 10.1175/1520-0485(2003)033<0945:PEACOV>2.0.CO;2.'

c. integral_wrt_depth_of_product_of_sea_water_density_and_salinity (1e-3 kg m-2)
'Product of grid cell thickness with density and salinity, summed over the depth of the ocean column. For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).'

Again my comments for this name are similar (the proposed units are fine). I suggest:
integral_of_product_of_sea_water_density_and_salinity_wrt_depth (1e-3 kg m-2)
'The phrase "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. The phrase "product_of_X_and_Y" means X*Y. The phrase "wrt" means "with respect to". Depth is the vertical distance below the surface. Sea water density is the in-situ density (not the potential density). [For Boussinesq models, density is constant Boussinesq reference density (reference_sea_water_density_for_boussinesq_approximation).] Sea water salinity is the salt content of sea water, often on the Practical Salinity Scale of 1978. However, the unqualified term 'salinity' is generic and does not necessarily imply any particular method of calculation. The units of salinity are dimensionless and the units attribute should normally be given as 1e-3 or 0.001 i.e. parts per thousand.'

What do you think?

3. Tendencies

There are 21 proposed tendency names which follow three patterns as follows:
tendency_of_sea_water_potential_temperature_expressed_as_heat_content[_due_to_PROCESS] (W m-2)
tendency_of_sea_water_conservative_temperature_expressed_as_heat_content[_due_to_PROCESS] (W m-2)
tendency_of_sea_water_salinity_expressed_as_salt_content[_due_to_PROCESS] (kg m-2 s-1).

In each case PROCESS is one of the following: advection, parameterized_mesoscale_advection, parameterized_eddy_advection, parameterized_mesoscale_diffusion, parameterized_submesoscale_advection, parameterized_dianeutral_mixing.

I have a number of general questions about these names that I would like to resolve before getting too bogged down in the details of the individual proposals. A full listing of all the outstanding OMIP proposals can be seen at http://cfeditor.ceda.ac.uk/proposals/1?status=active&namefilter=&proposerfilter=&descfilter=&unitfilter=&yearfilter=&commentfilter=OMIP&filter+and+display=Filter.

a. Heat content

We have two existing temperature_expressed_as_heat_content_names:
integral_of_sea_ice_temperature_wrt_depth_expressed_as_heat_content (J m-2)
integral_of_sea_water_potential_temperature_wrt_depth_expressed_as_heat_content (J m-2).
Are the proposed heat quantity names also column integral quantities or do they apply to a single grid cell? If the latter then I think we would need to say 'expressed_as_heat_content_of_ocean_layer'.

The units of the proposed names are W m-2 (i.e. J m-2 s-1). Perhaps it would be clearer simply to say 'tendency_of_heat_content'. They are not really temperature tendencies which would have units of K s-1. Some ocean models in CMIP6 use potential temperature as their prognostic variable and others use conservative temperature but the tendency of heat content is the same quantity in both cases, no matter how it is calculated. I suggest therefore that we don't need to distinguish between potential and conservative temperature in these names. If it is important to state which prognostic variable was used to calculate the heat content it would be better to do so in the long_name or comment attributes.

Bringing these two ideas together we would end up with names of the pattern tendency_of_heat_content_of_ocean_layer[_due_to_PROCESS] (W m-2). This has the advantage of halving the number of new heat_content names that we would need to introduce and simplifying the syntax. What do you think?

b. Salinity names

As with the heat content names, are these column integrals or layer quantities? (For now I will assume the latter).

Can we simplify tendency_of_sea_water_salinity_expressed_as_salt_content to be tendency_of_salt_content? As with the heat content names, I think it shouldn't be necessary to say which prognostic variable was used to calculate the quantity. (In theory it could be any of the salinities we have defined in CF).

Bringing these two suggestions together we would have names with the pattern 'tendency_of_salt_content_of_ocean_layer[_due_to_PROCESS] (kg m-2 s-1). What do you think?

c. Processes

The possible values of PROCESS for these names are taken from the following list: advection, parameterized_mesoscale_advection, parameterized_eddy_advection, parameterized_mesoscale_diffusion, parameterized_submesoscale_advection, parameterized_dianeutral_mixing.

Firstly, I would say that we don't need 'parameterized' in any of the names. We don't do this for other model parameterization schemes such as clouds, convection or radiation, so I think it is safe to assume that all sub-grid scale processes are parameterized without stating it explicitly.

Of the proposed processes, advection is the only one used in existing names.

The proposed tendency definitions describe 'eddy_advection' as 'parameterized eddy (e.g., submesoscale, mesoscale) advective processes' and the heat transport definitions say 'parameterized eddy advection (e.g., mesoscale, submesoscale, etc.) in an ocean model means the part due to a scheme representing parameterized eddy-induced advective effects not included in the resolved model velocity field.' We do have existing ocean names that refer to 'bolus_advection' defined as 'Bolus advection in an ocean model means the part due to a scheme representing eddy-induced effects not included in the velocity field' which sounds to me like the same thing. If they are the same, then we should either use 'bolus_advection' in the proposed names or change the existing names to use 'eddy_advection' to maintain consistency.

The proposed tendency definitions describe submesoscale_advection as 'parameterized submesoscale eddy advective processes' and the heat transport definitions describe mesoscale_advection as 'mesoscale eddy-induced effects not included in the resolved model velocity field.' Piecing this together, does this mean that 'mesoscale_advection' and 'submesoscale_advection' are both contributions to total 'eddy_advection'? In any case, if these are eddy advection processes I think it would make more sense to label them as such, i.e. mesoscale_eddy|bolus_advection and submesoscale_eddy|bolus advection.

We have existing  ocean 'diffusion' names but they make no reference to a spatial scale. I think it is fine to introduce more specific diffusion names if they are needed but we do need an explanation of what is meant by 'mesoscale' and 'submesoscale' - do they refer to particular spatial scales measured in kilometres or are they relative to the model resolution?

I think the key point with all these process names is the need to have at least a basic definition for each one. For example, what ocean processes does mesoscale_advection model as opposed to mesoscale_diffusion? Does mesoscale_diffusion in effect mean any mesoscale processes not modelled by mesoscale advection? What are the typical spatial scales represented by advection, mesoscale_advection, and submesoscale_advection? What is 'dianeutral_mixing' and on what spatial scale does it operate? It is important that we try to define these terms for the benefit of CF users who, like myself, are not experts in ocean modelling and also to prevent the possible future duplication of names due to the use of different terminology to refer to the same concept as exemplified by my question about bolus_advection / eddy_advection.

4. Stream functions

Four streamfunction names have been proposed:
 
ocean_meridional_overturning_mass_streamfunction_due_to_parameterized_mesoscale_advection (kg s-1)
ocean_y_overturning_mass_streamfunction_due_to_parameterized_mesoscale_advection (kg s-1)
ocean_meridional_overturning_mass_streamfunction_due_to_parameterized_submesoscale_advection (kg s-1)
ocean_y_overturning_mass_streamfunction_due_to_parameterized_submesoscale_advection (kg s-1)

The names in general look fine, as do the units. Whatever we agree about process names and definitions for the tendencies should also be applied to the streamfunction names.

5. Heat transport

Four heat transport names have been proposed:

northward_ocean_heat_transport_due_to_parameterized_mesoscale_advection (W)
northward_ocean_heat_transport_due_to_parameterized_submesoscale_advection (W)
northward_ocean_heat_transport_due_to_parameterized_eddy_advection (W)
northward_ocean_heat_transport_due_to_parameterized_mesoscale_diffusion (W)

The names in general look fine, as do the units. Whatever we agree about process names and definitions for the tendencies should also be applied to the heat transport names.

6. ocean_tracer_diffusivity_due_to_parameterized_mesoscale_advection (m2 s-1)
'The diffusivity used for parameterized mesoscale eddy advection (e.g.,Gent-McWilliams scheme). The specification of a physical process by the phrase "due_to_" process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.'

The name and units look okay to me. Whatever we agree about the process 'parameterized_mesoscale_advection' for the tendency names should be applied to this name too.

Best wishes,
Alison

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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.






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