[CF-metadata] additional standard name for ISMIP6

alison.pamment at stfc.ac.uk alison.pamment at stfc.ac.uk
Wed Jan 25 09:47:35 MST 2017


Dear Sophie,

I have been reviewing the status of the ISMIP6 standard name proposals. Many of the names can now be accepted for publication, others are very close to being accepted and I am aware that a few are still under discussion within the ISMIP6 group itself. I will be updating the standard name and area type tables shortly and will be including the 'accepted' names listed below. There will be another update in February by which time I hope we will have finalised more of the ISMIP6 names.

1. temperature_at_top_of_ice_sheet_model (K)
'The quantity with standard name temperature_at_top_of_ice_sheet_model is the upper boundary temperature that is used to force ice sheet models. It is the temperature at the interface between the ice sheet and the overlying medium which may be snow or the atmosphere. In all instances the temperature is that of the interface itself and not that of the medium above or below the interface.'

I think the name and units are now agreed. I have also made the wording of the definition consistent with that of proposal (2), which has been discussed earlier and agreed, so this name is now accepted for publication in the standard name table.

2. temperature_at_base_of_ice_sheet_model (K)
'The quantity with standard name temperature_at_base_of_ice_sheet_model is the lower boundary temperature that is used to force ice sheet models. Beneath ice shelves it is the temperature at the ice-ocean interface. Beneath grounded ice, it is the temperature at the ice-bedrock interface. In all instances the temperature is that of the interface itself and not that of the medium above or below the interface.'

I have used Sophie's suggestion of making the syntax of the name consistent with proposal (1). We have already agreed the units and the definition so this name is now accepted for publication in the standard name table.

3. land_ice_surface_melt_flux (kg m-2 s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The surface called "surface" means the lower boundary of the atmosphere. The land_ice_surface_melt_flux is the loss of ice mass resulting from surface melting. For an area-average, the cell_methods attribute should be used to specify whether the average is over the area of the whole grid cell or the area of land ice only. There is also a standard name for the quantity surface_snow_and_ice_melt_flux.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

4. land_ice_basal_specific_mass_balance_flux (kg m-2 s-1)
' “Land ice” means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Specific mass balance" means the net rate at which ice is added per unit area. A negative value means loss of ice. For an area-average, the cell_methods attribute should be used to specify whether the average is over the area of the whole grid cell or the area of land ice only. "Basal specific mass balance" means the net rate at which ice is added per unit area at the land ice base.'

The name, units and definition have been discussed and agreed. I have reordered the sentences in the definition but otherwise not changed the content, so this name is now accepted for publication in the standard name table.

I have also made small modifications to the definition of the existing name land_ice_surface_specific_mass_balance_flux to make it consistent with this proposal. I have separated out the definition of "specific mass balance" from the bit about surface processes and added the sentence advising about cell_methods.

5. land_ice_specific_mass_flux_due_to_calving (kg m-2 s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. "Specific mass flux due to calving" means the change in land ice mass per unit area resulting from iceberg calving. A negative value means loss of ice. For an area-average, the cell_methods attribute should be used to specify whether the average is over the area of the whole grid cell or the area of land ice only. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. 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, units and definition are now agreed. At Sophie's request I have added the sentence 'A negative value means mass loss'. This name is now accepted for publication in the standard name table.

6. land_ice_specific_mass_flux_due_to_calving_and_ice_front_melting (kg m-2 s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Specific mass flux due to calving and ice front melting" means the change in land ice mass per unit area resulting from iceberg calving and melting on the vertical ice front. A negative value means loss of ice. For an area-average, the cell_methods attribute should be used to specify whether the average is over the area of the whole grid cell or the area of land ice only. 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, units and definition are now agreed. At Sophie's request I have added the sentence 'A negative value means mass loss'. This name is now accepted for publication in the standard name table.

7. water_flux_into_sea_water_from_land_ice (kg m-2 s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. The water flux into sea water from land ice is the freshwater entering as a result of runoff from the surface and base of the ice and melting from the ice shelf base and vertical ice front. For an area-average, the cell_methods attribute should be used to specify whether the average is over the area of the whole grid cell or the area of the ocean portion only.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

8. land_ice_surface_x_velocity (m s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. A velocity is a vector quantity. "x" indicates a vector component along the grid x-axis, positive with increasing x. The surface called "surface" means the lower boundary of the atmosphere.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

9. land_ice_surface_y_velocity (m s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. A velocity is a vector quantity. "y" indicates a vector component along the grid y-axis, positive with increasing y. The surface called "surface" means the lower boundary of the atmosphere.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

10. land_ice_surface_upward_velocity (m s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. A velocity is a vector quantity. "Upward" indicates a vector component which is positive when directed upward (negative downward). The surface called "surface" means the lower boundary of the atmosphere.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

11. land_ice_basal_upward_velocity (m s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. A velocity is a vector quantity. "Upward" indicates a vector component which is positive when directed upward (negative downward).'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

> 
> > 12. magnitude_of_shear_stress_at_land_ice_base (Pa)
> > ' The magnitude of the shear stress at land ice base'
> > 13. magnitude_of_normal_stress_at_land_ice_base (Pa)
> > ' The magnitude of the normal stress at land ice base'
> > 14. magnitude_of_longitudinal_stress_at_land_ice_base (Pa)
> > ' The magnitude of the longitudinal stress at land ice base'
> >
> > The syntax of these three names is consistent with existing sea ice names.
> We have one existing "magnitude of stress" name,
> magnitude_of_surface_downward_stress, which represents the  magnitude of
> the x/y or eastward/northward surface stress components and specifies
> "downward", i.e. positive when momentum is transferred from upper to lower
> medium. Should your quantities be regarded as "downward" (momentum
> passing from the ice to the underlying surface) or "upward"? Units of Pa are
> fine.
> >
> > The definitions will of course depend on the sign conventions of the stresses:
> > ' "magnitude_of_X" means magnitude of a vector X. ["Downward" indicates a
> vector component which is positive when directed downward (negative
> upward).] OR [" Upward" indicates a vector component which is positive when
> directed upward (negative downward).]'
> > Please can you also provide some brief descriptions of ice shear stress,
> normal stress and longitudinal stress that I can add into the definitions?
> Answer Sophie: we are working of trying to find good definitions, so I will come
> back to you for this point.

These names are still under discussion.

15. upward_geothermal_heat_flux_at_ground_level_in_land_ice (W m-2)
'"Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. "ground_level" means the land surface (including beneath snow, ice and surface water, if any). In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. "Upward" indicates a vector component which is positive when directed upward (negative downward). The quantity with standard name upward_geothermal_heat_flux_at_ground_level_in_land_ice is the upward heat flux at the interface between the ice and bedrock. It does not include any heat flux from the ocean into an ice shelf.'

The name and units have already been agreed and I have written the definition to be consistent with our earlier discussion and existing standard names. This name is now accepted for publication in the standard name table.

> 
> > 16. land_ice_mass (kg)
> > 'The mass of land ice (glaciers, ice caps, ice sheet and ice shelves), computed
> as the ice volume times density.'
> >
> > The name and units are fine.
> >
> > We have two existing names, sea_ice_mass (currently undefined) and
> sea_water_mass defined as ' The quantity with standard name
> "sea_water_mass" is the total mass of liquid seawater in the global oceans,
> including enclosed seas.' Is your quantity globally integrated like the sea_water
> name or does it refer to the mass in an individual grid cell? We need to be clear
> in the definition. I think the method of calculation doesn't belong in the
> standard name definition (you could add it elsewhere, e.g., the comment
> attribute, for CMIP6 data).
> Answer Sophie: land_ice_mass is a quantity that is integrated for the whole ice
> sheet (i.e.: a 1 dimensional variable that varies with time), so it would be the
> total mass of the ice sheet, and therefore more similar to the
> “sea_water_mass"

The name and units are agreed. I suggest the following as the definition:
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. The quantity with standard name land_ice_mass is the total mass integrated over an area of land ice. The geographical extent of the ice over which the mass was calculated should be described by providing bounds on the horizontal coordinate variables."

OK?

This name is still under discussion.

> 
> > 17. land_ice_mass_not_displacing_sea_water (kg)
> > 'The mass of land ice that does not displace sea water if removed, computed
> as the ice volume times density. It excludes ice shelves, and components of the
> grounded ice sheet that would not displace sea level (for example, in regions
> where the ice rests on a bedrock that sits below sea level).'
> >
> > The name and units are fine.
> >
> > As per my comments on proposal 16, is this a globally integrated or local
> quantity? Again, I don't think the method of calculation belongs in the standard
> name definition but it could be reported elsewhere in the metadata.
> Answer Sophie: same answer as proposal 16

The name and units are agreed. I suggest the following as the definition:
' "Land ice not displacing sea water" means land ice that would not alter sea level if removed. It excludes ice shelves and grounded ice-sheets resting on bedrock that is below sea level. It includes glaciers, floating ice caps and ice-sheets resting on bedrock above sea level. The quantity with standard name land_ice_mass_not_displacing_sea_water is the total mass integrated over an area of land ice. The geographical extent of the ice over which the mass was calculated should be described by providing bounds on the horizontal coordinate variables.'

Is the bit about glaciers and ice caps correct? I put it in because the definition of land_ice usually refers to them and I want to be as clear as possible about how this name differs from the others.

This name is still under discussion.

> 
> > 18. grounded_ice_sheet_area (m2)
> > ' Total area of the grounded ice sheet, where grounded indicates that the ice
> rests on bedrock, and therefore excludes ice shelves.'
> >
> > Units of m2 are fine. This name brings to mind a recent discussion we had
> about two existing names, sea_ice_area and sea_ice_extent and I think there
> are two possible approaches for your name.
> >
> > The first approach would be to use the name as proposed, in which case it
> would need to be defined similarly to other X_area names:
> > ' "X_area" means the horizontal area occupied by X within the grid cell.
> "Grounded ice sheet" means glaciers, ice-caps and ice-sheets resting on
> bedrock. It does not include ice shelves.'
> > Taking this route means that the name itself does not represent a total. You
> would instead need to include a cell_methods of "area: sum" and coordinate
> variables with bounds to delineate the area over which the land ice total area
> was calculated.
> >
> > The second approach would be to have a name of
> grounded_ice_sheet_extent. You would need to supply coordinate variables
> with bounds to indicate the region of the earth over which the ice extent was
> calculated. By analogy with the sea_ice_extent name, this quantity would be
> regarded as a total area (so you wouldn't need the cell_methods), but it's
> important to note that it would be the total area of the grid cells containing
> land ice, rather than  that of the land ice itself. Following the sea_ice_extent
> definition, the definition of your quantity would then be something like the
> following (you may or may not need the wording about thresholds, depending
> on how you identify your grid cells):
> > ' The term grounded_ice_sheet_extent means the total area of all grid cells in
> which the grounded ice sheet area fraction equals or exceeds a threshold, often
> chosen to be 15 per cent. The threshold must be specified by supplying a
> coordinate variable or scalar coordinate variable with the standard name of
> grounded_ice_sheet_area_fraction. The horizontal domain over which the
> grounded ice sheet extent is calculated is described by the associated
> coordinate variables and coordinate bounds or by a coordinate variable or
> scalar coordinate variable with the standard name of "region" supplied
> according to section 6.1.1 of the CF conventions. "Grounded ice sheet" means
> glaciers, ice-caps and ice-sheets resting on bedrock. It does not include ice
> shelves.'
> >
> > Either approach would be OK with me, so the choice of which name to use
> really depends on the details of how you calculate the area
> Answer Sophie: What we want may be slightly different than what you propose
> as options. The quantity that we are after is a scalar value (not spatially
> resolved) for the total area covered by grounded ice sheet. The number that we
> are after can be calculated by multiplying grounded_ice_sheet_area_fraction
> with grid cell area and summing that over the entire grid.
>

Okay, so the quantity you need is the actual area of the grounded ice sheet. Unfortunately, we can't just call it 'area' because the definition of the existing names refers to the area within a grid cell. I think we would have to call it an integral. You say that it is summed over the entire grid, so is this a truly global quantity? If so, we could perhaps call it global_integral_of_grounded_ice_sheet_area by analogy with the existing global_average_sea_level names. If you want to be able to calculate the quantity for individual ice sheets then we'd have to call it something like integral_of_grounded_ice_sheet_wrt_xy, where xy essentially means the horizontal coordinates. You could then supply bounds on the coordinate variables to show the geographical extent of the area integral. What do you think?

This name is still under discussion.

> 
> > 19. floating_ice_shelf_area (m2)
> > 'Total area of the floating ice shelves, which is the land ice component that
> flows over sea water.'
> >
> > Units of m2 are fine. As with proposal 18, this name could be the one
> proposed, or floating_ice_shelf_extent, with the appropriate definitions and
> accompanying coordinates and cell_methods attributes as appropriate. Again
> the best choice depends on the details of how the area is calculated.
> Answer Sophie: same as with proposal 18.

I agree that proposals 18 and 19 are very similar. If we can agree (18) then this one will follow the same pattern.

This name is still under discussion.

20. grounded_ice_sheet_area_fraction (1)
' "X_area_fraction" means the fraction of horizontal area occupied by X. "Grounded ice sheet" indicates where the ice sheet rests over bedrock and is thus grounded. It excludes ice-caps, glaciers and floating ice shelves.

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

21. floating_ice_sheet_area_fraction (1)
' "X_area_fraction" means the fraction of horizontal area occupied by X. A "floating ice sheet", sometimes called an "ice shelf", indicates where the ice sheet is flowing over sea water.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

22. land_ice_runoff_flux (kg m-2 s-1)
'"Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. Runoff is the liquid water which drains from land. If not specified, "runoff" refers to the sum of surface runoff and subsurface drainage. Runoff flux over land ice is the difference between any available liquid water in the snowpack due to rainfall and melting minus any refreezing and liquid water retained in the snowpack. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

> 
> > 23. magnitude_of_land_ice_basal_drag (Pa)
> > 'The magnitude of basal drag at land ice base.'
> >
> > I assume this quantity is essentially a deceleration of the land ice flow
> towards the sea? Is it perhaps the sum of the stress terms in proposals 12 - 14?
> Please can you provide a bit more detail about what is causing the drag?
> >
> Answer Sophie: we are still working on the definitions, and will get back to you
> :)

This name is still under discussion.

> 
> > 24. tendency_of_land_ice_mass_due_to_surface_mass_balance (kg s-1)
> > 'The total surface mass balance flux over land ice is a spatial integration of
> the surface mass balance flux.'
> >
> > The name and units look sensible to me. You would need to use coordinate
> variables with bounds to describe the area over which the mass integration is
> calculated. We have the existing name
> land_ice_surface_specific_mass_balance_flux and clearly this new quantity is
> the spatial integration of that one.  Now I'm wondering what is really meant
> here by  'surface' - does it mean the lower boundary of the atmosphere or the
> upper boundary of the ice (which may have snow lying on it)? I'm guessing the
> latter, so we may need to use something like 'ice_surface' in both the new and
> existing names to distinguish from our usual use of 'surface'.
> >
> > I suggest the following wording for the definition:
> > ' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and
> also includes ice-shelves. Mass balance means the net rate at which ice is
> accumulated. A negative value means loss of ice. "tendency_of_X" means
> derivative of X with respect to time. 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 tendency in ice mass due to the [ice] surface mass
> balance is the spatial integral of the quantity with standard name
> land_ice_surface_specific_mass_balance_flux. The horizontal domain over
> which the quantity is calculated is described by the associated coordinate
> variables and coordinate bounds or by a coordinate variable or scalar
> coordinate variable with the standard name of "region" supplied according to
> section 6.1.1 of the CF conventions.'
> > We would also need some words about 'surface' or 'ice_surface' as
> appropriate. (The wording should also be updated if we decide to drop 'flux'
> from the mass balance name as discussed in proposal 4).
> 
> Answer Jonathan (10oct2016)
> I think surface may be correct in CF terms. For SMB, the snow and ice-sheet
> are not usually distinguished. If we mean exactly the mass added to the
> ice-sheet component for ISMIP6, beneath the snow pack, then it should be
> _at_top_of_ice_sheet_model, as in the interface temperature.
> 
> Answer Sophie: yes, this quantity is a spatial integration of the existing name
> land_ice_surface_specific_mass_balance_flux, which was in existence in the
> CF world before ISMIP6 (with definition "Land ice" means glaciers, ice-caps and
> ice-sheets resting on bedrock and also includes ice-shelves. Specific mass
> balance means the net rate at which ice is added per unit area at the land ice
> surface. A negative value means loss of ice. In accordance with common usage
> in geophysical disciplines, "flux" implies per unit area, called "flux density" in
> physics.).
> 
> I had assumed that the existing definition for
> land_ice_surface_specific_mass_balance_flux was what Jonathan describes
> above for SMB (or surface mass balance). If you think that it would be better to
> change the name (and perhaps, create a new name for what I thought was
> SMB), then please let me know. For consistency, would we then have to also
> replace “basal” by “_at_the_bottom_of_ice_sheet_model” in items such as 25
> below? Or perhaps, we could rename:
> land_ice_surface_specific_mass_balance_flux to
> land_ice_specific_surface_mass_balance_flux? Any suggestions more than
> welcomed!

Reading through Jonathan and Sophie's comments, it seems that I was trying to be more precise than is really necessary. If snow and ice are not generally distinguished for the purposes of surface mass balance then I think we shouldn't change the existing name land_ice_surface_specific_mass_balance_flux. So for this name we would have
tendency_of_land_ice_mass_due_to_surface_mass_balance (kg s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. "tendency_of_X" means derivative of X with respect to time. The surface called "surface" means the lower boundary of the atmosphere. "Mass balance" means the net rate at which ice is added. A negative value means loss of ice. The tendency in ice mass due to the surface mass balance is the spatial integral of the quantity with standard name land_ice_surface_specific_mass_balance_flux. The horizontal domain over which the quantity is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the CF conventions. 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.'

OK?

This name is still under discussion.

25. tendency_of_land_ice_mass_due_to_basal_mass_balance (kg s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. "Mass balance" means the net rate at which ice is accumulated. A negative value means loss of ice. "tendency_of_X" means derivative of X with respect to time. The tendency of land ice mass is the spatially integrated mass balance. 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 tendency in ice mass due to the basal mass balance is the spatial integral of the quantity with standard name land_ice_basal_specific_mass_balance_flux. The horizontal domain over which the quantity is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the
CF conventions.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

26. tendency_of_land_ice_mass_due_to_calving (kg s-1)
' "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves. "tendency_of_X" means derivative of X with respect to time. 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 tendency in ice mass due to calving is the spatial integral of the quantity named land_ice_specific_mass_flux_due_to_calving. The horizontal domain over which the quantity is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the CF conventions.'

The name, units and definition have all been agreed so this name is now accepted for publication in the standard name table.

A1. ice_sheet
'An area type of "ice sheet" indicates where ice sheets are present. It includes both grounded ice sheets resting over bedrock and ice shelves flowing over the ocean, but excludes ice-caps and glaciers (in contrast to land_ice, which includes all components).'

The area type and definition are agreed. This name is accepted for publication in the area_type table.

A2. floating_ice_shelf
'An area type of "floating ice shelf" indicates where ice shelves are present. Ice shelves are the component of ice sheets that flow over the ocean.'

The area type and definition are agreed. This name is accepted for publication in the area_type table.

A3. grounded_ice_sheet
'An area type of "grounded ice sheet" indicates where the ice sheet rest over bedrock and is thus grounded. It excludes ice-caps, glaciers and floating ice shelves.'

The area type and definition are agreed. This name is accepted for publication in the area_type table.

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.



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