[CF-metadata] additional standard name for ISMIP6

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 10 Oct 2016 17:26:50 +0100

Dear Alison and Sophie

> 4. land_ice_basal_specific_mass_balance_flux (kg m-2 s-1)
> 'Specific mass balance means the net rate at which ice is added per unit area at the land ice base. Computed as the total basal mass balance on the land ice portion of the grid cell divided by land ice area in the grid cell. A negative value means loss of ice.'
>
> The proposed name is consistent with the existing name land_ice_surface_specific_mass_balance_flux (which is why I suggested it in the first place!) and the units are fine. Looking again at the existing name and its definition, I am starting to wonder whether we really need the word "flux" at all (perhaps Jonathan Gregory can comment on this).

You're right that would more usually be called just basal mass balance, without
"flux". I can't remember whether we discussed it before. Including "flux" makes
it more obviously consistent with the many other "mass flux" names. I don't
have an opinion.

> For the definition I suggest the following (the bit in square brackets would only be needed if we keep "flux" in the name):
> ' "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 base.

Specific mass balance in general means the net rate at which ice is added per
unit area. This particular name, with "basal", is for the part of the mass
balance at the base.

> 5. land_ice_specific_mass_flux_due_to_calving (kg m-2 s-1)
> 'Loss of ice mass resulting from iceberg calving. Computed as the rate of mass loss by the ice shelf (in kg s-1) divided by the horizontal area of the ice sheet (m2) in the grid box.'
>
> I think calling this one a mass flux does make sense because it is a loss from the ice shelf and so there is a real flux from the shelf to floating icebergs. This is not a net quantity so it is a mass flux rather than a mass balance. However, I'm now thinking that we don't really need "specific" in this name because it's essentially just repeating the "per unit area" so perhaps this name should simply be land_ice_mass_flux_due_to_calving. Again I'd welcome comments on this point.

In the context of CF standard names I agree that mass flux would mean kg m-2
s-1. However in glaciology "mass flux due to calving" would often be under-
stood as kg s-1 (or Gt yr-1) and hence including "specific" is useful to be
clear what we mean.

I'd say the same for the next one

> 6. land_ice_specific_mass_flux_due_to_calving_and_ice_front_melting (kg m-2 s-1)

> 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?

I would like to clearer about this too. Sorry, Sophie! In general there could
be nine components of a 3D tensor, each one with two signs, because each refers
to two dimensions. In CF standard names, when we say "magnitude", we usually
mean an unsigned quantity e.g. speed cf. the vector velocity.

> 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 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.

Best wishes

Jonathan
Received on Mon Oct 10 2016 - 10:26:50 BST