Standard Names: Bedrock .. and heat fluxes out of bedrock

[martinjuckes]

There are 3 standard names which refer to bedrock, and a few more mention it in their definitions. Some of the wording appears to be ambiguous.

The intention of the term "bedrock_altitude" also needs clarification: it is supposed to give the altitude of bedrock beneath land ice and ocean: but if it really means "bedrock", why does it not apply to the altitude of bedrock beneath soil? I suspect that it is intended to refer to the height of solid earth beneath land-ice and ocean, which may be silt or rock. It appears to be primarily used in the land-ice context, in which case the neglect of the debris between ice and rock may be OK ... but what is the purpose of referring to the altitude of bedrock below ocean?

(1) In the standard names bedrock_altitude and associated tendency and change terms, bedrock is defined as "the solid Earth surface beneath land ice or ocean water." This definition is inappropriate for two reasons. Firstly, there will often be soil or sediment between ice or water and the bedrock and, secondly, bedrock may occur beneath the atmosphere. A more appropriate definition (from Enc. Britannica) is "solid rock that is typically buried beneath soil and other broken or unconsolidated material". There may be a need for a new term to represent the altitude of the solid earth.

(2) The term upward_geothermal_heat_flux_at_ground_level_in_land_ice is defined to be the upward heat flux in bedrock beneath ice, but there is likely to be a difference in the heat flux coming out of the ground and the heat flux into the ice due to horizontal transports at the interface associated with basal run-off (which is represented in some CMIP6 models).

There are comparable terms for sea ice, downwelling_shortwave_flux_in_sea_water_at_sea_ice_base and soil, upward_heat_flux_at_ground_level_in_soil, which might be useful in giving us patterns to follow.

upward_geothermal_heat_flux_in_bedrock_below_land_ice might work. I have used below_land_ice rather that _at_land_ice_base because there might be something between the land ice and the bedrock ... but is that what the people using this term want?

(3) "bedrock" also occurs in other standard name definitions. For example, land_ice_vertical_mean_x_velocity has the line "The vertical mean land ice velocity is the average from the bedrock to the surface of the ice" which clearly incorrect when the land ice is in the form of a floating ice shelf. Here, I think the it would be simpler to say "The vertical mean land ice velocity is the average from the base to the surface of the ice".

[martinjuckes]

A clarification on use of bedrock: up until version 12 of the standard name table, land_ice was defined as "resting on bedrock", but this was modified to include floating ice in version 13.

bedrock_altitude and associated terms were introduced in 2004, with reference to some discussions in the glaciology community.

The standard usage in geology appears to be that the lithosphere is made up of "regolith" (loose, heterogeneous superficial deposits) and "bedrock", and soil makes up part, but not all, of the regolith. There appears, however, to be a difference of usage when it comes to the solid Earth under the oceans, where lithosphere appears to mean only the rock layers, and sediment is considered as "lithogenous" (i.e. contributing to the creation of the lithosphere, but not part of it).

[feggleton]

Hi Martin,

Thank you for identifying and raising this issue, it was well spotted. Apologies this has taken some time to comment on, it seems you have done a fair bit of reading into this and I wanted to do the same (although I maybe spent more time trying to understand things!). I agree that the bedrock phrase needs amending. As you mentioned, when these terms and consequently the phrase was coined it was done only in relation to ice-sheet models and discussions at that time did not identify the need for it to be in other contexts. However, it is obviously important that any changes do not change the meaning of these terms and the changes are backwards compatible. As you said there are currently 3 names which contain the word bedrock and the phrase in their definitions. There are also 42 other names which contain some mention of bedrock in their definitions. However, I don't think it will be necessary to amend all of these, this will depend on the phrase used and the context as most are in reference to land ice and ice sheets. There are 6 phrases (which are used in the definitions) which mention bedrock, two of which are not in the land ice context.

grounded_ice_sheet : "Grounded ice sheet" indicates where the ice sheet rests over bedrock and is thus grounded. It excludes ice-caps, glaciers and floating ice shelves.

upward_geothermal_heat_flux_at_ground_level_in_land_ice: 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.

land_ice_verticalmean._velocity: The vertical mean land ice velocity is the average from the bedrock to the surface of the ice.

land_ice: "Land ice" means glaciers, ice-caps and ice-sheets resting on bedrock and also includes ice-shelves.

(two with no mention of land ice)

bedrock_altitude_change: The zero of bedrock altitude change is arbitrary.

bedrock: "Bedrock" is the solid Earth surface beneath land ice or ocean water. [only used in the 3 names mentioned]

'bedrock' looks to be the only phrase which should need changing in my opinion but this is open to discussion. I agree with your definitions of bedrock that it should mention something about being 'beneath soil' gravel etc. Perhaps suggesting something along the lines of:

"Bedrock" is the solid Earth surface beneath land ice, ocean water or soil.

However, a specialist in this subject may argue this is still too vague but this is just a starting point. I think in terms of ice sheet modelling the substance between the ice and bedrock is probably ignored as you say as it is likely negligible, but this isn't my topic of expertise. In some cold-based ice cases, soil, silt, sand and other things have been preserved however https://nsidc.org/cryosphere/icelights/2014/06/what-under-greenland%E2%80%99s-ice.

I am sceptical about the 'ocean water' part of this phrase also. I agree with your statement that it doesn't quite make sense. I also had not thought about the bedrock beneath the atmosphere point you make. In terms of changing the definition and creating a new term for solid earth I am not sure about this change and would need to seek a bit more advice in terms of changing the meaning of previous terms and therefore the data attributed to these.

Point 2 definitely needs further discussion. It would be great to get other opinions on this and see what people who have used this term think.

I agree with point 3, that does make sense to me. It would be good to also get other opinions on this. Your second post is very interesting and I think an important point here. Hopefully, this post will spark further discussion about these topics and we can start to think about the changes being agreed. Apologies if I am completely off with my comments and please do correct me if I have misunderstood anything as I am here to learn too.

Thanks, Fran

[JonathanGregory]

Thanks for this discussion. I think that for land ice you could say bedrock is the solid Earth surface beneath land ice or glacial till, since till is the usual term for the gunge underneath the land ice. Under the ocean, I think bedrock means the crystalline rock, beneath any consolidated sediment. The sea floor is the top of the consolidated sediment (or the bedrock, if there is no consolidated sediment), but beneath any unconsolidated sediment.

[martinjuckes]

Hello Fran, Jonathan,

thanks for those comments. I would be happy with the definition "Bedrock is the solid Earth surface beneath land ice, ocean water or soil", and I think we could treat this as a clarification rather than a change in meaning (since it appears clear that the term was never intended to refer to the sea floor).

For point (2) the problem is that there is definitely a horizontal transport of heat and water in the glacial till (or gunge layer -- as @JonathanGregory refers to it). So, there is going to be a difference between the heat coming out of the bedrock into the till and the heat going out of the till into the ice, even though the till layer is generally quite thin. There is some discussion of this in the documentation of the GLIMMER model : the flux of heat out of the bedrock differs from the flux into the ice by two terms: one is related to basal melting or freezing and the other is related to heat generated by friction at the ice base. I'll get in touch with the ISMIP6 team who asked for the upward_geothermal_heat_flux_at_ground_level_in_land_ice term (for CMIP6), to see if they can clarify the intention.

[feggleton]

Hi all,

I have added 3 names to the editor which are under discussion concerning a definition change, these are:

bedrock_altitude bedrock_altitude_change_due_to_isostatic_adjustment tendency_of_bedrock_altitude

Seen here: http://cfeditor.ceda.ac.uk/proposals/1

These are the only 3 names which include the bedrock phrase discussed above. If there is no further discussion in the next week or so I will accept the definition change on these 3 terms as the change is very minor and this discussion has been open a long time with no disagreement.

And just to clarify we are still waiting on clarification in reference to the other term being discussed here, upward_geothermal_heat_flux_at_ground_level_in_land_ice.

If there are any further points which need discusses which I have not mentioned, please bring them up.

[martinjuckes]

Hi Fran,

Thanks, those terms look good.

I've heard back from Sophie Nowicki who requested the 'upward_geothermal_heat_flux_at_ground_level_in_land_ice name for use in ISMIP6 back in a 2016 email discussion. The intention was definitely to represent the thermal heat flux into the ice, rather than out of the bedrock. These two are still the same in most ISMIP6 models, but not all: a few do represent horizontal transports. It is also clear that they want this heat flux for the grounded portion of the land ice.

upward_grounded_ice_sheet_basal_thermal_heat_flux might work (based on upward_sea_ice_basal_heat_flux and grounded_ice_sheet_area), but it is hard to know how best to arrange the terms.

upward_thermal_heat_flux_at_grounded_ice_sheet_base might be better.

[feggleton]

Hi Martin,

As there have been no further comments on the 3 terms agreed on above (bedrock_altitude, bedrock_altitude_change_due_to_isostatic_adjustment, tendency_of_bedrock_altitude) these will now be accepted into the next standard name table update (tbc). I have updated this in the editor. Thank you for getting those terms agreed.

We will keep this issue open to continue the discussion of the last term (upward_geothermal_heat_flux_at_ground_level_in_land_ice), Martin could you just clarify if this is just a discussion about change in term name and/or definition and suggest changes (doesn't have to be the final thoughts) and I will add this to the editor. This is a change to the existing term, isn't it?

Thanks,

Fran

[japamment]

Dear Martin, Fran, et al, @martinjuckes @feggleton

The updated definitions of the three bedrock names were published this week in Version 73 of the standard name table. It would be great if we can now resolve the question about upward_geothermal_heat_flux_at_ground_level_in_land_ice so that this issue can be concluded.

Martin suggested two alternatives for turning the existing name into an alias: upward_grounded_ice_sheet_basal_thermal_heat_flux or upward_thermal_heat_flux_at_grounded_ice_sheet_base. The second of these is consistent with the standard name Guidelines document whereas the first is not, so I would definitely prefer the second option. I notice that Martin says 'thermal' rather than 'geothermal' as in the existing name, but if we are actually referring to the total heat flux into the base of the ice then I agree it should be 'thermal'.

We have a total of 15 existing sea_ice and land_ice 'basal' terms. I think that we should also change these to be more consistent with the Guidelines document. The first few are completely straightforward:

basal_downward_heat_flux_in_sea_ice -> downward_heat_flux_at_sea_ice_base
downward_sea_ice_basal_salt_flux -> downward_salt_flux_at_sea_ice_base
land_ice_basal_drag -> drag_at_land_ice_base
land_ice_basal_melt_rate -> melt_rate_at_land_ice_base
land_ice_lwe_basal_melt_rate -> lwe_melt_rate_at_land_ice_base
land_ice_basal_specific_mass_balance_flux -> specific_mass_balance_flux_at_land_ice_base

For the next group, the question is whether to repeat 'sea | land_ice' at the end of the names, or is it obvious which base we are referring to? Although repetitive, the longer versions mean we can stick with standard terminology for the surfaces 'sea_ice_base' and 'land_ice_base' so that would be my preference. What do others think?

land_ice_basal_temperature -> land_ice_temperature_at_[land_ice]_base
land_ice_basal_upward_velocity -> land_ice_upward_velocity_at_[land_ice_]base
land_ice_basal_x_velocity -> land_ice_x_velocity_at_[land_ice_]base
land_ice_basal_y_velocity -> land_ice_y_velocity_at_[land_ice_]base
land_ice_basal_temperature -> land_ice_temperature_at_[land_ice_]base
sea_ice_basal_temperature -> sea_ice_temperature_at_[sea_ice_]base
tendency_of_land_ice_mass_due_to_basal_mass_balance -> tendency_of_land_ice_mass_due_to_mass_balance_at_[land_ice_]base
tendency_of_sea_ice_amount_due_to_basal_melting -> tendency_of_sea_ice_amount_due_to_melting_at_[sea_ice_]base
upward_sea_ice_basal_heat_flux -> upward_heat_flux_in_sea_ice_at_[sea_ice_]base

Lastly, the name sea_ice_basal_drag_coefficient_for_momentum_in_sea_water would become drag_coefficient_for_momentum_at_sea_ice_base[_in_sea_water] - the question here is whether we need _in_sea_water at the end because presumably the base of all sea_ice is in sea_water?

Best wishes, Alison

[martinjuckes]

Hello @japamment ,

my apologies for failing to respond for so long. Thanks for fixing the other standard names. For this last one, the use case in question is one in which the scientists can use "thermal" and "geothermal" almost interchangeably, but there is a clear intention that this should cover the component of the heat budget associated with heat flux into ice at its interface with bedrock and materials sitting on bedrock.

I agree with your preference for upward_thermal_heat_flux_at_grounded_ice_sheet_base.

For the flux/drag/rate terms I agree with your suggestions.

On the set with potential repetition, I think it would be good to avoid the repetition. I note that we also have temperature_at_base_of_ice_sheet_model and temperature_at_top_of_ice_sheet_model.

Could we follow this pattern:

land_ice_basal_temperature -> temperature_at_base_of_land_ice
land_ice_basal_upward_velocity -> upward_velocity_at_base_of_land_ice
land_ice_basal_x_velocity -> x_velocity_at_base_of_land_ice
land_ice_basal_y_velocity -> y_velocity_at_base_of_land_ice

etc? Alternatively, it may be worth keeping the basal forms in this case, as a convenient way of avoiding repetition.

For the last term, I think that the _in_sea_water suffix is determining the sign: it is the drag on the sea water from the sea ice, which is minus the drag on the sea ice from the sea water.

[JonathanGregory]

Dear @japamment and @martinjuckes

I like Martin's suggested pattern using _at_base_of_land_ice.

A heat flux is always "thermal", by definition. :-) If we mean the heat flux due to everything, then it should just be upward_heat_flux. If it were geothermal only, geothermal should be included.

Best wishes

Jonathan

[martinjuckes]

Dear @JonathanGregory ,

Thanks for the support on _at_base_of_.

On the use of thermal: the heat flux, in CF terms, combines radiative, latent, and sensible fluxes. I accept that thermal is wrong here, though I believe the "thermal" can be used as a synonym for "sensible", the latter would be the term to use here. However, after checking back on my conversation with the ISMIP6 team, it appears that we might as well have the total heat flux, so upward_heat_flux_at_grounded_ice_sheet_base will meet the requirement.

For consistency with other changes upward_heat_flux_at_base_of_grounded_ice_sheet may be better.

[feggleton]

Commenting here to see if we can move this forward to complete.

We only have 'upward_heat_flux_at_base_of_grounded_ice_sheet' to be agreed. In the cfeditor it is still under the original name upward_geothermal_heat_flux_at_ground_level_in_land_ice so this needs to be updated and then this can be resolved quite quickly @japamment