Standard names: proposal to add three new concepts related to seabed stress

[danibodc]

We have received a request for some new standard names from a colleague Lucy Bricheno who is producing various model outputs of seabed stress.

Proposer - Danielle Wright (NOC BODC) on behalf of Lucy Bricheno (NOC MSM)*

Date 05/12/2024

- Term sea_bed_downward_stress

- Description Ocean processes such as waves and currents can generate forces (or stress) on the sea bed. The surface called "sea bed" means the lower boundary of the ocean. "Stress" means the shear stress (force per unit area) exerted at the sea bed. A downward stress is a downward flux of momentum. "Downward" indicates a vector component which is positive when directed downward (as the water is acting on the sea bed).

- Units N m-2.

- Term sea_bed_downward_stress_due_to_dissipation_of_sea_surface_waves

- Description Ocean processes such as waves and currents can generate forces (or stress) on the sea bed. The surface called "sea bed" means the lower boundary of the ocean. "Stress" means the shear stress (force per unit area) exerted at the sea bed. A downward stress is a downward flux of momentum. "Downward" indicates a vector component which is positive when directed downward (as the water is acting on the sea bed). 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 phrase "dissipation_of_sea_surface_waves" means the stress associated with sea surface waves dissipation processes through bottom friction.

- Units N m-2.

- Term sea_bed_downward_stress_due_to_dissipation_of_ocean_current

- Description Ocean processes such as waves and currents can generate forces (or stress) on the sea bed. The surface called "sea bed" means the lower boundary of the ocean. "Stress" means the shear stress (force per unit area) exerted at the sea bed. A downward stress is a downward flux of momentum. "Downward" indicates a vector component which is positive when directed downward (as the water is acting on the sea bed). 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 phrase "dissipation_of_ocean_current" means the stress associated with ocean current dissipation processes through bottom friction.

- Units N m-2.

[github-actions]

Thank you for your proposal. These terms will be added to the cfeditor (http://cfeditor.ceda.ac.uk/proposals/1) shortly. Your proposal will then be reviewed and commented on by the community and Standard Names moderator.

[JonathanGregory]

Dear Danielle @danibodc

Thanks for the proposal. I have some questions:

• Are these scalar quantities or components of vectors? In the former case, we probably need to insert magnitude_of; in the latter case they should be donward_eastward and downward_northward.
• For consistency with existing standard names, they should say sea_floor rather than sea_bed.
• I feel that due_to_friction would be clearer than due_to_dissipation_of_ocean_current.

Best wishes

Jonathan

[bricheno]

Hi Jonathan, Danielle,

Thanks for getting this submitted. Agree to these suggestions. These stresses are scalar, so agree with the magnitude_of approach, and sea_floor makes good sense. It is not clear to me how best to distinguish between the sub levels of wave and current or combined stresses? The forces from wave and currents are both being dissipated by friction, but it is the source of the energy/ driver which is different.

[JonathanGregory]

Dear Lucy @bricheno

Yes, that's a good point about friction. In both cases, motion adjacent to the sea floor is being opposed by friction, as you say. When considering the motion near the bottom, how can you attribute it to contributions from different source of energy? I can see how one decomposes the net force or an acceleration into wind, pressure gradient, Coriolis, wave-breaking, etc., but how do you do it with the velocity?

Best wishes

Jonathan

[bricheno]

perhaps removing the dissipation entirely, and defining only the source of the force is better?

magnitude_of_sea_bed_downward_stress
magnitude_of_sea_bed_downward_stress_due_to_sea_surface_waves
magnitude_of_sea_bed_downward_stress_due_to_ocean_current

What do you think?
Lucy

[JonathanGregory]

Dear Lucy @bricheno

Yes, that approach would work, I think. For the first one, magnitude_of_sea_floor_downward_stress would be consistent with other standard names; there are several with sea_floor, and there is also magnitude_of_surface_downward_stress.

In practice, how are you distinguishing between these contributions to the stress? I'm still not clear how you identify which part of the sea water velocity at the sea floor is due to sea surface waves.

Best wishes

Jonathan

[bricheno]

Jonathan, I suppose the best way to describe it, is that the waves are exerting stress through an orbital velocity. Waves generate circular movement in the water column, which becomes more flattened due to interaction with the bottom. The effect is, that even though the waves aren't actually moving water anywhere, this back-and-forth motion still exerts a seabed stress. The currents, on the other hand do exert flow ( can be unidirectional, or oscillating tidal). This is an important difference for e.g. sediment - while waves can resuspend sediments, it is the currents that transport.

It is possible this is a little artificial, as it is coming from a model. But there are equivalents in observations - e.g. bottom landed Benthic Acoustic Stress Sensor which provide high-frequency data. I think that there is also a divide in the energy frequency spectrum - near-bottom high-frequency pressure fluctuations are primarily due to surface waves, while currents evolve over longer timescales. so this could be though of as dividing the process in the frequency space - like for an energy density spectrum?

Lucy

[JonathanGregory]

Dear Lucy

If the distinction could be made in terms of frequency or wavelength, then we would need only one standard name, because we would use a coordinate variable to specify the frequency ranges. That's a possible approach.

In your case, however, the distinction is really a binary choice, I guess. Perhaps you calculate one stress from a sea surface wave model, and the other from the resolved velocity of an ocean model? due_to_sea_surface_waves seems clear for the former. For the other, due_to_ocean_current seems less satisfactory to me, because it's unclear what that might include. "Current" could mean the time-mean velocity field, for instance, perhaps on large spatial scales. We could call it e.g. excluding_effect_of_sea_surface_waves, if that's an accurate description i.e. all other kinds of motion. Alternatively, if the wave part is not modelled by a velocity at all, we could make it explicitly a model quantity by calling it e.g. due_to_model_sea_water_velocity.

Best wishes

Jonathan

[bricheno]

Yes, that's right- they come from 2 different numerical models. I like "due_to_model_sea_water_velocity" instead of ocean current. So would it be satisfactory to use:

magnitude_of_sea_floor_downward_stress
magnitude_of_sea_floor_downward_stress_due_to_sea_surface_waves
magnitude_of_sea_floor_downward_stress_due_to_sea_water_velocity

I worry that making it explicitly 'model' closes the door to it being used elsewhere? but maybe this is not a real problem?

[JonathanGregory]

Generally we focus on the use-cases we have, and not worry so much about possible requirements in the future! If this distinction were needed for observations, it would have to be done in another way, such as using the frequency, which we discussed earlier. Alternatively if someone thought in future of a distinction which worked equally well for both obs and models, we could rename the standard name with due_to_model_sea_water_velocity by using an alias. There are a few other standard name which mention model.

I've just remembered that I had another point to raise---sorry for further complication! I think the stress you have in mind is the magnitude of the horizontal vector composed of the downward northward stress (the downward flux of northward momentum) and the downward eastward stress (or x and y, if not north/east). Is that right? The vector is the downward flux of horizontal momentum. It seems natural to call this "downward" stress, but is "downward" necessary, since it's an absolute value? (with no direction). The sea floor and the water experience the same magnitude of stress. Would it be correct and sufficient to call it sea_floor_horizontal_stress, without the magnitude_of_, which I suggested?

[bricheno]

yes, the downward could be dropped, as it's just a scalar. So are we converging on something like this?

sea_floor_horizontal_stress
sea_floor_horizontal_stress_due_to_sea_surface_waves
sea_floor_horizontal_stress_due_to_sea_water_velocity

[JonathanGregory]

Thanks, Lucy. Yes, I think sea_floor_horizontal_stress and sea_floor_horizontal_stress_due_to_sea_surface_waves would be good. For your use-case I do think we must be clear about the distinction between the effect of waves and other things. As we've discussed, we could do that by including model in sea_floor_horizontal_stress_due_to_model_sea_water_velocity. We can say in the description that this quantity does not include the stress due to any motions of the sea water that are not represented in the model sea water velocity. Would that be a correct description of the case? I appreciate your concern about not precluding future use for observational data. If there is a future use-case for that, we will think again. I'm confident something appropriate could be devised!

[bricheno]

Ok, I'm happy to go ahead with model as you suggest.

[JonathanGregory]

Dear Lucy

Thanks for your flexibility! Please could you post a new version of the standard names and their descriptions, that you and I have agreed? Maybe others will comment, and it would help the standard names team too.

Best wishes

Jonathan