[CF-metadata] CMIP6 Sea Ice MIP: General variables

[Dirk Notz]

Dear CF community,

to more specifically describe sea-ice output from climate-model
simulations for the CMIP6 Sea-Ice MIP (SIMIP), we would like to propose
the following new standard names, area types and regions. To ease
discussion, we will suggest variables that are likely to require some
discussion in follow-up emails.

New standard names:

1. proportion_of_model_time_steps_with_sea_ice_fraction_above_threshold
fraction of time steps which had sea ice during the averaging period of
typically one month. It allows one to differentiate half a month of 100%
sea-ice cover from a full month of 50 % sea-ice concentration.

2. sea_ice_mass_content [kg m-2]
describes the total mass of sea ice divided by grid-cell area.

3. sea_ice_basal_temperature [K]
the temperature at the interface, NOT the temperature within the
lowermost sea-ice model grid cell

4. sea_ice_salt_content [kg m-2]
the total mass of all salt in sea ice divided by grid cell area

5. sea_ice_melt_pond_thickness [m]
the volume of water in meltponds divided by meltpond covered area

6. thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area

7. ridged_sea_ice_thickness [m]
total volume of ridged sea ice divided by area of ridges

8. tendency_of_sea_ice_amount_due_to_thermodynamics [kg m-2 s-1]
total change in sea-ice mass from thermodynamic processes divided by
grid-cell area

9. tendency_of_sea_ice_amount_due_to_dynamics [kg m-2 s-1]
total change in sea-ice mass from dynamic processes divided by grid-cell
area

10. tendency_of_sea_ice_amount_due_to_lateral_melting [kg m-2 s-1]
total change in sea-ice mass from lateral melting divided by grid-cell area

11. surface_snow_sublimation_flux [kg m-2 s-1]
the rate of change of snow mass through sublimation and evaporation
divided by grid-cell area

12. tendency_of_surface_snow_amount_due_to_sea_ice_dynamics [kg m-2 s-1]
the rate of change of snow mass through advection with sea ice divided
by grid-cell area

13. tendency_of_surface_snow_amount_due_to_ice_conversion [kg m-2 s-1]
the rate of change of snow mass due to transformation of snow to sea ice
divided by grid-cell area

14. tendency_of_surface_snow_amount_due_to_drifting [kg m-2 s-1]
the rate of change of snow mass through wind drift of snow divided by
grid-cell area

15. sea_ice_basal_downwelling_shortwave_flux_in_sea_water [W m-2]
the downwelling shortwave flux underneath sea ice (always positive)

16. sea_ice_basal_net_downward_sensible_heat_flux [W m-2]
the net sensible heat flux under sea ice from the ocean

17. sea_ice_surface_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice surface

18. sea_ice_basal_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice base

19. salt_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of salt from water into sea ice divided by grid-cell area;
salt flux is upward (negative) during ice growth when salt is embedded
into the ice and downward (positive) during melt when salt from sea ice
is again released to the ocean

20. water_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of fresh water from water into sea ice divided by grid-cell
area; This flux is negative during ice growth (liquid water mass
decreases, hence upward flux of freshwater), positive during ice melt
(liquid water mass increases, hence downward flux of freshwater)

21. surface_drag_coefficient_for_momentum_in_sea_water [1]
Oceanic drag coefficient that is used to calculate the oceanic momentum
drag on sea ice

22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
X-component of force on sea ice caused by sea-surface tilt

23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
Y-component of force on sea ice caused by sea-surface tilt

24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
X-component of force on sea ice caused by coriolis force

25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
Y-component of force on sea ice caused by coriolis force

26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
X-component of force on sea ice caused by internal stress (divergence of
sigma)

27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]
Y-component of force on sea ice caused by internal stress (divergence of
sigma)

28. sea_ice_mass_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice mass transport through
given passages, positive into the Arctic Ocean

29. sea_ice_area_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice area transport through
given passages, positive into the Arctic Ocean


New standard regions:
31. Northern Hemisphere

32. Southern Hemisphere


New area types
33. snow_covered_sea_ice

34. ridged_sea_ice



Thank you very much for your feedback, guidance and input.

With best wishes, for the SIMIP Steering Committee,

Dirk Notz

[Jonathan Gregory]

Dear Dirk

Thanks for your proposals. Here are comments on some of them. The others are
new and fit existing patterns, I think.

Post by Dirk Notz
1. proportion_of_model_time_steps_with_sea_ice_fraction_above_threshold

I suggest it would be better to make this less model-specific by calling it
fraction_of_time_with_with_sea_ice_area_fraction_above_threshold
where I have inserted "area" as well for consistency with the existing name
sea_ice_area_fraction.

Post by Dirk Notz
2. sea_ice_mass_content [kg m-2]
describes the total mass of sea ice divided by grid-cell area.

This is the same as the existing sea_ice_amount (the area by which it is
divided is described by cell_methods, as in my other posting).

Post by Dirk Notz
5. sea_ice_melt_pond_thickness [m]
the volume of water in meltponds divided by meltpond covered area

OK, with a similar cell_methods issue: I think by default this would be
a local thickness, but it could be described as an area-mean.

Post by Dirk Notz
6. thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area

Is this ice floating on top of the meltpond?

Post by Dirk Notz
7. ridged_sea_ice_thickness [m]
total volume of ridged sea ice divided by area of ridges

Instead of a new standard name, could this be described by sea_ice_thickness
and "where" in cell_methods if we define a new area_type of ridged_sea_ice?

Post by Dirk Notz
11. surface_snow_sublimation_flux [kg m-2 s-1]
the rate of change of snow mass through sublimation and evaporation
divided by grid-cell area

Unless you specifically want to *exclude* ice, this could use the existing
surface_snow_and_ice_sublimation_flux.

Post by Dirk Notz
13. tendency_of_surface_snow_amount_due_to_ice_conversion [kg m-2 s-1]
the rate of change of snow mass due to transformation of snow to sea ice
divided by grid-cell area

I feel that due_to_ice_conversion is not specific enough because it doesn't
give the sense of the change. To be clear, would it be correct to say
due_to_conversion_of_snow_to_sea_ice
?

Post by Dirk Notz
15. sea_ice_basal_downwelling_shortwave_flux_in_sea_water [W m-2]
the downwelling shortwave flux underneath sea ice (always positive)

I think that
downwelling_shortwave_flux_in_sea_water_at_sea_ice_base
would be more natural because it's not really a property of the sea-ice.
The sea-ice base is a level.

Post by Dirk Notz
16. sea_ice_basal_net_downward_sensible_heat_flux [W m-2]
the net sensible heat flux under sea ice from the ocean

This has an existing name upward_sea_ice_basal_heat_flux.

Post by Dirk Notz
17. sea_ice_surface_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice surface
18. sea_ice_basal_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice base

We have an existing name of downward_heat_flux_in_sea_ice for the conductive
flux within the ice. To be specific about the level, we could prefix surface_
for 17 and suffix basal_ for 18.

Post by Dirk Notz
19. salt_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of salt from water into sea ice divided by grid-cell area;
salt flux is upward (negative) during ice growth when salt is embedded
into the ice and downward (positive) during melt when salt from sea ice
is again released to the ocean

This is like the existing name
virtual_salt_flux_into_sea_water_due_to_sea_ice_thermodynamics
isn't it, except not virtual.

Post by Dirk Notz
20. water_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of fresh water from water into sea ice divided by grid-cell
area; This flux is negative during ice growth (liquid water mass
decreases, hence upward flux of freshwater), positive during ice melt
(liquid water mass increases, hence downward flux of freshwater)

I think this is the same as the existing
water_flux_into_sea_water_due_to_sea_ice_thermodynamics

Post by Dirk Notz
21. surface_drag_coefficient_for_momentum_in_sea_water [1]
Oceanic drag coefficient that is used to calculate the oceanic momentum
drag on sea ice

Should that be surface_ (which means the top of the atmosphere) or perhaps
sea_ice_basal_?

Post by Dirk Notz
22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]

In other N m-2 = Pa names, we use "stress" rather than "specific force" but
I suppose this is different because they are not fluxes of momentum, but
divergences of momentum i.e. force per unit mass. Is "specific force" a
usual term for this? Could we say "coriolis_effect" rather than "term",
which sounds more algorithmic than geophysical?

Post by Dirk Notz
28. sea_ice_mass_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice mass transport through
given passages, positive into the Arctic Ocean

This exists under the name sea_ice_transport_across_line.

Best wishes

Jonathan

[Jonathan Gregory]

Dear Dirk

Post by Jonathan Gregory

Post by Dirk Notz
5. sea_ice_melt_pond_thickness [m]
the volume of water in meltponds divided by meltpond covered area

OK, with a similar cell_methods issue: I think by default this would be
a local thickness, but it could be described as an area-mean.

Our definition was simply based on the usual way of representing
melt-pond water in model simulations. However, as this is meant to be a
local thickness, we'd be happy to change the definition to simplyread
"Local thickness of melt ponds"

Perhaps to be clear we could say something like "Thickness of melt pond.
The cell_methods indicates whether it is a local thickness, an area-mean
thickness or some other statistic."

Post by Jonathan Gregory

Post by Dirk Notz
6. thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area

Is this ice floating on top of the meltpond?

Yes, it is. Do you believe that the definition should be changed?

I wasn't clear what the quantity is. We already have a standard name of
floating_ice_thickness, of which this is a special case. So we could use the
existing name and distinguish this case in cell_methods with
"area: mean where sea_ice_melt_pond". Would that be acceptable?

Post by Jonathan Gregory

Post by Dirk Notz
22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]

In other N m-2 = Pa names, we use "stress" rather than "specific force" but
I suppose this is different because they are not fluxes of momentum, but
divergences of momentum i.e. force per unit mass. Is "specific force" a
usual term for this? Could we say "coriolis_effect" rather than "term",
which sounds more algorithmic than geophysical?

In trying to follow CF_conventions, we adopted "specific" to point out
that these forces are calculated per unit area. We'd be very happy to
26+27 could then simply be referred to as "sea_ice_internal_stress_x"
and "sea_ice_internal_stress_y".
For 22-25, it would be non-standard in the sea-ice community to call
those "stress", as both coriolis and surface tilt act as body forces.
Standard terminology would be to simply refer to them as
"coriolis_force_on_sea_ice" and "sea_surface_tilt_force", but this is
probably not consistent with CF-conventions as units are force per area.
Do you have any other suggestion of how we could call these "forces per
sea-ice area"?

Thanks for explaining. We do already have some standard names with the longer
phrase per_unit_area in them, so maybe that would be the best pattern to
follow:
sea_ice_[xy]_force_per_unit_area_due_to_X
where X is sea_surface_tilt, coriolis_effect or internal_stress. Would
that be correct and make sense?

Best wishes

Jonathan

[Dirk Notz]

Dear Jonathan,

Post by Jonathan Gregory

Post by Jonathan Gregory

Post by Dirk Notz
5. sea_ice_melt_pond_thickness [m]
the volume of water in meltponds divided by meltpond covered area

Perhaps to be clear we could say something like "Thickness of melt pond.
The cell_methods indicates whether it is a local thickness, an area-mean
thickness or some other statistic."

This is good, thanks.

Post by Jonathan Gregory

Post by Jonathan Gregory

Post by Dirk Notz
6. thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area

Is this ice floating on top of the meltpond?

Yes, it is. Do you believe that the definition should be changed?

I wasn't clear what the quantity is. We already have a standard name of
floating_ice_thickness, of which this is a special case. So we could use the
existing name and distinguish this case in cell_methods with
"area: mean where sea_ice_melt_pond". Would that be acceptable?

This would work, I hadn't seen that before, thanks.

Post by Jonathan Gregory

Post by Jonathan Gregory

Post by Dirk Notz
22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]

Thanks for explaining. We do already have some standard names with the longer
phrase per_unit_area in them, so maybe that would be the best pattern to
sea_ice_[xy]_force_per_unit_area_due_to_X
where X is sea_surface_tilt, coriolis_effect or internal_stress. Would
that be correct and make sense?

This would be a good way of describing things. Hence, we'd suggest:

22. sea_ice_x_force_per_unit_area_due_to_sea_surface_tilt [N m-2]
23. sea_ice_y_force_per_unit_area_due_to_sea_surface_tilt [N m-2]
24. sea_ice_x_force_per_unit_area_due_to_coriolis_effect [N m-2]
25. sea_ice_y_force_per_unit_area_due_to_coriolis_effect [N m-2]
26. sea_ice_x_internal_stress [N m-2]
27. sea_ice_y_internal_stress [N m-2]

Best,

Dirk

[Dirk Notz]

Dear CF community,

here is my initial response to Jonathan, which I had accidentally only
copied to him.

Sorry.

Dirk


--------Forwarded message --------
Dear Jonathan,

thank you for your careful consideration of our proposal. This is truly
helpful.

Post by Jonathan Gregory

Post by Dirk Notz
1. proportion_of_model_time_steps_with_sea_ice_fraction_above_threshold

I suggest it would be better to make this less model-specific by calling it
fraction_of_time_with_with_sea_ice_area_fraction_above_threshold
where I have inserted "area" as well for consistency with the existing name
sea_ice_area_fraction.

I'm very happy with your suggestion
"fraction_of_time_with_sea_ice_area_fraction_above_threshold"

Post by Jonathan Gregory

Post by Dirk Notz
2. sea_ice_mass_content [kg m-2]
describes the total mass of sea ice divided by grid-cell area.

This is the same as the existing sea_ice_amount (the area by which it is
divided is described by cell_methods, as in my other posting).

I wasn't aware of the variable sea_ice_amount. Sorry for this oversight,
we then don't require sea_ice_mass_content as a new variable.

Post by Jonathan Gregory

Post by Dirk Notz
5. sea_ice_melt_pond_thickness [m]
the volume of water in meltponds divided by meltpond covered area

OK, with a similar cell_methods issue: I think by default this would be
a local thickness, but it could be described as an area-mean.

Our definition was simply based on the usual way of representing
melt-pond water in model simulations. However, as this is meant to be a
local thickness, we'd be happy to change the definition to simplyread
"Local thickness of melt ponds"

Post by Jonathan Gregory

Post by Dirk Notz
6. thickness_of_sea_ice_melt_pond_refrozen_ice [m]
the volume of refrozen ice in meltponds divided by meltpond covered area

Is this ice floating on top of the meltpond?

Yes, it is. Do you believe that the definition should be changed?

Post by Jonathan Gregory

Post by Dirk Notz
7. ridged_sea_ice_thickness [m]
total volume of ridged sea ice divided by area of ridges

Instead of a new standard name, could this be described by sea_ice_thickness
and "where" in cell_methods if we define a new area_type of ridged_sea_ice?

Yes, this would be possible. This is a good suggestion, thanks!

Post by Jonathan Gregory

Post by Dirk Notz
11. surface_snow_sublimation_flux [kg m-2 s-1]
the rate of change of snow mass through sublimation and evaporation
divided by grid-cell area

Unless you specifically want to *exclude* ice, this could use the existing
surface_snow_and_ice_sublimation_flux.

To be able to close the mass balance, we indeed would like to be able to
differentiate between sublimation of snow versus that of ice. However,
in iterating further with model developers, we have been made aware that
no model currently represents this distinction. We hence withdraw this
suggestion and are happy to adopt your suggestion.

Post by Jonathan Gregory

Post by Dirk Notz
13. tendency_of_surface_snow_amount_due_to_ice_conversion [kg m-2 s-1]
the rate of change of snow mass due to transformation of snow to sea ice
divided by grid-cell area

I feel that due_to_ice_conversion is not specific enough because it doesn't
give the sense of the change. To be clear, would it be correct to say
due_to_conversion_of_snow_to_sea_ice
?

This would be correct.

Post by Jonathan Gregory

Post by Dirk Notz
15. sea_ice_basal_downwelling_shortwave_flux_in_sea_water [W m-2]
the downwelling shortwave flux underneath sea ice (always positive)

I think that
downwelling_shortwave_flux_in_sea_water_at_sea_ice_base
would be more natural because it's not really a property of the sea-ice.
The sea-ice base is a level.

This is true - we certainly were overly sea-ice centric for this one. I
like your suggestion.

Post by Jonathan Gregory

Post by Dirk Notz
16. sea_ice_basal_net_downward_sensible_heat_flux [W m-2]
the net sensible heat flux under sea ice from the ocean

This has an existing name upward_sea_ice_basal_heat_flux.

Thanks, we were not aware of this and withdraw our proposal.

Post by Jonathan Gregory

Post by Dirk Notz
17. sea_ice_surface_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice surface
18. sea_ice_basal_net_downward_conductive_heat_flux [W m-2]
the net heat conduction flux at the ice base

We have an existing name of downward_heat_flux_in_sea_ice for the conductive
flux within the ice. To be specific about the level, we could prefix surface_
for 17 and suffix basal_ for 18.

This would work well, thanks.

Post by Jonathan Gregory

Post by Dirk Notz
19. salt_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of salt from water into sea ice divided by grid-cell area;
salt flux is upward (negative) during ice growth when salt is embedded
into the ice and downward (positive) during melt when salt from sea ice
is again released to the ocean

This is like the existing name
virtual_salt_flux_into_sea_water_due_to_sea_ice_thermodynamics
isn't it, except not virtual.

It is indeed, sorry that this slipped our attention.

Post by Jonathan Gregory

Post by Dirk Notz
20. water_flux_into_sea_water_from_sea_ice [kg m-2 s-1]
Total flux of fresh water from water into sea ice divided by grid-cell
area; This flux is negative during ice growth (liquid water mass
decreases, hence upward flux of freshwater), positive during ice melt
(liquid water mass increases, hence downward flux of freshwater)

I think this is the same as the existing
water_flux_into_sea_water_due_to_sea_ice_thermodynamics

Dito...

Post by Jonathan Gregory

Post by Dirk Notz
21. surface_drag_coefficient_for_momentum_in_sea_water [1]
Oceanic drag coefficient that is used to calculate the oceanic momentum
drag on sea ice

Should that be surface_ (which means the top of the atmosphere) or perhaps
sea_ice_basal_?

Sea_ice_basal_ is certainly the better and clearer terminology.

Post by Jonathan Gregory

Post by Dirk Notz
22. sea_ice_specific_x_force_due_to_sea_surface_tilt [N m-2]
23. sea_ice_specific_y_force_due_to_sea_surface_tilt [N m-2]
24. sea_ice_specific_x_force_due_to_coriolis_term [N m-2]
25. sea_ice_spcecific_y_force_due_to_coriolis_term [N m-2]
26. sea_ice_specific_x_force_due_to_internal_forces [N m-2]
27. sea_ice_specific_y_force_due_to_internal_forces [N m-2]

In other N m-2 = Pa names, we use "stress" rather than "specific force" but
I suppose this is different because they are not fluxes of momentum, but
divergences of momentum i.e. force per unit mass. Is "specific force" a
usual term for this? Could we say "coriolis_effect" rather than "term",
which sounds more algorithmic than geophysical?

In trying to follow CF_conventions, we adopted "specific" to point out
that these forces are calculated per unit area. We'd be very happy to
drop "specific":

26+27 could then simply be referred to as "sea_ice_internal_stress_x"
and "sea_ice_internal_stress_y".

For 22-25, it would be non-standard in the sea-ice community to call
those "stress", as both coriolis and surface tilt act as body forces.
Standard terminology would be to simply refer to them as
"coriolis_force_on_sea_ice" and "sea_surface_tilt_force", but this is
probably not consistent with CF-conventions as units are force per area.
Do you have any other suggestion of how we could call these "forces per
sea-ice area"?

Post by Jonathan Gregory

Post by Dirk Notz
28. sea_ice_mass_transport_across_line [kg s-1]
"net (sum of transport in all directions) sea ice mass transport through
given passages, positive into the Arctic Ocean

This exists under the name sea_ice_transport_across_line.

Thanks once again for pointing out the existing name, we then again
withdraw our proposal of a new standard name.

All the best,

Dirk