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

From: Dirk Notz <dirk.notz>
Date: Thu, 21 Jul 2016 15:44:20 +0200

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.

>> 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"

>
>> 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.

>
>> 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"

>
>> 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?


>
>> 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!

>
>> 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.

>
>> 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.

>
>> 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.

>
>> 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.

>
>> 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.

>
>> 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.

>
>> 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...

>> 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.

>
>> 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"?




>
>> 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
Received on Thu Jul 21 2016 - 07:44:20 BST