Will md.mask.ice_levelset evolve during a transient run?

SaiSan

Hi everyone,

I am running a continental ice sheet simulation with SIA. md.mask.ice_levelset is set to -1 where ice was there per initial condition and +1 where there were no ice. As the climate condition evolves, the ice sheet will advance or retreat in various places. Do I need to take care of the md.mask.ice_levelset during and/or after the transient run? Will the model automatically set md.mask.ice_levelset to -1 in places where there was no ice initially but later grew some, and to +1 where ice had disappeared?

Thank you!

Sai

mathieumorlighem

Hi @SaiSan

If you are running SIA, you don't need to worry about calving and retreat/advance. Just set the levelset as ice everywhere you think there may be ice, and assume that when the ice thickness is equal to md.masstransport.min_thickness, that means that there is no ice.
I hope this makes sense!
Mathieu

SaiSan

mathieumorlighem
Hi @mathieumorlighem

Thank you for your reply. By saying that "set the levelset as ice everywhere you think there may be ice", did you imply that if there is somewhere I did not set levelset to -1, no ice will ever appear on those places, even if smb or masstransport from elsewhere should later favor glaciation there?

Sai

SaiSan

For example, if in a particular year, the SMB becomes positive on a vertex initialized with ice_levelset=1 (no ice), will there be ice growing on this vertex, and will ice_levelset be automatically adjusted?
Alternatively, if mass transport should bring ice to a vertex initialized with ice_levelset=1 (no ice), will there be ice appearing on this vertex, and will ice_levelset be automatically adjusted?

mathieumorlighem

Hi SaiSan

you are right, the levelset will not reactivate new elements based on SMB alone. The level set is meant to deal with calving, for grounded ice you should activate everything (i.e. make the levelset negative) and rely on min_thickness to determine where there actually is ice. Sorry if that does not seem ideal...
Mathieu

SaiSan

mathieumorlighem
Oh that's totally fine. However, the clarification is absolutely crucial and I really appreciate it.
I would like to try two different appoaches in this case.
One is to set levelset=-1 in the whole continental domain, which is what you suggested if I understand correctly.
The other one in mind, which I am not 100% confident yet, is to set levelset=-1 only in currently glaciated areas, there surrounding areas and other places with negative but close to 0 SMB, stop the model once in a while, re-do this and restart, so that I can save some computation.
What do you think?

mathieumorlighem

yes that could work!

DominoAJones

@mathieumorlighem
I'm having a similar issue - tried expanding the domain to all regions where there should be advance and setting the masstransport.min_thickness as you suggested.

This ends up moving neumann BC to the margin of the mask (which is not the true front). As a result I cannot run a transient simulation anymore.

Do you have any suggestions?

mathieumorlighem

Hi @DominoAJones
I am not quite sure I understand your problem... the Neumann BC are applied where md.mask.ice_levelset crosses 0 which should be at the true front.
Could you tell us a bit more?
Mathieu

DominoAJones

Hi @mathieumorlighem

Thanks for the prompt response! The issue is when I make md.mask.ice_levelset=-1 everywhere there will be advance, then it does not cross 0 at the true front anymore. It only crosses 0 at the margin of the new mask (where there is no ice currently, but there should be with increased SMB forcing during a transient run).

How do I maintain the front where there is currently ice while allowing the front to advance?

Hopefully this clarifies a little bit, let me know if I need explain further/clearer. Domino

mathieumorlighem

Is your glacier land terminating or marine terminating?

DominoAJones

Marine terminating

mathieumorlighem

oh then you don't want to set the ice mask as a negative value over the ocean for sure. It will advance based on what you set as calving rate. The strategy to have a thin layer of ice to allow readvance is ONLY for grounded ice, where positive SMB may lead to regrowth locally (which is not the case over the ocean).

DominoAJones

Thank you! Appreciate the help!