[MITgcm-support] SHELFICE Boundary Layer Parameterization

[Daniel Goldberg]

Hi Sara

Thanks for the interest and sounds like a very interesting project -- i don't have anything to add regarding the calculation of tendency or vertical grid resolution, thanks for that Ken.

Just to add that if you are interested in resolving vertical movement of the ice-ocean interface on the scale of 10s to hundreds of meters, shelfice remeshing  would be a good tool to use, though if you are just interested in melt alone and a static ice-ocean interface there is no need. this paper (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JC013251) is a more detailed reference for remeshing than the docs (and note that you do not need to use a dynamic ice-shelf model to make use of it). If you have any queries about its use please contact me.

Best
Dan

On Wed, Jun 2, 2021 at 10:26 PM Ken Hughes <kenneth.hughes at oregonstate.edu<mailto:kenneth.hughes at oregonstate.edu>> wrote:

Hi Sara,


With the conventional approach, melting/freezing doesn't really occur. In each horizontal grid cell, the ice thickness remains constant. A melt rate is calculated, but only so that the correct amount of heat and salt are added into the ocean beneath the ice. "Melting", for example, removes heat from grid cell below the ice shelf and effectively removes salt (essentially equivalent to diluting with freshwater).


This addition/removal of heat/salt is what I think you're getting at when mentioning the tendency. If you know the temperature and salinity in the cell vertically adjacent to the ice, together with its velocity (to parameterize turbulent transfers), then you have everything you need to calculate the boundary. It can seem like a chicken–egg problem, but there are standard ways to deal with the heat flux into the ice to ensure the problem is solvable, e.g., the so-called three-equation formulation.


You mention you want a dynamic ice shell, so you might need to use the remeshing add on for the SHELFICE package. I haven't personally used it, but it is worth a look.


As for why we don't use a dense grid near the boundary, that's because the MITgcm works on a fixed vertical grid. Unless the ice shelf draft is everywhere the same, you can't always have a highly resolved grid below the ice.


If you haven't done so already, work through the Losch (2008) paper, which describes the physics behind the package.


Ken


On 2021-06-02 1:20 p.m., Miller, Sara G wrote:

[This email originated from outside of OSU. Use caution with links and attachments.]

Hello all,

I am trying to understand the treatment of the ice-ocean boundary layer in the SHELFICE package. My goal is to use MITgcm to model a dynamic, global ice shell over an ocean on Jupiter's moon Europa. I believe the SHELFICE package will be useful for me, but I do not fully understand the description given in the user manual regarding boundary layer modeling.

What is the total tendency and how is it used in the calculation of the boundary layer temperature at a future time step? The boundary layer temperature and freezing/melting information feels like a chicken-and-egg problem. It would make sense to me that you need to know the temperature in the BL to determine if you're in the freezing or melting regime, but MITgcm shows boundary layer temperature as a function of that total tendency term which I think captures the freezing/melting information.

More fundamentally, how do you melt the ice in MITgcm? Does it slowly melt until it passes the threshold to step up to the next grid, or does its pressure just change? That is, the ice base is always the first grid cell, and as it melts upwards the pressure term just changes with it. I am new to modeling, but I don’t understand why we’re working in fractions of a grid cell instead of just making the grid more densely-spaced in the boundary layer region.

I apologize for the volume of questions in a single email! I appreciate any and all wisdom/advice.

Best regards,
Sara

---
Sara G. Miller (she/her)
PhD Student
Georgia Institute of Technology



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Daniel Goldberg, PhD
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