[MITgcm-support] SHELFICE Boundary Layer Parameterization

[Ken Hughes]

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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