[MITgcm-support] [EXTERNAL] Treatment of Antarctic grounding zone regions

[Paul Holland - UKRI BAS]

Hi Enrico

Briefly, I always choose to 'dig' out the cavity in order to make sure that all of the floating ice has some ocean under it.  My logic is that we know the grounding line very well from satellite interferometry, but we do not know the seabed/ice bed very well at all, and so I always choose to sacrifice the seabed in order to preserve the grounding line.

When doing the 'digging', we usually choose to do it in such a way that the velocity points on the C grid have two cells open in the vertical.  This would in theory make sure that all ocean columns can be connected to each other with a very under-resolved overturning circulation.  This gets a bit complicated with partial cells.  You can see in Kaitlin Naughten's github code you refer to exactly how she is handling this.  Basically, you have to over-dig the tracer point bathymetry in order to make sure that the velocity points are open, however you want them to be.

The above logic of digging the seabed only applies to simulations with static ice.  If you had a coupled ice sheet model, it would be a bit strange to dig the seabed in different places as the model advances or retreats the ice.  In that case, we have adopted other strategies, including digging the ice instead of the seabed (i.e. shallowing the ice in order to preserve a modellable water column thickness) and other trickery with the streamice package.

I hope that helps a bit...

Cheers

Paul

-------------------
Message: 2
Date: Wed, 18 Nov 2020 06:55:51 -0800
From: Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
To: MITgcm Support <mitgcm-support at mitgcm.org>, Yoshihiro Nakayama
<Yoshihiro.Nakayama at lowtem.hokudai.ac.jp>, "Michael P. Schodlok"
<Michael.P.Schodlok at jpl.nasa.gov>
Subject: Re: [MITgcm-support] [EXTERNAL] Treatment of Antarctic
grounding zone regions
Message-ID: <C8C7DE4E-7A33-48F0-8803-32F35C681DBF at jpl.nasa.gov>
Content-Type: text/plain; charset="us-ascii"

Michael and Yoshi, do you have any advice to give for question below?
What decision did you make for your hi-res simulations, e.g., for:
https://www.nature.com/articles/s41598-019-53190-6

Thanks, Dimitris

> On Nov 18, 2020, at 6:47 AM, Pochini, Enrico <epochini at inogs.it> wrote:
>
> Dear community,
>
> I have a question for the polar oceanographers:
>
> I have noticed that in bathymetric reconstructions available for Antarctica (e.g. Bedmap2, RTopo-2) there are regions corresponding to the grounding zone of the biggest ice shelves where the water column thickness (resulting from the subtraction draft - bathy) equates exactly 1m.
>
> For my simulations I work in the Ross Sea at 5km resolution and vertical layers of a few tens of metres at the depth of the grounding zone. I am wondering whether this region should be cut out directly as non-resolvable in my vertical discretization, or whether, in a less conservative approach, the bathymetry should be deepened in order to resolve the region in at least one layer.
>
> Employing a preprocessing algorithm (https://github.com/knaughten/mitgcm_python <https://urldefense.us/v3/__https://github.com/knaughten/mitgcm_python__;!!PvBDto6Hs4WbVuu7!bWCWtDMcJoVTIrR5IAjyeN1eGS1eXJVRuH3eFS2l_JFJ3IhWUV5BBvLC7nnfv5WjG2XalrUIZ7A$>) I obtain a grounding-zone water-thickness of ~ 50-80 m, enough to fit ~1-2 layers.
>
> I am uncertain about which way to take, so I'm curious to hear how you treat such thin cavities in high resolution models in other Antarctic seas, whether you cut them or keep them, and how you make them resolvable without affecting too much the overall setup.
>
> Thanks!
>
> Enrico P.
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