[MITgcm-support] Increasing sea-ice/atmosphere drag

Tue Feb 12 05:04:38 EST 2019

Hi Martin,

Thanks for your ideas and sorry for my slow reply, I was trying them all out. I didn't find much sensitivity to mcPheeTaper but I did have good luck with frazilFrac. Setting frazilFrac=0.01 allowed the sea ice formation from supercooled water to spread out more, so the thick ice could be more easily exported rather than being trapped in one grid cell. Now the thick ice is typically in the range 1-5 m, and I can increase drag as much as I like without blowups. And having the value 0.01 instead of 0 does allow the supercooling to form sea ice eventually, so the water masses aren't too affected (unlike setting it to 0 which caused freshening of HSSW).

The fast sea ice velocity in this region was a consequence of the blowup, and is not typical of my simulations. The blowup in the cavity was right beside the thick ice, and likely a consequence of the numerical difficulties of a single strongly convecting column right beside the vorticity barrier of the ice shelf front. I do use z*-coordinates and real freshwater fluxes.

So I think that's solved my problem, thanks very much for your help.

All the best,

Kaitlin

Dr Kaitlin Naughten
British Antarctic Survey

________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
Sent: 04 February 2019 13:23:13
To: MITgcm Support
Subject: Re: [MITgcm-support] Increasing sea-ice/atmosphere drag

Hi Kaitlin,

I can’t claim that I have a great idea, but I have a few questions:
- why is the sea ice moving so fast (~3m/s)? Is this already a consequence of upcoming blow up?
- if not, can you figure out, what drives the ice locally?
- if the model blows up in the cavities, what is the connection to sea ice?
- do you use embedded sea ice (non-linear free surface options plus useRealFreshWaterFlux=.TRUE.) or “levitating” sea ice. With surface following z*-coordinates I can imagine strong gradients in coordinates and pressure gradient errors.
- have you tried playing with the basal melt/freezing parameters to reduce freezing of supercooled water? SEAICE_frazilFrac, SEAICE_mcPheeTaper and the like (see seaice_growth.F around line 1050 to see, what they really do). SEAICE_frazilFrac=0 should remove any sea ice formation from supercooled water (which means that this needs to be get rid off in some other way, also not good.), but even setting this to a very small value may help.

I don’t think that your drag (0.002) is terribly too high. Until very recently, this used to be the default value.
I think that the solver options will not help you.
If the ice growth cannot be stopped, you can always cap HEFF. This is unphysical, but so are 40m of ice.

Martin

> On 4. Feb 2019, at 12:16, Naughten, Kaitlin A. <kaight at bas.ac.uk> wrote:
>
> Hi everyone,
>
> I am running MITgcm on a Weddell Sea domain with sea ice and ice shelf cavities. I have a fresh bias in the cavity, which is likely because the katabatic winds in my atmospheric forcing are too weak and sea ice formation is therefore not strong enough. I am trying to counteract this by increasing the SEAICE_drag (between the atmosphere and the sea ice).
>
> Currently I have this set to 0.002 which is double the default value. If I push it any further, the model blows up due to extreme potential temperatures. This blowup seems to be happening just inside the cavity, just beside a region of very thick ice (~40 m) which forms due to supercooled water flowing out of the ice shelf cavity. I know this thick ice is due to the supercooled water, because if I switch off ice shelf thermodynamics it goes away. It doesn't seem to cause any problems for the rest of the simulation, except that now it seems to be stopping me from increasing sea ice drag. At the time of the blowup, the sea ice velocities in this region are also very strong (~3 m/s).
>
> I am using modified EVP and have tried increasing the number of EVP subcycles, as well as the alpha and beta parameters, but this does not help. I have two questions:
>
>        • Is there any other way I can improve the stability of the sea ice so that I can continue to increase drag without making it blow up? I have tried LSR instead of modified EVP but it makes the thick ice worse.
>        • Does anyone have suggestions for how to get rid of the thick ice? I have tried removing in-situ supercooling outside of cavities, but this significantly impacted the water mass properties on the shelf. I also tried setting SEAICEpressReplFac to 0 which made no real difference, and using very high sea ice diffusivities which just spread the thick ice out a tiny bit.
>
> Many thanks,
> Kaitlin
>
> Dr Kaitlin Naughten
> British Antarctic Survey
>
>
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