[MITgcm-support] Noise in vertical velocity field with ocean cooling/sea ice

[Lily Greig]

Thank you Martin,

I will give this a try :)

Best wishes,

Lily

________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
Sent: 17 February 2022 17:31
To: MITgcm Support <mitgcm-support at mitgcm.org>
Subject: Re: [MITgcm-support] Noise in vertical velocity field with ocean cooling/sea ice

Hi Lily,

hydrostatic simulations will always have noise issues in the vertical velocity (as the vertical velocity is the vertical integral of the horizontal velocity divergence, so any noise accumulates, and C-grids tend to have noisy horizontal fields), unless you have very smooth horizontal velocities.

I would use viscA4Grid to tune this noise away, also I would use a smaller viscAXGridMax of maybe 0.5 (1 is the limit; X=h,4). I wouldn’t use smag and leigth simultaneously, as the do the same thing, also the Smagorinsky parameterisation is for 3D isotropic turbulence, which you don’t have in your set up.
I suggest to start with this:

# viscC4leith = 2,
#viscC4leithD = 2,
 viscA4Grid = 0.01,
 viscA4GridMax = 0.5,
# only necessary if you use harmonic viscosity
 viscAhGridMax = 0.5,

and vary viscA4Grid until you are happy. Alternatively I would try  viscC2leith+D in addition to constant backround (or you can use viscC2leith+D = 2 + viscAhMin = 0.01.)

A trick is to use non-hydrostatic dynamics (with very few cg3d iterations O(40) to not make too expensive), because that introduces horizontal viscosity for vertical velocity.

There are a few regularization in KPP, but in experience they don’t do much, but maybe worth a try:

# personally, I think this should be default, but it isn't
C o reduce shear mxing by shsq**2/(shsq**2+1e-16) according to
C   Polzin (1996), JPO, 1409-1425), so that there will be no shear mixing
C   with very small shear
#undef KPP_SCALE_SHEARMIXING

# this improves things sometimes at high resolution.
#undef KPP_DO_NOT_MATCH_DIFFUSIVITIES
#ifndef KPP_DO_NOT_MATCH_DIFFUSIVITIES
C only makes sense if the diffusitivies are matched
# undef KPP_DO_NOT_MATCH_DERIVATIVES
#endif /*  KPP_DO_NOT_MATCH_DIFFUSIVITIES */

C o Include/exclude smooth regularization at the cost of changed results (maybe useful for AD-simulations)
C   With this flag defined, some MAX(var,phepsi) are replaced by var+phepsi
#undef KPP_SMOOTH_REGULARISATION


Hope that helps,

Martin

> On 17. Feb 2022, at 18:06, Lily Greig <l.greig at pgr.reading.ac.uk> wrote:
>
> Hi MITgcm community,
>
> I'm using MITgcm with set-up similar to Horvat et al. 2016 (75  by 75 km at 2 km resolution, zonally re-entrant channel, northern half sea ice covered, hydrostatic). The ocean is cooling everywhere, sea ice forming with thsice package. I am having some issues with grid scale noise and large values of the vertical velocity field. I have tried a few adjustments so far: I turned KPP smoothing options on, to no effect, and next I tried using different viscosities/tuning the viscosity constants using examples in the MLAdjust tutorial. Using modified Leith & Smagorinsky, noise reduction only happens in the model when I increase the viscosity constants by up to 2 orders of magnitude, but increasing them this much also dampens the dynamics, reducing magnitude of the zonal jet.
>
> For reference, I've attached an image with snapshots of zonal & vertical velocity fields, model day 105, for a range of viscosity constant values from 1.85 -> 250, all using modified Leith & Smagorinsky. The subplot titles e.g 'UVEL LS30' stand for velocity field + viscosities used (all Leith and Smag. here, or LS) + the value used for every viscosity constant. I've also attached an example data file to this email. Increasing the viscosity constants reduces the vertical velocity from ~ 4e-4 m/s to 1e-4 m/s, but also the zonal jet by ~ factor 2.
>
> I have also tried also using Leith & Smagorinsky individually, QGLeith, harmonic/biharmonic but all to little avail unfortunately. Any help would be much appreciated, if anyone has an idea of what could be the issue, or further adjustments to try.
>
> Many thanks,
>
> Lily
> <data><UVEL_WVEL_snap.png>_______________________________________________
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