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<div>Thank you Martin, </div>
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<div>I will give this a try :) </div>
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<div>Best wishes, </div>
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<div>Lily </div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> MITgcm-support <mitgcm-support-bounces@mitgcm.org> on behalf of Martin Losch <Martin.Losch@awi.de><br>
<b>Sent:</b> 17 February 2022 17:31<br>
<b>To:</b> MITgcm Support <mitgcm-support@mitgcm.org><br>
<b>Subject:</b> Re: [MITgcm-support] Noise in vertical velocity field with ocean cooling/sea ice</font>
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<div class="PlainText">Hi Lily,<br>
<br>
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.<br>
<br>
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. <br>
I suggest to start with this:<br>
<br>
# viscC4leith = 2,<br>
#viscC4leithD = 2,<br>
viscA4Grid = 0.01,<br>
viscA4GridMax = 0.5,<br>
# only necessary if you use harmonic viscosity<br>
viscAhGridMax = 0.5,<br>
<br>
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.)<br>
<br>
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.<br>
<br>
There are a few regularization in KPP, but in experience they don’t do much, but maybe worth a try:<br>
<br>
# personally, I think this should be default, but it isn't<br>
C o reduce shear mxing by shsq**2/(shsq**2+1e-16) according to<br>
C Polzin (1996), JPO, 1409-1425), so that there will be no shear mixing<br>
C with very small shear<br>
#undef KPP_SCALE_SHEARMIXING<br>
<br>
# this improves things sometimes at high resolution.<br>
#undef KPP_DO_NOT_MATCH_DIFFUSIVITIES<br>
#ifndef KPP_DO_NOT_MATCH_DIFFUSIVITIES<br>
C only makes sense if the diffusitivies are matched<br>
# undef KPP_DO_NOT_MATCH_DERIVATIVES<br>
#endif /* KPP_DO_NOT_MATCH_DIFFUSIVITIES */<br>
<br>
C o Include/exclude smooth regularization at the cost of changed results (maybe useful for AD-simulations)<br>
C With this flag defined, some MAX(var,phepsi) are replaced by var+phepsi<br>
#undef KPP_SMOOTH_REGULARISATION<br>
<br>
<br>
Hope that helps,<br>
<br>
Martin<br>
<br>
> On 17. Feb 2022, at 18:06, Lily Greig <l.greig@pgr.reading.ac.uk> wrote:<br>
> <br>
> Hi MITgcm community, <br>
> <br>
> 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. <br>
> <br>
> 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. <br>
> <br>
> 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.<br>
> <br>
> Many thanks, <br>
> <br>
> Lily <br>
> <data><UVEL_WVEL_snap.png>_______________________________________________<br>
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