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Hi Martin & all, </div>
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Thanks very much for your help, I have tried your suggestions however am still unfortunately having the noise issues, so am just prying for any further thoughts. I forgot to mention in my original email, but I am using a 2D set-up here (Nx=1), although an analogous
3D set-up with eddies also has noise. </div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">Following your recommendations with viscA4Grid, what I found is a similar picture to my previous experiments, in that I had to increase the value of <span style="font-size: 12pt; background-color: rgb(255, 255, 255); display: inline !important;">viscA4Grid</span><span style="font-size:14px;background-color:rgb(255, 255, 255);display:inline !important"> </span>a
lot, from 0.01 up to around 0.5 in order for the noise in the vertical velocity field to reduce. At such high values of viscosity, the zonal jet is also damped an undesirable amount. </font></div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">I have tried your suggestions with <span style="font-family: Calibri, Helvetica, sans-serif; font-size: 12pt; background-color: rgb(255, 255, 255); display: inline !important;">viscC2leith+D which
also didn't reduce the noise much. </span>I also ran using non-hydrostatic dynamics, which made only a very small difference to the noise.</font></div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">Also just to mention for the KPP options you described, I could not find a KPP_SCALE_SHEARMIXING flag anywhere in the code. </font></div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">If you did have any further recommendations, it would be great to hear. Thanks a lot. </font></div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">Best wishes, </font></div>
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<div style=""><font face="Calibri, Arial, Helvetica, sans-serif">Lily </font></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Lily Greig <l.greig@pgr.reading.ac.uk><br>
<b>Sent:</b> 17 February 2022 18:02<br>
<b>To:</b> mitgcm-support@mitgcm.org <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>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="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><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="x_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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