[MITgcm-support] Irregular Velocity Fields Near Basin Boundaries in MITgcm

[Martin Losch]

Hi Elian,

just a hunch, but your domain stretches from 3degN to 67degN, where the zonal (dx) spacing will have decreased from 1*cos(3deg) ~100km to 1*cos(64deg)~50km, the dy spacing will always be ~111km, so you’ll have non-square grid cells. At 1deg resolution it shouldn’t matter that much, but this could be a problem for the cfl-number, especially the one for viscosity. I’d try to use viscAhGrid instead of viscAh (see mom_common/mom_calc_visc.F or the corresponding part in the documentation), instead of viscAh=2e4, you could use viscAhGrid=0.0065, if I got it right. The vertical velocity is just a consequence of the noise in the horizontal velocities (as it is computed from the divergence of the horizontal velocities)

I would turn off the CD-scheme (this may increase the noise, but a smoothing method may mask problems that you want to see for debugging). In general, I usually use viscA4Grid ~ 0.01 instead.

Did you try synchronous time stepping (i.e. only use deltaT, but not deltaTtracer/Clock, and what’s missing is deltaTFreeSurf)?

Is the striping only in zonal direction?

unrelated: globalFiles does not always work, is not necessary here, useSingleCpuIO is enough.

Martin

> On 3. Apr 2025, at 21:18, Vanderborght, E.Y.P. (Elian) <e.y.p.vanderborght at uu.nl> wrote:
> 
> Hi Jody,
>  Thanks for the reply. The tracer field look fine, I also checked the convection adjustment index and there are no irregularities found here.
>  Best regards,
> Elian
>  Van: MITgcm-support <mitgcm-support-bounces at mitgcm.org> Namens Jody Klymak
> Verzonden: donderdag 3 april 2025 20:41
> Aan: mitgcm-support at mitgcm.org
> Onderwerp: Re: [MITgcm-support] Irregular Velocity Fields Near Basin Boundaries in MITgcm
>  U ontvangt niet vaak e-mail van jklymak at uvic.ca. Ontdek waarom dit belangrijk is
> CAUTION: This email originated from outside of Utrecht University. Do not click links or open attachments unless you recognize the sender and know the content is safe.
>  Hi Elian,
>  I would look at your tracer fields.  Is that maybe just (poorly resolved) convection?
>  Cheers,   Jody
>  From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of "Vanderborght, E.Y.P. (Elian)" <e.y.p.vanderborght at uu.nl>
> Reply-To: "mitgcm-support at mitgcm.org" <MITgcm-support at mitgcm.org>
> Date: Thursday, April 3, 2025 at 8:25 AM
> To: MITgcm Support <MITgcm-support at mitgcm.org>
> Subject: [MITgcm-support] Irregular Velocity Fields Near Basin Boundaries in MITgcm
>  You don't often get email from e.y.p.vanderborght at uu.nl. Learn why this is important
> Dear MITgcm community,
> I am running MITgcm in a rectangular basin with a single-hemisphere setup. However, I am encountering irregularities in both vertical and horizontal velocity fields near the basin boundaries (see attached figure).
> Initially, I suspected insufficient horizontal viscosity. While increasing it to unrealistically high values reduced the issue, it did not resolve it entirely. I have also tested the following:
>  
>     • Decreasing time-step,
>     • Using different advection schemes,
>     • Increasing horizontal diffusion,
>     • Increasing vertical viscosity.
>  None of these approaches have fully addressed the problem. I have also reviewed studies using similar setups, but they do not report encountering this issue. Does anyone have insights into what might be causing this or suggestions for a fix? I have attached my data file and SIZE.h for reference.
> Any help would be greatly appreciated.
>  Best regards,
> Elian
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