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

[Vanderborght, E.Y.P. (Elian)]

Dear Martin,

Thank you very much for your response and suggestions. I’ve implemented all the proposed changes. Unfortunately, the output still shows irregularities near the northern boundary, even after significantly increasing viscAhGrid. Switching to synchronous time stepping also did not resolve the issue. The striping persists across all three components of the velocity field.

Any further ideas are appreciated!

Best regards,
Elian
-----Oorspronkelijk bericht-----
Van: MITgcm-support <mitgcm-support-bounces at mitgcm.org> Namens Martin Losch
Verzonden: vrijdag 4 april 2025 8:12
Aan: MITgcm Support <mitgcm-support at mitgcm.org>
Onderwerp: Re: [MITgcm-support] Irregular Velocity Fields Near Basin Boundaries in MITgcm

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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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