[MITgcm-support] Help with internal wave breaking and shoaling parameterization package

Fri Aug 26 11:40:39 EDT 2022

Hi Nadav, I just discussed answers to your questions with group in cc.

1. We used MITgcm in hydrostatic configuration.

2. We used KPP because it was the parameterization used in the global model that provided the boundary conditions, so mostly for inertia reasons and due to lack of bandwidth to explore all possible options.  But you are right in pointing out that background diffusivity and viscosity in KPP is meant, in part, to represent internal wave breaking.  Note that below the surface boundary layer, KPP includes a Richardson-number based representation of mixing.  This is why it makes sense to turn off the background diffusivity and viscosity terms when you are starting to explicitly resolve internal waves in the simulation.

3. Breaking of IWs in our simulations is parameterized by the KPP Richardson-number based term, since our resolution is insufficient to resolve actual breaking.  With the exception of the southwest corner of our domain, IWs do not have much opportunity to shoal in our open-ocean regional simulations.  I think this is a big difference with your domain, where much of the vertical mixing physics in your specific case may be driven by interactions with bottom bathymetry.

4. For realistic representation of vertical shear, more vertical levels is definitely better.  If you can afford to increase vertical levels, you definitely should, and compare with the coarser-vertical-resolution simulations to establish impact of vertical resolution on the solution.

Cheers,

Dimitris Menemenlis

On Aug 26, 2022, at 5:11 AM, Nadav Mantel <nadav.mantel at mail.huji.ac.il<mailto:nadav.mantel at mail.huji.ac.il>> wrote:

Hi Dimitris,

Thank you for responding to my email.
I have a few questions regarding the paper and the study.
1. In the study did you run a non-hydrostatic or hydrostatic model?
2. Why in the first place use KPP in IW parameterizations? Obviously it is an extremely well known and widely used vertical parameterization package but isn't meant to include IW in the first place. What was the motivation not to research KL10, or one of the richardson packages (PP81, MY82, GGL90) which were designed for this reason?
3. If I understood correctly, in the study you researched IW presence, its shear and other physical aspects of the IW spectrum, yet you didn't look at the shoaling and breaking of said IWs, once again, if I understood correctly because I might be missing some crucial point. Would the findings of the study (remove KPP background parameters) also hold for this type of parameterization?
4. In the study you use high vertical resolutions. I have "inherited" a model which uses 32 layers, in an elongated and rather deep basin. In your opinion, should I increase vertical layers or do the MITgcm packages overcome the reduced resolutions?

Thanks so much for your input and for sharing the paper,
Nadav

On Thu, Aug 25, 2022 at 11:15 PM Menemenlis, Dimitris (US 329B) <dimitris.menemenlis at jpl.nasa.gov<mailto:dimitris.menemenlis at jpl.nasa.gov>> wrote:
Hi Navad, here is a recently-published paper that may be relevant to some of the questions that you ask: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022GL099614

But note that the above study pertains to the representation of internal waves in an open ocean region, as opposed to a narrow, elongated gulf.

Dimitris Menemenlis

On Aug 25, 2022, at 2:49 AM, Nadav Mantel <nadav.mantel at mail.huji.ac.il> wrote:

Hey everyone,

This is a topic that comes up every now and then but I would still really appreciate someone's help.
I am running a non-hydrostatic regional model for the Gulf of Aqaba\Eilat, an elongated semi-enclosed basin [~200 km long, ~30 km wide ~1000 m deep] with an entry through a shallow sill in the south. The model runs with a horizontal resolution of 300 m and 32 vertical layers with the  differences in depth of 5,10, 12.5, 15, 17.5, ... m and a time step of 10s. I'm forcing tides through the southern boundary using the obcs package. The barotropic flow crossing the Straits of Tiran (our shallow sill) creates large amplitude (50 m in observations) internal waves along the gulf.

We would like to parameterize the shoaling and general breaking of the internal waves using one of the many packages MITgcm has to offer but we have had some troubles.

We first intended to use the KL10 package, yet we had uncharacteristic seasonal mixing and when checking the viscosity output we had weird results where in some places we got viscosity coefficients of 3 [m^2/s] whereas the rest of the gulf is much smaller. It could be possibly due to non-linear EOS, I saw in the mitgcm bug tracker that there are issues with anything but linear EOS use in the package.

We moved on to the richardson number packages, starting with pp81 as it is the simplest. Our problem was that we consistently got the maximum viscosity (a default of 1) in almost all of the second layer, creating once more uncharacteristic mixing with very large diffusivities (60 m^2/s).

We thought about using KPP as it was the package used in simulating the gulf up until now, but it wasn't designed for this purpose and we are skeptical it would produce good results.

Kind of at a loss at this point, and it would be really amazing to get someone's feedback via zoom call on their experiences with internal wave parameterization using any package for regional models.

Thanks,
Nadav Mantel
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