[MITgcm-support] Noise in vertical velocity field with ocean cooling/sea ice

[Lily Greig]

Hi MITgcm community,

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.

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.

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.

Many thanks,

Lily
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