[MITgcm-support] High vertical velocity/noise next to land

[David Munday]

Dear MITgcm supporters,

I'm using MITgcm in a narrow sector model (20 degrees longitude by 120 degrees latitude with the southern-most 20 degrees as a re-entrant "circumpolar" channel) and want to use as similar a configuration at as wide a range of resolutions as possible. Unfortunately, I'm getting quite noisy velocity fields at half-degree and finer grid spacing, which most visibly manifests as very strong (~1E-4 m/s) grid-scale oscillations in vertical velocity (the configuration is hydrostatic).

At my coarsest resolution of 2x2 degrees I get quite strong upwelling (~1E-6 m/s) next to my western boundary, in the first grid cell, and comparable downwelling in the first grid cell next to the eastern and northern boundaries. I don't think this is the Veronis effect, since I'm using GM to do my temperature/salinity dissipation, and my western boundary currents are about 2-3 grid cells wide. At half-degree and finer, these coherent bands break down into grid scale oscillations and the horizontal velocities start to visibly show the noise that's causing the strong vertical velocities. My first thought was that there might be a problem with my boundary conditions, and so I tried freeslip conditions on the walls to no avail.

I've searched through the support archive to try and get some help and found a number of suggestions that had been made previously. From those discussions I got a few suggestions to try, such as switching to the vector invariant form of the momentum equations, using the Jamart wet point options, highOrderViscosity advection, and making sure that I've got staggerTimeStep set appropriately. I've also fooled around with the different tracer advection schemes. None of that really helped.

I've had the most success with using a Shaprio filter, although I only know what is said about them in the documentation and I spent last friday fairly blindly fiddling to see if I could get better results. The Shapiro filter causes the vertical velocity to form bands of alternating upwelling/downwelling for 2-3 grid cells next to the boundaries, although there are grid scale oscillations still pleasant. The CD scheme does work to prevent the worst of the noise occurring, but makes the flow very viscous, so much so that I can reduce my viscosity by more than an order of magnitude without causing the solution to explode! Just increasing viscosity, by a factor of ten doesn't help, If I wack up the GM co-efficient to suppress all eddy activity the noise remains in coherent bands...Except now I don't have eddies :(

Has anyone got some suggestions for what I'm doing wrong? I've attached the data file that I'm using at half degree without any of the things I've tried over the last three weeks included. For anyone that's familiar with it, I tried a carbon copy of PARM01 from the quarter degree set-up in the repository and that didn't improve the noise.

Thanks for reading my long-winded cry for help, fingers crossed,

Dave

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Dr. David R. Munday
University of Oxford
Atmospheric, Oceanic, and Planetary Physics
Clarendon Laboratory
Parks Road
Oxford
OX1 3PU
U.K.                 01865 272 093

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"But I, being poor, have only my dreams;
I have spread my dreams under your feet;
Tread softly because you tread on my dreams."
     - W.B. Yeats, He Wishes for the Cloths of Heaven -

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