[MITgcm-support] numerical instability in a non-hydrostatic simulation

[Guangyu Xu]

Hello Jean-Michel,

Thanks a lot for your response. I have found the cause of the issue. It is the temperature advection scheme (33) used. Those short-wave-length oscillations of therms reflect ’numerical dispersion’. I switched to a lower-order advection scheme (77) and those artifacts disappeared. However, the wake behind the platform also becomes a lot smoother. The scheme 77 is more diffusive than 33 and hence may have smoothed out turbulent features to some degree. On the other hand, the increased numerical diffusion suppresses small-scale instability. 

Guangyu



> On Jun 20, 2026, at 7:57 PM, Jean-Michel Campin <jmc at mit.edu> wrote:
> 
> Hi Guangyu,
> 
> I don't know where the noise comes from, but just to get an idea of resolution
> and domain size: 
> what are the value of Nx,Ny,Nr ? (from the parameter file "data", it's not obvious 
> since tRef,delX,delY, delR are set from a bin file).
> And are you using a variabl resolution in X ? in Z ?
> 
> Cheers,
> Jean-Michel
> 
> On Thu, Jun 11, 2026 at 09:18:01AM -0700, Guangyu Xu wrote:
>> Hello fellow MITgcm users,
>> 
>> I have been running MITgcm simulations to investigate the wakes behind
>> surface platforms. Attached are a plot of the vertical temperature
>> cross-section from a non-hydrostatic run and the runtime model settings.
>> The two vertical bars near 0 and 0.5 km w-e distances are the subsurface
>> structures of two platforms. The initial temperature field is horizontally
>> homogeneous. The model forcing consists of a constant eastward flow of 0.2
>> cm/s. There is no surface forcing or Coriolis.
>> 
>> I noticed that the oscillations of isotherms around 0.5 km w-e distance
>> have very short wavelengths, which span only two grid cells. These
>> oscillations are numerical artifacts, and they are not present in the
>> results of hydrostatic runs.
>> 
>> I am interested to know if any of you have seen similar artifacts in
>> non-hydrothermal simulations. Any suggestions on how to eliminate or reduce
>> these numerical noises would be greatly appreciated.
>> 
>> Many thanks!
>> Guangyu
> 
> 
> 
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org <mailto:MITgcm-support at mitgcm.org>
>> https://urldefense.com/v3/__http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support__;!!K-Hz7m0Vt54!gYwLYFk5G73TbHHaZtfzXruH4khyKSG71jH-deYi3Tz8Ik4YltrpTuyUHm_Z2mH6uU3_yRcmtrI$
> 
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org <mailto:MITgcm-support at mitgcm.org>
> https://urldefense.com/v3/__http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support__;!!K-Hz7m0Vt54!gYwLYFk5G73TbHHaZtfzXruH4khyKSG71jH-deYi3Tz8Ik4YltrpTuyUHm_Z2mH6uU3_yRcmtrI$

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.mitgcm.org/pipermail/mitgcm-support/attachments/20260621/9163ff72/attachment.html>