[MITgcm-support] Tracer instabilities when I reduce hFacMinDr

[Martin Losch]

Hi Kailin,

this looks like a difficult problem to solve. Here are my ad-hoc ideas:

The unphysically negative temperatures are most likely just the consequence of your instability and then the flux-limiters (which are never perfect) can’t help. They end to mask problems in the initial state, which is not helpful either. Maybe you want to try a non-flux-limited scheme to see where the problem starts.

When you checked the clf-numbers, did you also check advcfl_W_hf_max? In contrast to advcfl_wvel_max, this number takes into account the hFacC (so it will be larger for smaller hFacC, i.e. hFacMinDr)?
Also are your instabilities correlated (in space) with rough or steep topography?

The minimum cell thickness is computed like this (as you probably already know):
MAX( hFacMin, MIN(hFacMinDr*recip_drF(k),oneRL) )
so that for thin cells (10m) you’ll have a minimum thickness of 5m, but for thicker cells you’ll have drF*hFacMin, which is larger than 5m for drF>50m. Does this tansition have anything to do with your instability pattern?

Did you have look at the vertical structure of your instabilities. Often not only the horizontal but also the vertical viscosity/diffusivity can cause cfl-like problems.

What happens if you decrease your time step even further? Does the problem go away?
Try larger viscosities (for momentum) …

Martin


> On 24. Oct 2018, at 12:47, Naughten, Kaitlin A. <kaight at bas.ac.uk> wrote:
> 
> Hi everyone,
> 
> I'm currently running an ice shelf configuration with hFacMin=0.1, hFacMinDr=20, and vertical resolution ranging from 10 m (top few layers) to 25 m (in the cavity) to 100+ m (deep ocean). I'm trying to decrease hFacMinDr so I can get a better representation of the ice shelf base, as the current value of 20 m combined with the 25 m resolution in the cavity doesn't allow the partial cells to do very much.
> 
> I'm trying to get hFacMinDr=5 working. I expected to have to reduce the timestep to prevent vertical CFL errors, and at this point I have reduced the timestep by a factor of 4 (which should exactly compensate for the thinner cells, given the CFL criterion is linear, correct?) However, the simulation still looks very strange, in particular the bottom water temperature and salinity in a few patches on the continental shelf (not in the cavity): it looks like checkerboard instabilities or false extrema, even though I'm using a flux-limited advection scheme (33). In some cells, the temperature goes down to -7 C.
> 
> I checked the CFL terms in the monitor output, and the vertical terms are on the order of 1e-2. This is similar to my previous simulations with hFacMinDr=20. and timestep 4 times larger. So, why am I still seeing these instabilities? Are they still CFL errors (despite what the monitor terms seem to suggest) or could they be something else?
> 
> I am using the linear free surface until I find a stable timestep for hFacMinDr=5, so we can rule out any problems related to nonlinear free surface or r*.
> 
> Many thanks,
> Kaitlin Naughten
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