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Vertical migration and CFL criterion

I was wondering if enabling vertical migration simulation with big amplitude between day and night location in the water column (400 to 100 m depth) could result in breaking CFL criterion,
I am using NEMO3D model with a resolution of 0.083 degree, and still break occasionally CFL crierion on W with number consistently between 1 and 1.1 while I set dt to 180s...
if I set dt to 1800 s, then I got a lot these warnings with values mainly between 1 and 3, only on W again,
should I just ignore these warnings?
I am using the ichtyop version shared in a recent post (Coupling Copernicus Ocean data products (OPA-NEMO)),

Ichthyop version: 
Ichthyop 3.3alpha
Hydrodynamic dataset: 

Also, is vertical migration working when backtracking particles? it's not obvious when I look at the ouputs,

I would say that CFL between 1 and 1.1 is not a big deal. If you are concerned about it, however, then decrease the timestep a bit more and it should be OK.

Hi Christophe,
I am a bit concerned because I have seen in a previous post that Philippe said that it was a bug in th program in computing the vertical velocity, and I wonder if something similar is happening here, I don't see why vertical velocity would be so important that particle skip a layer over 180 s, another problem that I guess is related is that a lot of particles get beached in the ground (at 500 m depth),
I am going to run several simulations and I would to be sure everything is working that I don't need to rerun them and also I would prefer to set dt to something like 1800 s (which is consistent considering the horizontal resolution of the model that I use) to reduce computing time,

Hi Boris.

I can have a look at that. However, the problem for the vertical is that although W values are small, the layer thickness is also small compared to the horizontal spacing (a hundred meters compared to a hundred kilometers). Which could explain the CFL issue with W.


Hi Nicolas,
I sent you by email a link to access the file,
I am now doing backtracking simulations to estimate sources, then I will do forward tracking including vertical nmigration behaviour, if I keep the coastline behavior parameter on 'beaching' a lot of particles beach in the sea bottom, which is odd, so I set it on 'bouncing' but then I usually get error message at varying time during the simulation,
you will see that the simulation produces continously CFL broken errors,