[MITgcm-support] Smaller deltaTmom speeds up the model?!

[Naughten, Kaitlin A.]

Hi everyone,


I am experimenting with timesteps and have discovered something strange. I have deltaT=600, and for the first few months of simulation I need to have deltaTmom=60 so that some areas with an unstable water column in the initial conditions don't cause the model to blow up. After the first few months, I can remove deltaTmom=60 (so it defaults back to deltaT=600) and the model is stable.


But when I do this, the model slows down! To confirm this was the case, I ran two simulations starting from the same timestep (1 year in). The simulation with deltaTmom=60 completed 22464 timesteps in 20 minutes. The simulation with deltaTmom=deltaT=600 only completed 8496 timesteps.


I have two questions about this:


1) Is this behaviour as counterintuitive as it seems? I'm wondering if larger momentum timesteps are somehow causing the elliptic solver to converge more slowly, which more than compensates for the reduced number of timesteps?


2) Since a smaller deltaTmom is so much faster, is there anything wrong with having a different value for deltaTmom and deltaT? Some of my colleagues thought that having different timesteps for ocean tracers and momentum might be dangerously unphysical.


Many thanks,

Kaitlin Naughten

________________________________
This message (and any attachments) is for the recipient only. NERC is subject to the Freedom of Information Act 2000 and the contents of this email and any reply you make may be disclosed by NERC unless it is exempt from release under the Act. Any material supplied to NERC may be stored in an electronic records management system.
________________________________
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.mitgcm.org/pipermail/mitgcm-support/attachments/20180619/a3c1b515/attachment.html>