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

[Martin Losch]

Hi Kaitlin,

see answers below

> On 19. Jun 2018, at 15:12, Naughten, Kaitlin A. <kaight at bas.ac.uk> wrote:
> 
> 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?
you can see what time is spent where by inspecting the profiling at the end of STDOUT.0000. If the elliptic solver really uses all the time in your case with deltaTmom=deltaT, then that should be obvious from the profiling numbers.
I think, deltaTfreeSurf is also set to deltaTmom (please check in STDOUT.0000), so that your elliptic solver now sees a 10 times shorter timesteps, i.e. the tendencies are 10 times smaller and the route to convergence is shorter, so I wouldn’t be surprised, if that’s really what’s happening.
> 
> 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.
You’ll damp all of your fast waves and probably suppress the seasonal cycle. But it also modifies your water masses and stratification. See relevant papers about tracer acceleration (e.g.,
Danabasoglu, G., 2004: A comparison of global ocean general circulation model solutions obtained with synchronous and accelerated integration methods. Ocean Modelling, 7, 323- 341.
Bryan, K. 1984: Accelerating the convergence to equilibrium of ocean-climate models. Journal of Physical Oceanography, 14(4), 666-67
) for a discussion.

Martin

> 
> Many thanks,
> Kaitlin Naughten
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