Opened 11 years ago

Closed 9 years ago

#285 closed help (fixed)

responding to a run with negative pressure

Reported by: mjm Owned by: um_support
Component: UM Model Keywords: instablilty, HadCM3, negative pressure
Cc: m.mineter@… Platform:
UM Version: 4.5

Description

Hello!

A run fell over with negative pressure after 8 or 9 model years.
I recall that this happened once before, in a run we later abandoned, so I'd like to capture best practice from you in case it happens again again.

I realise the negative pressure indicated numerical instability, but
I'd not want to do the whole run at (say) halved timestep.

An option is perhaps to restart with dump files from before the crash, and with a smaller timestep for a period then change the timestep back. But I'm not sure how the means are managed across runs, and if this will cause confusion in the annual and decadal means for example.

I;m also a bit uneasy in principle about say halving the timestep (for the atmosphere i assume, only) half way through a run for a bit of the run.

It might also just be that we have made an error and so this would not arise if we had the run set up correctly - so we are also checking this. (Is this always the explanation of "negative pressure"?)

So general guidance would be welcome! (Not asking you to debug the specific run - at least yet!)

Regards
Mike

Change History (2)

comment:1 Changed 11 years ago by lois

Hello Mike,

I am not sure that there is any standard guidance for these problems are the result of many different issues. You will have to manage the annual and decadal mean problem in the best way you can!

My suggestion would be to go back to the last dump and reconfigure and see if this reconfigured dump smooths out whatever is causing problems with a NRUN. Just because it doesn't crash where it did previously doesn't mean that it will not crash again.

If it is crashing again then you could try reducing the timestep for a short period until it overcomes whatever issue it causing the crash.

Sorry for the bland suggestions!

Lois

comment:2 Changed 9 years ago by lois

  • Resolution set to fixed
  • Status changed from new to closed
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