Error: negative mass in set_thermodynamic

Unified Model
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1

Hello

I’m running an RA3 nesting suite forced by ERA5 down to 200m (u-cr028). Everything runs smoothly down to 500m but for 200m it fails after ~45mins model time with the error:

?  Error code: 100
?  Error from routine: set_thermodynamic
?  Error message: A total of     2 points had negative mass in set_thermodynamic. This indicates the pressure fields are inconsistent between different levels and the model is about to fail.
?  Error from processor: 808
?  Error number: 41

I’ve taken my timesteps from here:

https://code.metoffice.gov.uk/trac/rmed/wiki/subkm

I previously attempted to run with an 8s timestep and saw from previous questions reducing the timestep is often the fix for this, at 6s the model runs for more time before crashing with fewer points failing. Is there much point in reducing the timestep further or is it more likely there’s something awry with my inputs or physics?

It’s a big domain so I’m reluctant to keep trying reduced time steps unless this is a sensible approach and the reduction 8s to 6s was likely too small to overcome the stability issue?

2

Hi Helen

I guess you’ve been here: https://code.metoffice.gov.uk/trac/um/wiki/KnownUMFailurePoints#Pointswithnegativemassinr2_set_thermodynamic

(it doesn’t help much.)

Reduction of the timestep need not be permanent, if the model runs on past the problem, it might be OK to increase the timestep again. Not much to lose by reducing it further at this stage

Grenville

3

yep read that and a few of the older threads. OK, I’ll reduce down to 4s for this CRUN and hope it’s just a spin-up hiccup!

4

reducing the timestep to 4s worked, I’ll try increasing back to 6s for the next 6 hours and edit this post if that’s successful for completeness as I couldn’t see in previous questions if people were successful in increasing the timestep back.

5

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