Reverting the global ice particle limiter to local limiter

[oyvindseland]

What is the feature/what would you like to discuss?

Taken from the planning discussions at #186

Reverting the global maximum ice particle number maximum back to a local limiter in time and space(nimax) This parameter was used in an erroneous way in CESM2 and NorESM2. This was corrected in later versions of NorESM2.x The global limiter was found to give a better ECS in particular when working on LGM simulations, but has no particular physical argument to it. At the present time it is set to 1 cm-3. While re-introducing the old parameterisation is relatively straightforward from a code point of view it may need shadow files for PUMAS. The shadow file will likely be the same as for including secondary ice formation so these tasks can likely be connected. May have a large impact on the model climate sensitivity.

Is there anyone in particular you want to be part of this conversation?

No response

Will this change (regression test) answers?

Yes

Will you be implementing this enhancement yourself?

Yes, but I will need some help

[gold2718]

@oyvindseland, can you select a Milestone so we know where this fix is going to go?

[oyvindseland]

We also need a general NorESM3 milestone

[oyvindseland]

I started changing the parameterisation by commenting out the existing. As a first test I only ran 1 month and was surprised that turning off the limiter decreased the global peak for ice particle numbers. I also found a reduction over large regions. I checked that turning off was not don in the wrong way by doing a test in which I increased the global limiter by 1e4. This increased limit gave identical results to the one that commented out the limiter. I presume that it must be just a fluke caused by a change in the meteorological patterns. The first month do indeed have large changes in the patterns compared to default, so I have started a longer simulation to see if I can understand impact.