[Liwg-core] update on snow resetting

Lipscomb, William Henry lipscomb at lanl.gov
Wed Mar 29 18:15:38 MDT 2017

Hi all,

Thanks to Bill S. for the clarification of spin-up procedures.

I’m OK with the plan Bill S. suggests.  My understanding is that CLM will spin up starting from bare snow in task 3B.  It’s still possible that glacier-ablation-area biases could contaminate 3B, if the atmosphere is too cold as a result of sitting over glacier columns with 10-m snow in task 2 when it ought to be sitting over glacier columns with seasonal snow.  But since Greenland glacier ablation zones occupy a relatively small area, I can live with that bias.


Bill L.

On Mar 29, 2017, at 9:08 AM, Miren Vizcaino <M.Vizcaino at tudelft.nl<mailto:M.Vizcaino at tudelft.nl>> wrote:

On Mar 29, 2017, at 9:17 AM, Lenaerts, J.T.M. (Jan) <j.lenaerts at uu.nl<mailto:j.lenaerts at uu.nl>> wrote:

Hi Bill,

Thanks for your update and clarification. If I understand you correctly, the Task 3B of the spinup will - in any case - include a IG procedure with BG forcing (similarly to what Miren suggested to do yesterday) with a fully erased snow ‘memory’ for the entire globe.

Hi Jan, I am pretty sure that there will be snow ‘memory’ in the atmospheric forcing - so we have to be careful. miren

So that means we actually need to do less work with a better result: great!

I follow your recommendation as there is little added value to spend a lot of time to get it ‘as best as we can’ for Task 2A, and a simple tweak will suffice.

All the best,


On 29 Mar 2017, at 03:11, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:

Hi all,

I talked more with Dave about the snow resetting, and he clarified some important points regarding the spinup procedure and timeline.

First: I have misunderstood the spinup process for the "real" CESM2 run. Dave clarified that we could start from bare ground for the final iteration, spinning CLM up under coupler history output (Task 3B in the attached pdf).

However: There could be some value in having the upcoming iteration of the spinup (Task 2A) have better snow conditions, to the extent that this will feed back on the atmosphere and thus determine the atmospheric forcings that will be used for Task 3B.

To say that differently, in case it's confusing: A procedure for resetting the snow pack may not actually be used directly for the "final" CESM2 spinup, but it could be useful for these initial pre-spinups, in order to get the most consistency as we build towards the final spinup. In addition, it could be useful for future development iterations (post-CESM2 release), which may use a procedure more like what we've been doing for the last year and a half: using offline spinups to start up a coupled run.

Based on this, and based on the tight timeline for making this useful for Task 2A, I would recommend that we fall back on the simpler procedure of resetting all snow over tundra (i.e., non-glacier) points to some small value (35 mm?), while leaving glacier points alone. This would mean NOT implementing the idea of determining whether each point is an ablation or accumulation point.

This recommendation is based on a few factors: (1) To be useful for Task 2A – which is really the time when this needs to be applied – this needs to be in place, ideally on the CLM trunk, by the end of this week; (2) We don't need to get the initial state exactly right at this stage, and this resetting procedure is irrelevant for the final spinup – so we just want to get something close to "right".

Does that seem reasonable? If so, then Leo, I'd still love your help confirming the implementation for this resetting procedure, confirming that this works as intended, and helping to determine how long we need to let the model run for in this "snow deletion" mode in order to remove the whole snow pack.

Bill S

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Jan Lenaerts
IMAU, Utrecht University || University of Colorado
@lenaertsjan<https://twitter.com/lenaertsjan> || website<http://www.colorado.edu/lab/icesheetclimate/>

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William Lipscomb
Los Alamos National Laboratory
Group T-3, MS B216
Los Alamos, NM 87545

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