[Liwg-core] update on snow resetting

Bill Sacks sacks at ucar.edu
Tue Mar 28 19:11:21 MDT 2017

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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