[Liwg-core] latest #134 with reset snow thickness simulates N ablation areas

Lenaerts, J.T.M. (Jan) j.lenaerts at uu.nl
Wed Mar 22 13:54:13 MDT 2017


Hi all,

Thanks Raymond and Miren for the great work.

I am a bit cautious on using this 35 mm. In this case, i.e. without repartitioning and with N_MELT=0.5, this seems to work, but I wonder what happens as soon as one of these two is reset to default values.

Switching off repartitioning is undesirable, since that would create a huge discrepancy between land and glaciers (assuming we only switch it off for glaciers). We still have some room to play with N_MELT.

Cheers,

Jan








On 22 Mar 2017, at 17:39, Jeremy Fyke <garmeson.lanl at gmail.com<mailto:garmeson.lanl at gmail.com>> wrote:

Hi Miren

This is great.  To what extent do you think this represents an equilibrated snow pack/ablation area?  Ie are you guys confident the snow pack isn't 'filling in' still, and that this ablation area would persist indefinitely if this simulation was continued?

Jeremy




On Wed, Mar 22, 2017 at 10:13 AM Miren Vizcaino <M.Vizcaino at tudelft.nl<mailto:M.Vizcaino at tudelft.nl>> wrote:
Hi Bill, and All

The latest simulation is able to sustain N Greenland ablation areas and seasonal snow over ice-free areas

https://www.dropbox.com/s/da8ez62hipf04ns/134_newinitialization.pdf?dl=0

Raymond is running another resetting simulation with the repartition on.

For the resetting technique, please check with Leo if anything remains unclear. Raymond was applying Leo’s capping approach that I assume will adjust all variables.

One important remaining issue is the mismatch between downscaled SMB<0 and QICE_MELT. Bill and Bill, do you have any ideas of what is going on here? Could it be related to still having acab=0 over the

accumulation areas?

- integrated ablation for the downscaled SMB (sum of SMB<0) = 70 Gt; however, integrated QICE_MELT=300 Gt (maybe due to differences in how the 1-SCF is included in the computation?)


Regarding refreezing, the spatial map and amounts seem to largely agree with RACMO (please see slide 6 of the PDF). The % using melt as the sum of QICE_MELT and snow melt is in the right ballpark.

I’d like very much to hear from Jan, Leo and/or Michiel on the thickness initialization amount. My take here is that, giving the delicate mass and energy balance in northern Greenland, and that we don’t start the runs in September, it is critical to follow Raymond’s recommendation.

Thanks, Miren





On Mar 22, 2017, at 3:28 PM, Raymond Sellevold - CITG <R.Sellevold-1 at tudelft.nl<mailto:R.Sellevold-1 at tudelft.nl>> wrote:

Hi,

Yes, I will also do an experiment with n_melt = 1, to see the effect of resetting the snowpack more clearly.

There have been several experiments by Leo where the snow has been reset to 100mm over the tundra, with the outcome that the snow keeps on growing. The reason seems to be because the total melt in this region is the same as the yearly snowfall and when areas that have a total snowfall of 35mm w.e. between end of melt season and January are initialised with 100mm in January, you end up with more snow than the model is able to melt in the summer, thereby causing the permanent snow cover. The same story applies for the northern ablation areas.

Currently, I have adapted Leo’s method of doing this: h2osno_max is set to 35.0 and I run the model for one day, such that the model itself calculates the snow. I figured this was a better idea then editing the restart files since h2osno is not the only variable related to snow.

Raymond

On 21 Mar 2017, at 22:16, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:

Hi Miren and others,

Thanks a lot for this.

This sounds promising, but it also sounds like we need a clean experiment to just see the effect of resetting the snow pack. In addition to turning the repartitioning back on, should n_melt be kept at 1 for this?

I'm also wondering if a reset value of 100 mm would have the desired effects, since that was the value that I thought we agreed on. I guess we could use 35 mm, but it seems confusing to have this reset value differ from the cold start value, which is currently set at 100 mm. Should we also switch the cold start value to 35 mm in this case? I'm interested to hear what others think about this – particularly Leo and Jan, who had thoughts in the past about the appropriate snow initial conditions to avoid undesirable heating of soil under snow.


Also: I realized that there could be some subtleties with how exactly we should reset the snow pack. If we decide to move ahead with this solution, I'd like a more detailed description of how this should be done. Specifically: it's not clear to me that just resetting h2osno is enough, because there are a lot of different snow variables, and resetting h2osno without resetting some other variables might cause things to be in an inconsistent state.

Related variables that are integrated across the snow column are int_snow_col, snow_depth_col, and possibly others. And then there are many variables that are stored layer-by-layer for the snow pack. Does anyone know if any of these variables need to be reset for things to be in a consistent state? (From a quick look through the code, it looks like it should be okay not to reset int_snow_col, but I'm not positive of that, and I'm not sure about the other variables.)

Thanks,
Bill S



On Mar 20, 2017, at 10:41 AM, Miren Vizcaino <M.Vizcaino at tudelft.nl<mailto:M.Vizcaino at tudelft.nl>> wrote:

Hi,

Raymond did a new run with #134 physics and reset of initial snow thickness to 0.35mm over the GrIS and n_melt=0.5 and rainfall repartition off.

The simulations has gone through 20 years. Ablation areas of GrIS are well captured and amount to 10% of ice sheet area.

Other highlights:

- Seasonal snow cover over tundra

- Some high accumulation areas in the SE have reached maximum snow thickness (10 m w.e.)

- integrated ablation for the downscaled SMB (sum of SMB<0) = 70 Gt; however, integrated QICE_MELT=300 Gt (maybe due to differences in how the 1-SCF is included in the computation?)

- refreezing amounts to 40% of snowmelt + QICE_MELT + rainfall and 57% of snowmelt + downscaled ice melt + rainfall.  Still a high ratio. Will this ratio change as snow thickness equilibrates ?

Next steps:

-  sensitivity to switching on rainfall repartition

Thanks, Miren












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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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