[Liwg-core] target: ablation areas

Kampenhout, L. van (Leo) L.vanKampenhout at uu.nl
Thu Jan 26 08:49:31 MST 2017


Hi all,

I’ve started two new experiments B6 and B7 where I perturb the surface temperature, as per Bill Lipscomb’s suggestion. That is, I only apply the perturbation over downscaled columns, so grid cells without any glaciers are not affected.
https://docs.google.com/document/d/1l6SU0PPba8eOuB8fiI3LYVDdyDMrwZkQf2lyh7Ztaqw/edit?usp=sharing

The runs haven’t finished their first year yet, but the first year already reveals a significant response:

B3 total JJA melt = 182.2 Gt (average over 6 years)
B6 total JJA melt = 179.8 Gt
B7 total JJA melt = 261.7 Gt

B6 melt exceeds B3 melt in June and July, but suffers from a cold August. I’ll send an update to these numbers when the runs have reached at least 6 years of runtime (probably tomorrow). Arguably 6 years is not long enough for the model to adjust, but it hopefully gives an insight into the sensitivity.

In the meantime, I’ve been looking at some regional differences, see this Powerpoint<https://drive.google.com/file/d/0BwZ8r8pyf3aUUnhxZFNvS2w1aUk/view?usp=sharing>. It demonstrates that the regional differences are large, with the West least biased, and the North & East most.




Regarding revisiting snow density: if snow density would inhibit melt, we would see a large ground heat fluxes (GHF), or not? Although GHF is regionally a little on the high side, it does not explain the melt biases. Indeed, the GHF could even be a little high due to the changes that we made in this run, which make the soil a little warmer w.r.t. standard 129 (which has the fractional snow cover bug). Moreover, in my head lower surface densities correspond to higher albedo, which could be a bad thing. However, I’m willing to give this a try if there is a clear wish to do so. Personally, I believe the 129 is still a little too cold, so when it would warm we start to see a much improved SMB simulation. It’s not only the ice sheet that is cold, also the tundra in the North. But then, I could be wrong.

Cheers,
Leo



On 26 Jan 2017, at 14:53, Lipscomb, William Henry <lipscomb at lanl.gov<mailto:lipscomb at lanl.gov>> wrote:

Hi Miren,

I agree that if we had sufficient snow melt to expose bare ice in (at least most of) these ablation areas, we’d have a much improved SMB simulation.  Without more snow melt, it’s hard to see how we get there.

The snow physics is much improved from CESM1 (thanks to Leo and Jan), and I’d be very reluctant to backtrack.  My sense is that the snow albedo has been pretty well validated, and any tuning we did could have undesirable effects outside ice sheets, so that’s not a good option.  We do now have a fixed treatment of fractional snow (thanks again to Leo), which should give us weighting similar to RACMO.  I like Leo’s idea of revisiting the snow density (looking at thermal effects that could inhibit melt).

With respect to sub-grid simulation, our potential tuning knobs include elevation-based downscaling of temperature, radiation and precip.  Currently we downscale temperature and LW, but not SW or precip. (To be precise, we downscale the phase of precip via the temperature ramp, but not the amount.)
* I’m not aware of a simple, well validated downscaling scheme for either SW or precip—is there such a scheme that we’ve overlooked?
* Leo has been experimenting with LW, and my understanding is that turning off the downscaling makes the melt smaller rather than bigger.
* Has anyone looked at sensitivity to the prescribed temperature lapse rate?  I’d be surprised if it made a big difference, but you never know.

My guess is that climate forcing is the major issue.  So I think it’s a good idea to run the column experiments you mention and to contact Dave B.

Since many of the recent runs have been short, I’m not quite clear on how much the initial snow thickness matters and how long it takes the model to equilibrate.  So I also agree it would be good to look at Hinit.  Could you please remind me what value of Hinit we’ve been using for Greenland?

Thanks,

Bill L.

On Jan 26, 2017, at 2:52 AM, Miren Vizcaino <M.Vizcaino at tudelft.nl<mailto:M.Vizcaino at tudelft.nl>> wrote:

Morning, -

Let’s plan on immediate steps for the LIWG towards CESM2.0

Target: having sufficient snow melt in (all) the observed ablation areas as to expose bare ice.

Motivation: ablation areas expose bare ice at some point during the melt season (this is confirmed by remote sensing - I checked with Stef - and regional modeling - this is how RACMO operates. He told me there is no super-imposed ice in the model, so ice albedo comes from getting rid of the snow. There is some weighting in RACMO between bare ice albedos - around 0.4, that come from MODIS- and snow albedos, when the snow thickness is low)

Where we are now: difficulties to get the ablation areas in the north and northwest, and too narrow SW ablation area

Please let me know asap if you disagree with target.

There are three ingredients here:

1- snow physics
2- climate forcing
3- sub-grid simulation

1-

It could be worthwhile to revisit some of the changes that I made to fresh snow density as perhaps this prevents early melt and associated albedo changes.  But I doubt whether this solves the bias completely.

Leo, would you like to discuss how to do some testing here?

2-

I'd like to think that CESM suffers from a cold bias in the North and East of about 2 degrees:
https://www.dropbox.com/s/wqu2n4f6ad5lemb/maps_JJA_b.e20.B1850.f09_g16.pi_control.all.129_liwg_Hmax1m_0019-0021.pdf?dl=0


Raymond is going to look to the sensitivity of snow melt to the forcing in N Greenland with the column model

I’ll contact Dave Bailey

I vaguely remember from CESM1.0 that there was N cold bias and permanent snow pack over the tundra, - same problem?-, but ablation areas where ok. Perhaps the bias in snow melt from wrong densities explains this.

3-  Hinit, Hmax : here we have a bunch of runs, none very conclusive, because of bugs, multiple things changed, too short runs (e.g., B5 is only 3-years-long).

Hinit seems to matter, and should be 0.1-0.5 m w.e. both for ice-sheet and tundra - For Hmax, let’s address it at a later stage

Thanks, Miren


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---
William Lipscomb
Los Alamos National Laboratory
Group T-3, MS B216
Los Alamos, NM 87545
505-667-0395




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