[Liwg-core] target: ablation areas
Lipscomb, William Henry
lipscomb at lanl.gov
Thu Jan 26 06:53:27 MST 2017
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?
On Jan 26, 2017, at 2:52 AM, Miren Vizcaino <M.Vizcaino at tudelft.nl<mailto:M.Vizcaino at tudelft.nl>> wrote:
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
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?
I'd like to think that CESM suffers from a cold bias in the North and East of about 2 degrees:
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
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