[Liwg-core] Rain-snow repartitioning

Marcus Löfverström marcusl at ucar.edu
Wed Jan 11 18:31:44 MST 2017


Hi Bill,

I am intrigued by this idea. It will of course require some carefully
crafted sentences when formulating this in the model documentation but I
think it could be a way to defend having a different partitioning over
glaciated surfaces.

*Question primarily for Jan and Leo*: what was the rational for choosing 0C
and -5C as end values in the ramp you guys tested the other day? Perhaps
you mentioned it in an earlier email but I seem to have missed out on most
of the discussion about these experiments -- how they were set up and what
results you got. Did you have a reason for selecting precisely these values
or was it more of "a fishing expedition" (as I reviewer once wrote when I
didn't motivate a parameter choice in a paper). Is it possible to run a few
shorter simulations (perhaps 10 yr F cases) where you test the impact of
different choices along the line of Bill S's email above? Sorry for nagging
if you have already done that.

Best,
Marcus

On Wed, Jan 11, 2017 at 5:35 PM, Lipscomb, William Henry <lipscomb at lanl.gov>
wrote:

> Hi Bill,
>
> Thanks for the papers.  I’ll look at them this evening.
>
> One other thought to add to the mix:  For glacier landunits we’re doing a
> fairly crude temperature adjustment (globally uniform lapse rate).  Using a
> somewhat unphysical ramp—or a ramp different from that used in other
> landunits—could perhaps be rationalized as a fix for a crude lapse rate.
>
> But I feel like I’m handwaving.  I’d be happy to have some backup from the
> European folks tomorrow :-)
>
> Bill L.
>
>
> On Jan 11, 2017, at 3:31 PM, Bill Sacks <sacks at ucar.edu> wrote:
>
> Thanks for your thoughts Bill and Jan.
>
> I did a quick literature search, which turned up the attached papers. I
> have given these only the briefest of skims, mostly looking at the figures,
> but they suggest to me that – for the land surface as a whole – the current
> ramp from 0 - 2 C is more physically justifiable than a -2 to 2 C ramp. In
> fact, if anything, the ramp should be shifted more in the *positive*
> direction – say from 0 - 3 C. Someone else should look at these more
> closely to make sure I'm interpreting them correctly. There's still the
> possibility that the relationship could be different over the ice sheet, I
> suppose, as well as the possibility that we can acknowledge that we're
> making this change in order to get good answers despite lack of physical
> justification....
>
> Bill S
>
>
>
> <Dai-2008-Geophysical_Research_Letters.pdf>
> <Sims and Liu 2015.pdf>
> <Stewart et al 2015.pdf>
>
> On Jan 11, 2017, at 3:09 PM, Lipscomb, William Henry <lipscomb at lanl.gov>
> wrote:
>
> Hi Bill S.,
>
> You raise some good questions.  I think the answer to (1) is yes.  We
> would be exchanging an excess of snow for an excess of rain.  My reasoning
> would be that it may be better to get the right answer (good SMB) for the
> wrong reason (excessive rain) than the wrong answer (poor SMB) for a
> different wrong reason (excessive snow).  Either way, we have too much
> precip, and we’re tuning to limit the damage.
>
> A subtlety, though, is whether the recently improved precip is causally
> related to colder temperatures.  If so, then the knobs we’re using to
> reduce precip are not very effective knobs for reducing SMB.  Do others
> have thoughts on this?
>
> As for (2), I’d expect that in warmer climates we’ll see more rain.  With
> modified repartitioning, we'd be starting from a different preindustrial
> baseline, with the usual caveats about possible differences in climate
> sensitivity when you have a different baseline.
>
> I haven’t looked at Antarctica.  Maybe Jan and Leo have?
>
> Thanks,
>
> Bill L.
>
>
>
>
>
> On Jan 11, 2017, at 2:39 PM, Bill Sacks <sacks at ucar.edu> wrote:
>
> Hi all (just including LIWG folks here),
>
> I was just giving Bette a recap of our discussion on rain-snow
> repartitioning, and this raised a couple of questions for us:
>
> (1) For Greenland, is it correct that a change to the rain-snow
> partitioning would lead to more rain, which is a degradation relative to
> observations – but that Jan and Bill L feel we can (and should) live with
> this degradation in order to boost the melt?
>
> (2) Particularly if (1) is true: Since it feels like this achieves the
> right answer (net SMB) for the wrong reasons (too much rain): Has any
> thought been given to whether this makes sense in a climate change (future
> or paleo) world?
>
> (3) Has anyone looked at what this change to rain-snow partitioning does
> over Antarctica? We want to be sure that we're not substantially degrading
> the Antarctica SMB from this change....
>
> Thanks,
> Bill S
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>
> ---
> William Lipscomb
> Los Alamos National Laboratory
> Group T-3, MS B216
> Los Alamos, NM 87545
> 505-667-0395 <(505)%20667-0395>
>
>
>
>
>
>
> ---
> William Lipscomb
> Los Alamos National Laboratory
> Group T-3, MS B216
> Los Alamos, NM 87545
> 505-667-0395 <(505)%20667-0395>
>
>
>
>
>
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-- 
Marcus Löfverström (PhD)
Post-doctoral researcher
National Center for Atmospheric Research
1850 Table Mesa Dr.
80305 Boulder, CO, USA

https://sites.google.com/site/lofverstrom/
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