[Liwg-core] energy budget issue for glaciated landunits

Kampenhout, L. van (Leo) L.vanKampenhout at uu.nl
Tue Jan 24 14:13:56 MST 2017


Hi Bill and others,

I’ve been doing some more experiments with LW downscaling:
https://docs.google.com/document/d/1l6SU0PPba8eOuB8fiI3LYVDdyDMrwZkQf2lyh7Ztaqw/edit#

As I mentioned in a previous email, I found that it has little effect on the energy balance in offline runs (only about 5 Gt/yr).
However in coupled mode, there is a big response when I turn off LW downscaling : it is halved (cf. run B3 and B4).

I can’t really explain this at the moment, it could even be caused by natural variability , but I suspect a real difference between coupled and offline runs.
I’ll keep working on this a bit more, any suggestions welcome!

Leo



On 23 Jan 2017, at 16:48, Lipscomb, William Henry <lipscomb at lanl.gov<mailto:lipscomb at lanl.gov>> wrote:

Hi Leo,

Your finding about the downscaled LW is very interesting.  When we added the LW downscaling option, I was imagining giving greater LW (and greater melt) to lower-elevation ECs, relative to high-elevation ECs.  I think this suggestion originally came from Jeff Ridley at the Hadley Centre.  But I can see how the downscaling would redistribute LW energy from glaciated to bare-land columns, which could be a bad thing.

I’d suggest that we first try turning off the LW radiation downscaling entirely, and see what happens.  Then we can come back to the idea of an ice-column-only downscaling.  I would expect that an ice-column-only downscaling would maximize melt in low-lying glacier ECs, though as you say the physical basis for this (without a corresponding treatment of SW) is a little shaky.

Thanks,

Bill L.

On Jan 23, 2017, at 8:13 AM, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:

Hi Leo,

Okay, thanks for the explanation of your thoughts. I'll let Bill L chime in here if he has anything more to add, but my understanding is that the current parameterization implicitly assumes that cloud fraction, cloud height and most other properties such as cloud liquid water content are the same for different elevation classes / bare land. In that case, downwelling longwave radiation is a function of cloud temperature. The current adjustment is based on differences in surface temperature, which won't be exactly the same as cloud temperature, but seems like a reasonable approximation given the limited information we have.

Do you see problems with this?

If you feel this is problematic, then I'd probably argue for just turning off longwave radiation downscaling entirely, rather than applying it only over ice-covered regions. This can be done via the namelist parameter, glcmec_downscale_longwave.

Thanks,
Bill S

On Jan 23, 2017, at 6:33 AM, Kampenhout, L. van (Leo) <L.vanKampenhout at uu.nl<mailto:L.vanKampenhout at uu.nl>> wrote:

Hi Bill,

I'm adding liwg-core to the cc list here, to include others in the discussion.

that is probably a good idea, thanks.

Within the CISM domain, the elevation of the bare land portion of the grid cell comes from CISM. Jeremy and I discussed this extensively, and this felt like the most consistent way to handle downscaling. Are you suggesting continuing to do other aspects of the downscaling over bare land columns, but not the longwave downscaling? What would the physical justification be for not doing the longwave downscaling over the bare land columns?

Downwelling longwave radiation is controlled primarily by the existence and thickness of clouds. So by downscaling longwave radiation you’re downscaling a cloud property. However, clouds also modulate shortwave radiation and this is not accounted for, which makes the current approach inconsistent. I believe this inconsistency currently leads to a net loss of radiation over the ice sheet, with a considerable magnitude (~ 0.5 W/m2 year round, equivalent to about 80 Gt/year of melt).

Now that I think about it, an alternative approach could be to introduce a lapse rate for SW transmittance, based on .e.g. Figure 6 of this paper<http://www.the-cryosphere.net/10/2379/2016/tc-10-2379-2016.pdf>. This could compensate the LW radiation loss by an increase in SW radiation. Unsure how to relate the SW transmittance to SW downwelling though.

Leo



On 23 Jan 2017, at 13:43, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:

Hi Leo,

I'm adding liwg-core to the cc list here, to include others in the discussion.

Within the CISM domain, the elevation of the bare land portion of the grid cell comes from CISM. Jeremy and I discussed this extensively, and this felt like the most consistent way to handle downscaling. Are you suggesting continuing to do other aspects of the downscaling over bare land columns, but not the longwave downscaling? What would the physical justification be for not doing the longwave downscaling over the bare land columns?

Bill S

On Jan 23, 2017, at 5:30 AM, Kampenhout, L. van (Leo) <L.vanKampenhout at uu.nl<mailto:L.vanKampenhout at uu.nl>> wrote:

Hi Bill,

I was wondering about the topographic height of the bare land portion of the grid cell. Is this set by CISM or is is equal to the grid cell mean height? Either way, I think in general this height will be lower than that of the glaciated portion, so more radiation goes the the bare land (low elev) than to the glacier part (high elev). To my current understanding, this is a design decision with big implications ( -5 W/m2 over the entire ice sheet) so we may need to reconsider this. My proposal would be to turn off downscaling for the bare land columns, just for the glaciated columns. Has this ever been tested? If not, I would like to test the effect of this.

Thanks,
Leo



On 21 Jan 2017, at 14:41, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:

Hi Leo,

If I understand the problem right: You need to look at grid cell means for the longwave radiation downscaling to conserve: the adjustments are made over the bare land portion of the grid cell, too, so those need to be included in your averages to get conservation.

Bill S

On Jan 20, 2017, at 6:12 PM, Kampenhout, L. van (Leo) <L.vanKampenhout at uu.nl<mailto:L.vanKampenhout at uu.nl>> wrote:

Hi Bill,


To make sure I understand:  It’s currently the case that in glacier columns with fractional snow cover, any shortwave radiation that ought to be going into the ice (or “soil”) is simply thrown away?  And the fix, at least for now, is to apply all this radiation to the snow cover until the snow is gone?

That is correct. Following this reasoning, the imbalance is greatest when the fractional snow cover is smallest, right before bare ice appears.

Looking at your pdf plot, it seems there is a mean ice-sheet-wide imbalance of ~0.1 W/m2 during summer and fall.  Based on a back-of-the-envelope calculation, that translates to a melting deficit of ~5 Gt/yr, which is less than I was hoping.  But obviously the imbalance is much bigger locally, which could lead to important feedbacks.  Also, with it being late on a Friday, I could have made a mistake somewhere on my envelope :-)


I get the same value as you do, with the same caveat.

There was a second finding in my exercise which I did not mention in my email because I’m unsure what to think of this yet. That is, the longwave downscaling also causes a SEB residual but only when integrating the _ICE energy flux variables, not with the grid-cell average variables. According to the code documentation, this cannot be true because there is a normalisation in place that forces the downscaled longwave to exactly equal the gridcell value. However, when I turn off LW downscaling the residual disappears.

That said, the residual is about 0.5 W/m2 throughout the year with your Stefan-Boltzmann downscaling approach, and over 0.6 W/m2 with a lapse rate approach. Using the same envelope as before, this translates to about 80-100 Gt/yr of additional melt that we’re missing out on, excluding feedbacks. I don’t have time to look into this over the weekend, but I’m happy to do some more analysis next week.

Cheers,
Leo


Thanks again,

Bill L.


On Jan 20, 2017, at 3:21 PM, David Lawrence <dlawren at ucar.edu<mailto:dlawren at ucar.edu>> wrote:

Thanks Leo,

I read through your presentation and I think your logic is sound.  But, as Bill suggests, Sean is really the one who should review this because he knows this code really well.  I think he is in a meeting this afternoon, but I hope he can look at this on Monday at the latest.

Cheers,

Dave

On Fri, Jan 20, 2017 at 2:55 PM, Bill Sacks <sacks at ucar.edu<mailto:sacks at ucar.edu>> wrote:
Hi Leo,

Thank you very much for your careful analysis and detailed write-up of this problem. I have only skimmed through this and will let others – particularly Sean – comment in more detail.

I'm not sure if this is related, but I have vague recollections of a discussion with Sean a few years ago about how the fractional snow parameterization should be treated for glacier_mec columns. I recall our feeling that at least some parts of this parameterization should not apply to glacier_mec columns, since they already have some treatment of subgrid snow variability through the multiple elevation class scheme. I can't recall any more details right now, though maybe Sean remembers more?

Bill S

On Jan 20, 2017, at 2:51 PM, Kampenhout, L. van (Leo) <L.vanKampenhout at uu.nl<mailto:L.vanKampenhout at uu.nl>> wrote:

Forgot to attach the source code that contains the fix


On 20 Jan 2017, at 22:32, Kampenhout, L. van (Leo) <L.vanKampenhout at uu.nl<mailto:L.vanKampenhout at uu.nl>> wrote:


Hi Dave, Sean and Bill,
CC others,

While doing the surface energy analysis for Greenland, I discovered that the surface energy budget was not fully closed. This holds both at the grid-cell integrated level as well as the glacier level (for this, I use the  variables ending in ‘_ICE’).

See attachment seb.pdf, panel SEB residual. Note that the imbalance occurs in summer, during the melt season.

I reproduced this problem in a single column run, the results are attached. I believe the problem lies with the implementation of fractional snow and how glaciers are handled, namely by setting frac_sno_eff = 1.0. I hope you agree with the analysis. I also suggested a workaround which appears to work without problems in another ongoing BG run (2+ years). We are seeing slightly higher melt rates now over Greenland, which is good. How substantial the changes are is left to be seen.

In the long-term a better solution may be desired.

Cheers,
Leo



<swrad_bug_glaciers.pptx>

<seb.pdf>

<SurfaceRadiationMod.F90>



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











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





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