[MITgcm-support] spurious energy sink in AIM?

[Brian Rose]

Hi David,

Thanks for the suggestions.  I just checked the Shapiro filter numbers.  The KE sink does go down at warmer global mean temperatures, and so it helps explain a small part of my imbalance.  But it varies by only 0.15 W/m^2 over the whole array, whereas the variations in TOA radiative imbalance are > 1 W/m^2.  

Surface friction is maybe a more likely culprit, but naturally I didn't save the appropriate diagnostics for that, so I need to re-run some test cases.

I would be surprised though if the connection with large-scale precip is really just coincidence...  the radiative imbalance correlates better with precip than it does with global mean temperature.  Very fishy.

I will report back later.

Brian

On May 26, 2011, at 6:34 PM, David Ferreira wrote:

> Hi Brian, 
> A quick thought: there is friction and numerical viscosity (Shapiro filter) in the model. Both remove kinetic energy and, strickly speaking, this sink of KE should be fed back into the temperature equation as a warming (we discussed doing this with JMC, but never took the time).
> Anyway, I have a note that the surface friction and the Shapiro filter represent a sink of 1.2 and 0.6 W/m^2, respectively in Aqua. This seems to fit your numbers.
> Could this help with your problem ?
> Now there is no obvious connection with the large scale precipitation, but maybe this is just "coincidence". Both friction     and large scale precip decrease with global temp ? (weaker meridional temp gradient, then less eddies,  and then less friction and large-scale precip).
> Cheers,
> david
> 
> On 5/26/11 2:24 PM, Brian Rose wrote:
>> 
>> To those MITgcmers familiar with the atmospheric physics pkg/aim_v23:
>> 
>> I may have found evidence of a bug violating energy conservation, and it seems to be related to the large-scale precipitation parameterization.
>> 
>> Briefly, the model consistently equilibrates with a global mean imbalance in radiative fluxes at the TOA (top of atmosphere) of order 1 - 2 W/m^2 (excess absorbed shortwave over outgoing longwave).  A large ensemble of runs shows that the TOA imbalance varies systematically with the global mean large-scale precip rate.
>> 
>> Some details:
>> - I'm running AIM and pkg/thsice on a global C24 grid with aquaplanet geometry.  The same TOA imbalance appears in slab ocean runs (AIM coupled to a 60 m mixed layer) and in fully coupled runs that have reached equilibrium. 
>> - I compute TOA imbalance based on AIM diagnostic output fields TSR and OLR averaged over 20 year snapshot runs, taking an area-weighted global average of TSR-OLR.
>> - I have a large array of runs in a slab ocean configuration.  I vary a control parameter that leads to changes in global mean temperature.
>> -  All runs have had ample time to equilibrate, usually about 100 years or more.  There is no temperature drift.
>> - The TOA imbalance (net flux in to the system) systematically decreases as the climate warms (see first figure below).
>> -  There is no such imbalance in the net heat flux across the sea surface.
>> -  These runs are all warm enough to be sea ice-free (although pkg/thsis is enabled)
>> - After some sleuthing, I figured out that there is a systematic shift from large-scale to convective precip as the climate warms.  There is a very strong correlation between the TOA imbalance and the large-scale precip rate (see second figure).
>> -  The slope of the graph in figure 2 suggests that the radiative imbalance would tend to zero in the limit of zero large-scale precip.
>> - It thus appears that the large-scale precip is behaving as an energy sink (and also therefore acting a spurious positive feedback in the model).
>> 
>> I tried two simple tests to reduce the imbalance:
>> (1)  set aim_energPrecip=.FALSE.  (normally I have this set to .TRUE.)
>> (2)  increase QSMAX by an order of magnitude over its default value
>> Both tests were run out to equilibrium.  Both had basically no effect on the TOA imbalance.
>> 
>> So I know the imbalance is very well correlated with large-scale precip, but I don't understand why.
>> Any bright ideas?
>> 
>> I am attaching a typical data.aimphys file for my setup.  (there are a few unfamiliar switches in here, belonging to my custom implementation of a specified 'q-flux' to the ocean slab)
>> 
>> Thanks
>> Brian
>> 
>> ---------------------------------------------
>> Brian E. J. Rose, PhD
>> NOAA Climate and Global Change Postdoctoral Fellow
>> Department of Atmospheric Sciences
>> University of Washington
>> 
>> office:  ATG 318   phone:  (206) 543-4596
>> email:  brose at atmos.washington.edu
>> ---------------------------------------------
>> 
>> 
>> 
>> 
>> 
>> 
>> 
>> 
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