[MITgcm-support] spurious energy sink in AIM?

Mon May 30 21:25:51 EDT 2011

Brian,
Good to hear the budget can be "closed".
That said, I don't think the large-scale precip is driven by convergence 
associated with surface drag. It's mainly due to northward advection of 
warm/moist air in synoptic scale systems, which then cools and condensates.

Both the KE sink and the large-scale precip increase with the eddy 
activity, but I' m not convinced there is a direct relationship between 
the two.
david

On 5/27/11 3:14 PM, Brian Rose wrote:
> Update:
>
> Turns out David is right, it's surface friction that's responsible for 
> the energy sink.  Frictional KE sink is of order 1 W/m^2 globally, and 
> correlates very closely with changes in large-scale precip.
>
> In case anyone is interested, I'm attaching two new plots:
> - one shows the very close correlation between large-scale precip and 
> frictional heating.
> - the second shows that the energy budget is (mostly) closed by adding 
> the "missing" heating from surface friction and Shapiro filter to the 
> radiative imbalance.  The residual looks like random scatter around 
> about +0.1 W/m^2 bias.
>
> I suppose that the large-scale precip is being driven fairly directly 
> by convergence associated with surface drag, which explains the 
> closely correlation.
>
> My problem now is that this introduces a spurious positive feedback 
> into the model, since eddies, friction and large-scale precip go down 
> in warmer climates.
>
> Any thoughts on how difficult it would be to implement frictional 
> heating, and thus move closer to total energy conservation?
>
> Brian
>
>
>
>
>
>
>
> On May 27, 2011, at 11:10 AM, Brian Rose wrote:
>
>> 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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