[MITgcm-support] 2deg global experiment: advection schemes

[Martin Losch]

How embarrassing for me,
but my statement remains valid: "beats me" (o:

Martin

On Nov 7, 2005, at 5:01 PM, Samar Khatiwala wrote:

> Hi Martin
>
> You may want to reread the threads I started on advection schemes 
> (some time
> in Sept 2004 and again in April 2005 I think). JMC had some useful 
> recommendations
> to make, notably the use of staggerTimeStep when using a 
> forward-in-time method
> (like the DSTXX schemes). The reason has to do with stability due to 
> internal waves.
> In a more idealized setting, I too got dramatically different 
> circulations (more so than
> yours) with different advection schemes.
>
> To quote a well known expert on the MITgcm:
>
> "Samar,
> beats me, but the different advection schemes can be dramatically 
> different in their result (have a look at the documentation 
> http://mitgcm.org/sealion/online_documents/node80.html ).
> Martin"
>
> Samar
>
> On Nov 7, 2005, at 6:47 AM, Martin Losch wrote:
>
>> Hi there,
>>
>> I would like to share my experience with this (and maybe get some 
>> general advice):
>>
>> I am running a 2deg global ocean model (much like the 2deg ECCO 
>> configuration, that is, from -80 to 80 deg, 23 layers, GM, KPP) and I 
>> make the interesting observation, that the solution is very sensitive 
>> to the advection scheme (and the parameter staggerTimeStep) that I 
>> use.
>> I have reason to believe that the default 2nd-order scheme is 
>> inadequate for my purposes (I restore T and S along the southern 
>> boundary in order to model the effects of the Antartic shelf ice 
>> caverns and thus get sharp gradients there, that lead to weird T and 
>> S properties near the southern boundary), so I tried 3rd-order 
>> upwind, DST3 with an without flux limiter, and 2nd-order with flux 
>> limiter.
>> First of all, when staggerTimeStep=.false. (default), then these 
>> advection schemes (together with a set of GM, diffusivitity, 
>> viscosity parameters that produced a reasonable circulation with the 
>> default 2nd-order scheme: 133Sv ACC, 20Sv MOC, etc) give me a 
>> dramatically intensified circulation: ACC order 400-500Sv, etc. 
>> watermasses change drastically (not the patterns, just the horizontal 
>> and vertical gradients, which explains the stronger circulation).
>> Second, staggerTimeStep=.true. brings down the values to a realistic 
>> 126-130Sv (and realistic MOC, etc), except for DST3 without flux 
>> limiter, where the ACC is still O(250Sv).
>>
>> 2 years ago, Jean-Michel recommended to use the staggerTimeStep flag 
>> with higher order schemes, so I am not so surprised that this option 
>> salvages most of the runs, but still I am surprised that these 
>> advection schemes have such a fundamental impact on the solution.
>> I had expected that the additional diffusion introduced by these 
>> schemes (in particular the flux limited schemes) would actually 
>> reduce the circulation (as they mostly do with 
>> staggerTimeStep=.true.), but not increase it.
>>
>> Was I wrong to keep the GM-diffusivity and viscosity parameters while 
>> switching the advection shemes?
>>
>> Martin
>>
>> Martin Losch // Alfred-Wegener-Institut für Polar- und Meeresforschung
>> Postfach 120161, 27515 Bremerhaven, Germany
>> mailto:mlosch at awi-bremerhaven.de // Tel./Fax: ++49(471)4831-1872/1797
>> http://www.awi-bremerhaven.de/People/show?mlosch
>>
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>
>
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