[MITgcm-support] Shapiro filter configuration

[Yuan Lian]

Hi Jean-Michel,

Yes, I know the power "n" is for tuning the strength of Shaipro
filter, but only noticed later Shapiro filter will be turned off is 
"nShapT=0"
for temperature (same for u and v) as Shapiro filter only applies when
"nShapT>0".

I am using C18 generated from matlab script. I need some quick test
so I didn't bump up the resolution to C32. It is quite interesting to
see the corner effects are gone if I turn off Shapiro filters for u and v.
I think this has something to do with the radiative heating. Unlike
the Earth, Titan receives very weak solar heating. Both temperature and
winds change very slowly. I think Shapiro filter actually
induces larger U and V near corners than what generates it (typically
the equator to pole temperature contrast is merely a few kelvin on Titan).

Here is my configuration file data.shap that doesn't give any sort of
corner problems. However, if I change nShapUV=4 then I got strong corner
effects which eventually crash the simulation.

&SHAP_PARM01
  shap_filt_uvStar=.FALSE.,
  shap_filt_TrStagg=.TRUE.,
  Shap_funct=2,
  nShapT=0,
  nShapUV=0,
  nShapTrPhys=0,
  nShapUVPhys=0,
  Shap_uvLength=-1.,
  Shap_Trtau=1200.,
  Shap_uvtau=1200.,
&

Best,
Yuan


On 6/27/11 5:06 AM, Jean-Michel Campin wrote:
> Hi Yuan,
>
> The nShap[T,UV] parameter correspond to the power "n" in section 2.20
> "Shapiro Filter" of the manual. The larger "n" is, the more scale
> selective the filter is (the lesser effect it has on larger scale
> feature).
>
> regarding corner effects:
> - which cube-sphere grid are you using ? There are some CS-32 grid-files
>    that behave better than others regarding this issue.
> - hard to tell much more without knowing what are the shapiro parameter
>    (data.shap) you are using.
>
> Cheers,
> Jean-Michel
>
> On Sat, Jun 25, 2011 at 09:34:16PM -0700, Yuan Lian wrote:
>> OK, Just figured out nShapT=0 will turn off Shapiro filter for
>> active tracers.
>> nShapT=1 is an extremely bad option, which smooths out almost everything.
>>
>> In atmosperhic modeling with cube-sphere grid, I noticed strong
>> corner effect
>> occurs if Shapiro filter is stronger, say nShapUV=2 causes much
>> larger pressure
>> anomaly around corners than nShapUV=4. Setting nShapUV=0 actually totally
>> removes the corner effects. Is there any reason why this happens?
>> Isn't Shapiro
>> filter supposed to smooth the wind vectors (so the vertical winds I
>> assume) than
>> exaggerating them?
>>
>> Thanks!
>>
>> Yuan
>>
>>
>> On 6/25/11 7:15 PM, Yuan Lian wrote:
>>> Hi,
>>>
>>> Does anyone by chance know the difference between nShapT=1 and
>>> nShapT=0? I noticed nShapT=1
>>> has stronger smoothing effect than nShapT=0. Are they supposed to
>>> behave in this way? I though
>>> nShapT=1 would have weaker smoothing.
>>>
>>> Thanks!
>>>
>>> Best,
>>> Yuan
>>>
>>>
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