[MITgcm-support] sponge layer

[David Wang]

Matt,

I might not use the exactly correct terminology (so that confused Lex...).

Okay, I need that within some width near a domain boundary there will be 
an extra relaxation term for temperature tendency equation, which is

	-(Tmodel - Tstar)/\tau

where Tmodel is the model temperature and Tstar is some temperature 
climatology interpolated onto the right time stamp. \tau is the function 
of the distance from the boundary. I think this treatment is often 
employed in tropical ocean modeling.

(Lex, this is so-call "relaxation", which is essentially a Newtonian 
damping towards some prescribed values, Tstar)

While in the current obcs code, there is such a term as above, but Tstar 
is calculated as the linear combination of model temperature and 
prescribed boundary temperature:

           trelax (i.e., Tstar) =(
      &      float(spongeThickness-jsl)*OBNt(i,kLev,bi,bj)
      &    + float(jsl)*theta(i,j,kLev,bi,bj) )
      &    / float(spongeThickness)

I hope I'm saying more clearly than I was...

Thanks, D.W.

Matthew Mazloff wrote:
> Hi David,
> 
> To restore to a value use obcs prescribe and make the location an  "open 
> boundary"; the model will set the field to the prescribed value  at this 
> open boundary at the end of every forward step.
> 
> Sponge layers are designed to allow a smooth transition to a  prescribed 
> value (I think they will still work with obcs_prescribe  off).  For an 
> artificial restoring force the difference between the  model and the 
> prescribed data (divided by some timescale) is used.   The MITgcm does 
> this with the timescale falling off linearly over the  sponge layer 
> thickness.
> 
> It is not clear to me if you want something else besides this?
> 
> -Matt
> 
> 
> On Dec 14, 2005, at 4:59 PM, David Wang wrote:
> 
>> Hello...
>>
>> I'm going to implement "sponge layers" near artificial domain  
>> boundaries within which model temperature may be restored to some  
>> temperature climatology (say Levitus94) at different relaxation  
>> timescales. I've already checked out the sponge layer option in the  
>> obcs pkg, where the relaxation temperature is the linear  combination 
>> of model temperature and the prescribed temperature at  the boundary. 
>> This is not really what I want. If I need to write  some extra code to 
>> achieve my purpose (which is not uncommon),  could anyone provide any 
>> guidelines... or did I miss anything?
>>
>> btw, I did notice the 3-dim relaxation code emerged in the latest  
>> checkpoint, but its relaxation timescale is set to be spatically  
>> constant...
>>
>> Thanks, David
>>
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> 
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-- 
Daiwei (David) Wang	 Graduate Student
Lamont-Doherty Earth Observatory of Columbia University
103B Oceanography
61 Route 9W, Palisades, NY 10964-8000
Voice: (845) 365-8694