[MITgcm-support] sponge layer

[Patrick Heimbach]

David,

the 3-dim. relaxation code checked in a few days ago
is independent of the obcs code.

My understanding is, this code would come in handy for what you
need, if you could use a spatially varying relax. constant, 
since it would enable you to mimic an extended sponge layer 
(not just relaxed to a 2-d slice, but to an extended 3-d strip),
and suppress relaxation where tau(i,j,k) is zero
(the checked-in code does allow for interpolation in time).
Is this correct?
Well, if so, should be easy to do.

-Patrick
 


On Wed, 2005-12-14 at 18:10, David Wang wrote:
> 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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> >> MITgcm-support at mitgcm.org
> >> http://mitgcm.org/mailman/listinfo/mitgcm-support
> > 
> > 
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-- 
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Patrick Heimbach   Massachusetts Institute of Technology
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