[MITgcm-support] useOBCSsponge

[Matthew Mazloff]

Hi Dmitri,

I have used sponge layers in hydrostatic runs without any problems.

Actually, I don't think (as the model is currently coded) you can 
prescribe W on the open boundary for non-hydrostatic runs.  I would 
think the model would crash at runtime if you tried; there is no W file 
name in obcs_readparms.F

      NAMELIST /OBCS_PARM01/
     &          OB_Jnorth,OB_Jsouth,OB_Ieast,OB_Iwest,
     &          useOrlanskiNorth,useOrlanskiSouth,
     &          useOrlanskiEast,useOrlanskiWest,
     &          OBNuFile,OBNvFile,OBNtFile,OBNsFile,
     &          OBSuFile,OBSvFile,OBStFile,OBSsFile,
     &          OBEuFile,OBEvFile,OBEtFile,OBEsFile,
     &          OBWuFile,OBWvFile,OBWtFile,OBWsFile,
     &          useOBCSsponge, useOBCSbalance, useOBCSprescribe

It actually appears that W on the open boundary is set to zero in 
non-hydrostatic runs.  I am not sure why the model forces W to zero on 
the boundaries and maybe someone else can fill us in.  I am guessing 
there is some technical reason or I have missed something and am just 
wrong about this.

After thinking about it, I don't think forcing w=0 on open boundaries is 
extremely significant in ocean modeling.  It is definitely not the most 
realistic way to deal with the boundary; you are correct that it may 
cause sharp gradients in W.  But whether these unphysical gradients are 
significant is another question.  For ocean modeling with reasonable 
surface forcing, large W values are likely only to occur in regions near 
solid boundaries where vorticity generated by the vortex stretching 
associated with the vertical velocity can be dissipated, or in fronts 
where the isopycnals are steeply inclined.  If the model adequately 
resolves these boundary and frontal regions, unphysical gradients may 
have a large effect.  Open boundaries, however, occur mostly in the open 
ocean (and hopefully not in fronts) where it is likely that W is small.  
If W is small next to the open boundary, a zero W on the open boundary 
is likely insignificant.  If the sponge layer is causing W and W 
gradients to be significant in the ocean interior, I would guess the 
prescribed open boundary condition is not very physical.  Perhaps this 
is why noone has taken the time to find a way to calculate (restore) W 
in open boundary (sponge layer) regions.  Just a guess.  Of course, I do 
not know what experiment you are running.  Are you doing something where 
W is significant near your open boundary and needs to be prescribed?  If 
so, I think you will need to do some coding and i hope someone else can 
advise you on the best way.

-Matt



Dmitri Leonov wrote:

> Hello,
>
> Is the "useOBCSsponge" option ready to be used: in general and with 
> non-hydrostatic dynamics in particular?
>
> Only u and v (also t and s but not w) are relaxed over 
> 'spongeThickness' grid points.
> But in the non-hydrostatic mode, w is prescribed at the boundary. That 
> may cause a sharp gradient of w (?)
> Does it mean that some modifications are needed (adding another 
> subroutine to obcs_sponge.F, to be called from CALC_GW)?
>
> Thanks,
> Dmitri
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20050814/88a12cac/attachment.htm>