[MITgcm-support] OBCS settings

Tue Sep 12 19:41:24 EDT 2006

Hi Matt,

Thanks for that.  I had made sure that all unfilled 'water' cells in my initialisation theta and salinity fields were replaced with a local nonzero mean, but to double check, i've replaced these with dummy fields of  theta=1, salinity=33 for the entire model domain.  And then:

With closed boundaries, this runs fine (my first 'STOP NORMAL END', yay!).

With open boundaries, the wierd edge effects kick in again;  this is for sure a problem with my boundaries, not with T,S.

One crucial piece of information could be that i don't initialise the velocities at the open boundaries, so perhaps the more pertinent question is:  if OB[E/w][u/v/t/s]files are required, then 
a) do i understand correctly from the package code that this means allowing OBCSprescribe, and
b) do the u/v/t/s files contain simple 2-d binary fields of u/v/t/s )dims [1 x ny]?
c) does exf need to be switched on for this, and if so
d) is it possible to prescribe cyclic (e.g. 1 year's worth only) fields for OBCS but a longer time series for other 
packages (KPP & seaice)?  and finally
e) is it possible to use daily u/v/t/s files run from MITs ongoing global model runs (or is there some other source i've not found that gives global, depth-distributed u/v/w fields) ?

sorry for the deluge of questions- i've only found 1 /verification example that uses obcs and didn't manage to glean much from it.

If anyone can point out an example that would help, i'd be grateful.

To your original question, some of my wierd edge effects    ( with dummy initialisation fields of theta=1, S=33, a model domain from -50 to -80 (south), 140 to 260 E, 1 degree isotropic resolution, nx = 120, ny = 81, nz = 21)  :

T, S at timestep 0 feature a band 1 cell wide at the eastern boundary which is set to 24 at iz=0 and changes with depth; i think it's being set to the tref for each depth --- maybe the default behaviour with non-prescribed boundaries?  According to the T/S fields shown in state.0000...nc this makes a big discrepancy at the eastern edge, as the neighbouring cells contain theta values closer to +-1.

min/max OBNu,v,t,s OBEu,v and OBWu,v  are +- 1 E+32 at all timesteps,
min/max OBEt,s and OBWt,s are  4 to 24 and 34.58 to 34.9 respectively

U,V and W go crazy at the EW edges, with strong negative velocities in the northern corners and positive ones in the southern corners.  At the next 2 timesteps, these get crazier, with interleaved NS strips of strong + and - velocities, but the velocities at the boundaries themselves are zero.  This is also true if i switch nonslipsides to true.

There's more, but that seems plenty to begin with!
Thanks again,
jill.

----- Original Message -----
From: Matthew Mazloff <mmazloff at mit.edu>
Date: Tuesday, September 12, 2006 1:33 am
Subject: Re: [MITgcm-support] OBCS settings

> Hi Jill
> 
> One thing to watch for is unphysical prescribed values.  For 
> example,  
> I have made open boundary conditions which resulted in zero 
> salinity  
> values being prescribed on some wet points.  If this occurs at 
> depth  
> you will get some substantial upwelling.  What kind of "weird edge  
> effects" are you seeing?
> 
> -Matt
> 
> 
> On Sep 10, 2006, at 9:09 PM, <jschwarz at awi-bremerhaven.de> wrote:
> 
> > Hi all,
> >
> > I'm getting some wierd edge effects in a model of the Ross Sea, 
> and  
> > as they only appear when i open the East and West boundaries i  
> > presume i'm setting the boundaries wrong.  Could someone verify  
> > that the following is correct:
> >
> > model domain:  nx=120; ny=81;   Southern edge is coastline;
> > objective:  open entire eastern and western boundaries, keep the  
> > northern boundary closed;
> >
> > in data.obcs:
> > OB_Ieast = 81*-1,
> > OB_Iwest=81*1,
> > OB_Jnorth=120*0,
> >
> > (and OBCS switched on in packages.conf etc etc - the boundaries 
> are  
> > definitely being read in and are echoed into the mitgcmuv output 
> file)>
> > I've tried some variations on this with rather spectacular  
> > effects.. but they point to the above being what i want;  only 
> the  
> > text in OBCS.h doesn't seem to say the same thing.. i'm a bit  
> > confused.
> >
> > Many thanks for any assistance!
> > jill.
> > _______________________________________________
> > MITgcm-support mailing list
> > MITgcm-support at mitgcm.org
> > http://mitgcm.org/mailman/listinfo/mitgcm-support
> 
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