[MITgcm-support] Questions about the OBCS Package

[Matthew Mazloff]

Hi Ben

The default is a Dirichlet boundary condition…plain and simple :o)

Matt






> On Jun 11, 2016, at 10:39 PM, Benjamin Ocampo <rurik at ualberta.ca> wrote:
> 
> Hi Matt:
> 
> The system I am simulating is a 120x120x80 curvilinear grid system with heating from below. 
> Since I want to simulate convection with rotation and no boundary conditions on the sides, 
> the OBCS package seems appropriate. The model spans roughly 40^o min. along the 
> latitude (a square polar cap region of minimum length R_sphere*40^o).
> 
> I could try to simulate the entire sphere but then the efficiency of the model drops off
> significantly due to the fine grid (preferred method) needed to simulate convection. 
> The regional model with the Orlanski configuration does indeed work so I have no 
> problem with it.
> 
> As for i), to further clarify the question:
> 
> I) For the default OBCS, what mathematical model is used to simulate open
>    boundary condition on the sides? Or how is it being simulated? 
> 
> The reason why I ask this question is twofold: 
> 
>  - To understand the mathematics and the algorithm in how the default 
>    open boundary condition is being simulated at sides. 
>  - To compare the mathematics of the default to the sponge and Orlanski
>     algorithm.
> 
> This will help me further understand the mathematics behind regional
> modelling in general.
> 
> Cheers,
> Benjamin
> 
> 
> 
> 
> 
> 
> On Sat, Jun 11, 2016 at 9:17 AM, Matthew Mazloff <mmazloff at ucsd.edu <mailto:mmazloff at ucsd.edu>> wrote:
> Hi Benjamin
> 
> i) The state will simply be prescribed on the open boundary.
> 
> ii) sorry, I don’t use it
> 
> 
> I’m not sure your setup — you could provide more info. But most likely my advice will be to make a restoring region that is perhaps 10 cells thick and has a bathymetry with no gradient normal to the boundary. (That way you will not be forcing a normal flow that immediately has divergence from topographic gradients.) Prescribe the normal flow and T and S (and SSH if necessary for your setup) that you want. Set tangential flow to 0 and have a restoring on this tangential flow, which will damp the open boundary waveguide.
> 
> These things I am suggested are intended to limit unphysical vertical flows.
> 
> Open boundaries are no fun,
> Matt
> 
> 
> 
> >
> > On Jun 10, 2016, at 9:45 AM, Benjamin Ocampo <rurik at ualberta.ca <mailto:rurik at ualberta.ca>> wrote:
> >
> > Hi All:
> >
> > I have a couple questions about the OBCS package with regards to
> > regional modelling at the poles:
> >
> > i)   If I do not use Orlanski, Stevenson or Sponge configuration
> >      with the OBCS package, what method or algorithm does
> >      OBCS use to simulate Open Boundary Conditions (OBC)?
> >
> > ii)  Although I have read [Orlanski, 1976], I am still a little bit
> >      confused with the paper. As far as my understanding goes,
> >      the Orlanski OBC solves for the time dependent phase speed
> >      C of the fluid characteristic variable phi at grid points lying on
> >      (N-1) where N is the index locating the horizontal boundary
> >      sets the C at horizontal boundaries so long as C is b/w some
> >      range. Algorithm wise, what are the advantages and
> >      disadvantages of using the Orlanski configuration? When is
> >      the Orlanski configuration appropriate to use.
> >
> > Cheers,
> > Benjamin
> >
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