[MITgcm-support] Free exchange across OBC

[Jody Klymak]

Hi Neil,

If you make T_b = T_i then dT/dx = 0 and there are no pressure gradients, so you can’t have T_b set by the interior *and* have the correct pressure gradients.  In fact, as you are finding, it is very difficult to radiate arbitrary signals out an open boundary.  Orlanski does OK if the flow is dominated by one phase speed perpendicular to the boundary, but that is almost never the case in three-D.  

If I had your problem, I would simply telescope the open boundaries "far enough away" from the region of interest that the boundaries do not affect the local solution and then apply a sponge at those boundaries to some background level that will absorb any effects that radiate in.  “Far enough away” is probably a couple of mode-1 horizontal wavelengths (maybe 400 km in the open ocean), but if your internal response is relatively modest you may be able to get away with a smaller domain.

Cheers,   Jody

> On 2 Apr 2015, at  7:24 AM, Neil Fraser <Neil.Fraser at sams.ac.uk> wrote:
> 
> Hi Everyone,
>  
> I am running as experiment to study the effects of certain wind events on the stratification inside my domain. My domain has 3 open boundaries, with the northern edge bounded by coastline.
>  
> My problem is this: if I keep the boundary forcing constant (with or without the sponge method), the stratification at the boundary is then constant in time and affects any changes in the stratification of the interior. However, if I prescribe time-varying boundary conditions I must assume prior knowledge of how the stratification changes with these wind events, which is exactly what I am trying to study.
>  
> Is there a configuration allowing free exchange between inside the domain and outside? i.e. allow water to advect in and out of the domain, with velocities determined by  pressure gradients between boundary points and adjacent interior points, and with T and S of any inflow matching that of the boundary on the previous timestep.
>  
> Right now I have Orlanski and Balance turned on, and I am seeing some unwanted boundary effects propagating into the domain.
>  
> Any help greatly appreciated,
>  
> Cheers,
>  
> Neil
> The Scottish Association for Marine Science (SAMS) is registered in Scotland as a Company Limited by Guarantee (SC009292) and is a registered charity (9206). SAMS has two actively trading wholly owned subsidiary companies: SAMS Research Services Ltd (SC224404) and SAMS Ltd (SC306912). All Companies in the group are registered in Scotland and share a registered office at Scottish Marine Institute, Oban Argyll PA37 1QA. The content of this message may contain personal views which are not the views of SAMS unless specifically stated. Please note that all email traffic is monitored for purposes of security and spam filtering. As such individual emails may be examined in more detail. _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org <mailto:MITgcm-support at mitgcm.org>
> http://mitgcm.org/mailman/listinfo/mitgcm-support <http://mitgcm.org/mailman/listinfo/mitgcm-support>
--
Jody Klymak    
http://web.uvic.ca/~jklymak/





-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20150402/155e7493/attachment.htm>