[MITgcm-support] orlanski and prescribe bc's

[Martin Losch]

Hi Dave,

see comments below:

On May 5, 2011, at 5:48 PM, Sutherland David wrote:

> Hi all,
> 
> I am working on setting up a high-resolution, regional application of the MITgcm, where the forcing is imposed at the boundaries (from a larger domain ROMS model). Tides are important, as are the BC's for T and S, and we are interested in using the MITgcm b/c of its nonhydrostatic and finite volume codes. I have got the model to run using OBCSprescribe, but have to use the OBCSbalance routine to ensure that eta doesn't drop 10's of meters over the run. 
The balancing is code is very convenient in many cases. If you don't want that, you can, however, balance your flow "offline" if you have "simple" forcing, like periodic tides:  make sure that you net volume flux into the domain in zero over one forcing cycle. You need to take into account the grid geometry precisely (including the correct hFac's) and use double precision as input data.
> 
> I am wondering about a different approach of prescribing velocities (and hence the tides) on 1 incoming boundary and letting them radiate away at the other boundary. But, I'd also like to keep prescribing T and S at all boundaries. My understanding is if you turn on OBCSprescribe, this supersedes all other BC's- so I need to modify some of the obcs code to allow some boundaries to be prescribed (1 u/v, all T/S) and others to radiate (1 u/v)? 
You can use different boundary conditions on different boundaries. With the prescripe-flag, nothing is overwritten until you actually specify a file. The Orlanski radiation conditions do modify T/S as well. I am not sure, what will happen when you use Orlanski on one boundary and then overwrite the hydrography via the prescribe flag, but it should a least work technically (and you might get garbage after some integration time).
Also check out the Stevens boundary conditions, only implemented for eastern and western at the moment, that do something similar to what you want. They are intended to be used in large scale circulation runs (there is no velocity "radiation" conditions), but you could use the code as a template for you modifications (as it is called AFTER obcs_prescribe_read).

Martin