[MITgcm-support] Volume conservation with OBCS

[Martin Losch]

Hi David,

not that I am an expert on the Orlanski-BCs in the code, but I don't  
think that there is anything in them that ensures exact conservation  
of total volume. So before the flow reaches the eastern boundary,  
there is probably no outflow (can't you check that from the content  
of state.*.nc (dumpFreq > 0. ) or any other diagnostics that you are  
using?).

I don't know the size of your domain, but I estimated this: if the  
domain is (lx,ly,lz) = (50km,20km,1km), then 5h of inflow of 10cm/s  
would give a volume increase of 3.6e10m^3, corresponding to 36m of SL  
rise. Probably you did something similar to estimate your expected SL  
rise in the case of no outflow.

This is what I would try:
1. sanity check: turn off orlanski, initialize your entire domain  
with u=0.1, v=0. and have u_west=0.1 inflow and u_east=0.1 outflow.  
Along northern and southern boundaries specify v=0.1; (this is  
similar to verification/exp4). With this configuration (if your  
topography if constant along the open boundaries), you should not  
have any SL-rise.
2. everything (especially the homogeneous initialization) the same as  
1. but turn on orlanski in the east, and see what happens.
3. there are flags in the code (see end of obcs_calc), that make the  
code balance each of the 4 OB inflows indiviually (that is no net  
inflow at each of the OBs). That's not what you want, but you could  
modify this code to enforce a global balance, at the cost of breaking  
the orlanski "dynamics" along the boundaries.

Martin


On 30 Jun 2008, at 21:57, David Hebert wrote:

> Dear all,
>
> I am trying to simulate a flow over sumberged mountains. I have set  
> up my simulation so that the western boundary is prescribed with a  
> constant velocity (0.1 m/s), and the remaining boundaries are  
> orlanski. I am also using the exactConserv flag for free surface.  
> Also, I use exf to start the simulation from rest and ramp up the  
> boundary over one hour.
>
> My question is that my surface height, eta, rises unrealistically  
> to 25m after 5 hours of simulation. It seems that I have flow  
> coming in the domain, but not out. Is there something in the  
> orlanski boundary condition that would cause this? I have set the  
> CMAX to 0.49 and cVelTimeScale to 40.0, which is 10 timesteps.
>
> Any help/advice from experienced users is greatly appreciated.
>
> Thanks,
>
> David
>
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