Default Boundary Conditons?

[black]

What are the default boundary conditions for ROMS? I wouldn't think ROMS would have default BC's but when looking at several of the example configurations I noticed that some (Seamount in particular) do not specify BCs. Why? And what is ROMS doing when no boundary condtions are assgined?

[shchepet]

This reply is not a to be understood as statement of policy, but rather explanation of de-facto settled practices.

If no boundary conditions are specified, then it means that they are CLOSED everywhere. It means: (1) no normal flow; (2) Neumann (zero-gradient extrapolation rule) for T,S; and (3) either free-slip, or no-slip for tangential velocity, depending on input parameter gamma2.

Starting with this default, the boundary conditions may be SELECTIVELY changed to either

OPEN [by setting OBC_WEST ...EAST ....SOUTH ...NORTH] on each side independently;

or

PERIODIC [EW_... or NS_PERIODIC] obviously pairwise, i.e., EW_PERIODIC affects both east and west sides.

Within the OPEN class there is a rich choice of sub-classes (Orlanski, Flather, clamped, reduced physics etc... --- some of them experimental, and some even ill-posed).

In principle, THERE MUST BE NO DEFAULT within the OPEN class, i.e., if you say OPEN you must also specify WHAT KIND OF open boundary you want.

...But, for a very long time, and as a manner of bad policy, we made GRADIENT boundary conditions to play the role of default open boundary. Then we were receiving e-mails saying "I defined open boundary [and nothing else] and model blows up!" Of course it blows up, because it should blow up --- otherwise there is something wrong with the code.

...I checked the most recent official Rutgers code and found that it is still possible to specify SOUTH_M2GRADIENT which would result in code like

Code: Select all

        DO i=Istr,Iend
          vbar(i,Jstr,kout)=vbar(i,Jstr+1,kout)
#  ifdef MASKING
          vbar(i,Jstr,kout)=vbar(i,Jstr,kout)*GRID(ng)%vmask(i,Jstr)
#  endif
        ENDDO

and subsequent blow up because of fundamental ill-posedness.

[black]

Thank you for the explanation. That was exactly the information I was looking for.
Now is there a central reference that explains the options for open boundary conditions and when each one is appropriate. I figure I will just start trying them and see which one gives me good results (unfortunately the understanding of which will probably come much later).

[kate]

Maybe you should tell us what problem you are trying to solve and then we can suggest things to try. Walls work, the periodic boundaries work, open boundaries are a big mess of trial and error. If you can define your problem in terms of a closed domain, go for it.

For open boundaries, do you need tides? Do you have boundary information to provide the model? We've had far better luck with daily info from a larger domain than with monthly climatologies.

[goutam]

If no boundary conditions are specified, then it means that they are CLOSED everywhere. It means: (1) no normal flow; (2) Neumann (zero-gradient extrapolation rule) for T,S; and (3) either free-slip, or no-slip for tangential velocity, depending on input parameter gamma2.

Starting with this default, the boundary conditions may be SELECTIVELY changed to either

OPEN [by setting OBC_WEST ...EAST ....SOUTH ...NORTH] on each side independently;

or

PERIODIC [EW_... or NS_PERIODIC] obviously pairwise, i.e., EW_PERIODIC affects both east and west sides.

I am using ROMS 2.2. In cppdefs.h, I don't find
OBC_WEST
OBC_EAST
OBC_SOUTH
OBC_NORTH

what I find is
EASTERN_WALL
WESTERN_WALL
NORTHERN_WALL
SOUTHERN_WALL

So does this mean that in ROMS 2.2, if no boundary conditions are specified, then it is open everywhere, unlike as stated above?

Thanks
Goutam

[kate]

In Include/globaldefs.h:

#if (defined NORTH_M3RADIATION && defined NORTH_M3NUDGING) || \
defined NORTH_M3CLAMPED
# define NORTH_M3OBC
#endif

# if defined WEST_FSOBC || defined EAST_FSOBC || \
defined SOUTH_FSOBC || defined NORTH_FSOBC || \
defined WEST_M2OBC || defined EAST_M2OBC || \
defined SOUTH_M2OBC || defined NORTH_M2OBC || \
defined WEST_M3OBC || defined EAST_M3OBC || \
defined SOUTH_M3OBC || defined NORTH_M3OBC || \
defined WEST_TOBC || defined EAST_TOBC || \
defined SOUTH_TOBC || defined NORTH_TOBC
# define OBC
# endif

If OBC is #defined, it will look for an open boundary condition file.

In Nonlinear/t3dbc_im.F:

# if defined WEST_TRADIATION
# elif defined WEST_TCLAMPED
# elif defined WEST_TGRADIENT
# else
!
! Western edge, closed boundary condition.
!
!# endif

The default is closed. You can always check the t3dbc_im.f90 file to see what's left after the C preprocessor has been through.

[goutam]

So if the default is closed then what is the use of these options in cppdefs.h :-
EASTERN_WALL
WESTERN_WALL
NORTHERN_WALL
SOUTHERN_WALL

Can you please explain?
Thanks
Goutam

[kate]

grep is your friend in these situations. Doing a grep for EASTERN_WALL on all the files will show you where it is used. It is used in the biharmonic operators when setting the boundary contition on the Laplacian intermediate result.