Issue with Sea Surface Height (Zeta) Boundary Condition in ROMS Model

[hyc006]

Hello ROMS Discussion Community,

I'm encountering a persistent issue with the sea surface height (zeta) output from my ROMS model, particularly regarding the boundary conditions. My aim is to fix the sea surface height to zero at the north and south boundaries using the Chapman boundary condition, as illustrated below:

Code: Select all

!                   W       S       E       N
!                   e       o       a       o
!                   s       u       s       r
!                   t       t       t       t
!                           h               h
!
!                   1       2       3       4

   LBC(isFsur) ==   Per     Cha     Per     Cha         ! free-surface

However, I'm observing discrepancies between the zeta values at the boundary in the ROMS output and the specified boundary condition. Just like I show in figure below.
Despite adjusting the Nudging/relaxation time scales to large values, the boundary issue persists:

Code: Select all

! Nudging/relaxation time scales, inverse scales will be computed internally, [1:Ngrids].

       TNUDG == 10*365.0d0                    ! days
       ZNUDG == 10*365.0d0                      ! days
      M2NUDG == 10*365.0d0                      ! days
      M3NUDG == 10*365.0d0                      ! days

! Factor between passive (outflow) and active (inflow) open boundary conditions, [1:Ngrids].
! If OBCFAC > 1, nudging on inflow is stronger than on outflow (recommended).

      OBCFAC == 40*365.0d0                      ! nondimensional

Now, I'm contemplating whether adjusting the sponge layer might resolve this issue. Here's the section in question:

Code: Select all

! Logical switches (TRUE/FALSE) to increase/decrease horizontal viscosity and/or diffusivity
! in specific areas of the application domain (like sponge areas) for the desired application grid.

    LuvSponge == F                              ! horizontal momentum
LtracerSponge == F F                            ! temperature, salinity, inert

My question is whether modifying the sponge layer is necessary, or if there are other parameters or settings that I should consider adjusting to address this boundary discrepancy. Additionally, should I explore any modifications in the .h file?

Any insights or guidance would be greatly appreciated.

Thank you in advance!

Hsin-Yi Chen

[wilkin]

The Chapman condition does not "fix" zeta on the boundary. It is designed to radiate out barotropic waves generated inside the domain.

Read ... https://www.myroms.org/wiki/Boundary_Co ... _condition

It is designed to be used in conjunction with the Flather condition. Read ...https://www.myroms.org/wiki/Boundary_Co ... _condition.

Notice that the user imposed zeta on the boundary does not enter into Chapman, only Flather.