unrealistic boundary current when modeling internal tide

[gaokun227]

Dear all,

I am modeling the M2 internal tide in South China Sea (the domain could be seen in the following figure). The only forcing I used is tidal forcing, which is derived from TPXO7.1. The _FSChapman, _M2Flather, _M3Radiation and _TSRadiation OBCs are applied at all of the four open boundaries. The initial T/S are horizontally uniform, so there should be no geostrophic currents in the initial fields.

My problem is that after several days of simulation, some strong currents appear at the open boundaries. The first figure is the daily averaged surface layer temperature and velocity field at the 10th day, while the second is the surface layer daily averaged temperature and velocity field at the 30th day.



Do you have any idea about where these “unrealistic” boundary currents originate from? Thanks in advance.

Kun

[kate]

What does your bathymetry look like? What is the source of it and did you smooth it at all?

[gaokun227]

What does your bathymetry look like? What is the source of it and did you smooth it at all?

Hi Kate, the bathymetry in my model (see the following figure) is derived from etopo2, with the horizontal resolution as 1/20*1/20 degree. And it is smoothed with the max slope parameter (r=grad(h)/h) as 0.25.

[mrayson]

Hi Kun,

I ran into a similar problem when trying to model internal tides on the Australian north west shelf. I was setting up the model using the same methodology (horizontally uniform TS). I found that you also need to set _TNUDGING and _M3NUDGING to constrain the tracers and baroclinic velocities at the boundaries when using the radiation conditions. You will also need to set the nudging time scales parameters in the *.in file.

I used:
TNUDG == 60.0d0 60.0d0
M3NUDG == 60.0d0

and

OBCFAC == 60.0d0

I was nudging the M3 to zero and TS back to the initial conditions at the boundaries.

Matt

[gaokun227]

Hi Kun,

I ran into a similar problem when trying to model internal tides on the Australian north west shelf. I was setting up the model using the same methodology (horizontally uniform TS). I found that you also need to set _TNUDGING and _M3NUDGING to constrain the tracers and baroclinic velocities at the boundaries when using the radiation conditions. You will also need to set the nudging time scales parameters in the *.in file.

I used:
TNUDG == 60.0d0 60.0d0
M3NUDG == 60.0d0

and

OBCFAC == 60.0d0

I was nudging the M3 to zero and TS back to the initial conditions at the boundaries.

Matt

Hi Matt, thanks a lot for your suggestion. I tried out your method. It is a good way to constrain the model. The boundary currents are reduced, however, not completely vanished. Do you have any idea about the source of these currents? I believe only if we know where this problem comes from, could it be fully solved.

Best,

Kun

[feroda]

Could you tell us the CPPs you used in your model?

[gaokun227]

Could you tell us the CPPs you used in your model?

Hi feroda, the CPP options I activated are:

Code: Select all

 ADD_FSOBC           Adding tidal elevation to proccesed OBC data.
 ADD_M2OBC           Adding tidal currents to proccesed OBC data.
 ANA_BSFLUX          Analytical kinematic bottom salinity flux.
 ANA_BTFLUX          Analytical kinematic bottom temperature flux.
 ANA_FSOBC           Analytical free-surface boundary conditions.
 ANA_M2OBC           Analytical 2D momentum boundary conditions.
 ANA_SMFLUX          Analytical kinematic surface momentum flux.
 ANA_SSFLUX          Analytical kinematic surface salinity flux.
 ANA_STFLUX          Analytical kinematic surface temperature flux.
 ASSUMED_SHAPE       Using assumed-shape arrays.
 AVERAGES            Writing out time-averaged fields.
 DIFF_GRID           Horizontal diffusion coefficient scaled by grid size.
 DJ_GRADPS           Parabolic Splines density Jacobian (Shchepetkin, 2002).
 DOUBLE_PRECISION    Double precision arithmetic.
 EAST_FSCHAPMAN      Eastern edge, free-surface, Chapman condition.
 EAST_M2FLATHER      Eastern edge, 2D momentum, Flather condition.
 EAST_M3NUDGING      Eastern edge, 3D momentum, passive/active outflow/inflow.
 EAST_M3RADIATION    Eastern edge, 3D momentum, radiation condition.
 EAST_TNUDGING       Eastern edge, tracers, passive/active outflow/inflow.
 EAST_TRADIATION     Eastern edge, tracers, radiation condition.
 MASKING             Land/Sea masking.
 MIX_GEO_TS          Mixing of tracers along geopotential surfaces.
 MIX_S_UV            Mixing of momentum along constant S-surfaces.
 MPI                 MPI distributed-memory configuration.
 NONLINEAR           Nonlinear Model.
 NONLIN_EOS          Nonlinear Equation of State for seawater.
 NORTH_FSCHAPMAN     Northern edge, free-surface, Chapman condition.
 NORTH_M2FLATHER     Northern edge, 2D momentum, Flather condition.
 NORTH_M3NUDGING     Northern edge, 3D momentum, passive/active outflow/inflow.
 NORTH_M3RADIATION   Northern edge, 3D momentum, radiation condition.
 NORTH_TNUDGING      Northern edge, tracers, passive/active outflow/inflow.
 NORTH_TRADIATION    Northern edge, tracers, radiation condition.
 POWER_LAW           Power-law shape time-averaging barotropic filter.
 PROFILE             Time profiling activated .
 RADIATION_2D        Use tangential phase speed in radiation conditions.
 RAMP_TIDES          Ramping tidal forcing for one day.
 !RST_SINGLE         Double precision fields in restart NetCDF file.
 SALINITY            Using salinity.
 SOLVE3D             Solving 3D Primitive Equations.
 SOUTH_FSCHAPMAN     Southern edge, free-surface, Chapman condition.
 SOUTH_M2FLATHER     Southern edge, 2D momentum, Flather condition.
 SOUTH_M3NUDGING     Southern edge, 3D momentum, passive/active outflow/inflow.
 SOUTH_M3RADIATION   Southern edge, 3D momentum, radiation condition.
 SOUTH_TNUDGING      Southern edge, tracers, passive/active outflow/inflow.
 SOUTH_TRADIATION    Southern edge, tracers, radiation condition.
 SPLINES             Conservative parabolic spline reconstruction.
 SSH_TIDES           Add tidal elevation to SSH climatology.
 TS_U3HADVECTION     Third-order upstream horizontal advection of tracers.
 TS_C4VADVECTION     Fourth-order centered vertical advection of tracers.
 TS_DIF2             Harmonic mixing of tracers.
 UV_ADV              Advection of momentum.
 UV_COR              Coriolis term.
 UV_U3HADVECTION     Third-order upstream horizontal advection of 3D momentum.
 UV_C4VADVECTION     Fourth-order centered vertical advection of momentum.
 UV_QDRAG            Quadratic bottom stress.
 UV_TIDES            Add tidal currents to 2D momentum climatologies.
 UV_VIS2             Harmonic mixing of momentum.
 VAR_RHO_2D          Variable density barotropic mode.
 VISC_GRID           Horizontal viscosity coefficient scaled by grid size.
 WEST_FSCHAPMAN      Western edge, free-surface, Chapman condition.
 WEST_M2FLATHER      Western edge, 2D momentum, Flather condition.
 WEST_M3NUDGING      Western edge, 3D momentum, passive/active outflow/inflow.
 WEST_M3RADIATION    Western edge, 3D momentum, radiation condition.
 WEST_TNUDGING       Western edge, tracers, passive/active outflow/inflow.
 WEST_TRADIATION     Western edge, tracers, radiation condition.

[susonic]

Have you ever tried to run the model without tidal forcing under same condition?

[gaokun227]

Have you ever tried to run the model without tidal forcing under same condition?

The tidal forcing is the only forcing I used in my model. The ocean will stay at rest if the tidal forcing is removed,

[eivinds]

Have you tested that the ocean actually remains at rest when unforced? I had huge problems when simulating motionless stratified unforced water in a bounded region with sloping bathymetry. Artificial currents where generated near sloping sigmalayers.

[feroda]

Actually, I am pretty curious of your model domain. Comparing tidal currents with topography, it is obvious that the currents largely follow the steep bathymetry slop. Why don't you expand your western boundary to southeast coast of China? It is also worthwhile to think about if you want to include the Taiwan Strait, as the tidal currents there might be stronger than those in your present model domain.