ROMS tide amplitude problem

[CBian]

Hello, ROMS users!

When I simulated the tide in Bohai and Yellow Sea, I confront a problem: the amplitude of the tide is about 30% less than the real amplitude near the coast （fig. a）.

First, I thought maybe the error was caused by the topography. Therefore, I try the Etopo5 topography data. However, the amplitude result is similar with the old one, also 30% less than real. So the topo should be Ok here. Below is the topo of the domain.
Longitude: 117.5-127E Latitude:35-41N



Second, I tried to change the quadratic drag coefficient. In this domain, the drag coefficient should be 2.5x10-3. When I decrease the drag coefficient to 0.8x10-3 (fig.b), the simulated M2 tide amplitude looks proper near coast comparing with the correct M2 tide(fig.c).



However, I think this drag coefficient is too small compare to actual 2.5x10-3. There must be other reasons lead to the small amplitude near coast. The tide forcing data used in my ROMS model is from OTIS (OTPS7.2, under direction of Wiki ROMS). I only have a south boundary in my domain. In figure d, I compare the M2 tide amplitude from ROMS zeta result to the input tidal forcing M2 tide amplitude. These two should be same, but they are not. And there is a obvious different around 22th and 23th points. The input tidal forcing amplitude is about 1.3m, but the ROMS output results are 0m. I checked the mask_rho, and found these two points are water points, but the ROMS output zeta are all 0m at these two points.

I am really confused.
What may be the reasons I always got small amplitude near coast?

If I use an extremely small quadratic drag coefficient 0.8x10-3 in this domain, can ROMS still work well when I calculate the currents and sediment transport later?

Here is my model setting:
ROMS version:461M
Resolution: 1/12 degree (about 8km) N=30 layers, dt=600, ndtfast=30, hmin=5m, Tcline=3m.
Maximum grid stiffness ratios: rx0 = 2.364813E-01 (Beckmann and Haidvogel)
rx1 = 6.975034E+00 (Haney)

Activated C-preprocessing Options:

ECSS East China Seas/ sediment transport
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.
CURVGRID Orthogonal curvilinear grid.
DOUBLE_PRECISION Double precision arithmetic.
EAST_FSCHAPMAN Eastern edge, free-surface, Chapman condition.
EAST_M2FLATHER Eastern edge, 2D momentum, Flather condition.
EAST_M3RADIATION Eastern edge, 3D momentum, radiation condition.
EAST_TRADIATION Eastern edge, tracers, radiation condition.
MASKING Land/Sea masking.
MIX_S_TS Mixing of tracers along constant S-surfaces.
MIX_S_UV Mixing of momentum along constant S-surfaces.
MPI MPI distributed-memory configuration.
MY25_MIXING Mellor/Yamada Level-2.5 mixing closure.
NONLINEAR Nonlinear Model.
!NONLIN_EOS Linear Equation of State for seawater.
NORTH_FSCHAPMAN Northern edge, free-surface, Chapman condition.
NORTH_M2FLATHER Northern edge, 2D momentum, Flather condition.
NORTH_M3RADIATION Northern edge, 3D momentum, radiation condition.
NORTH_TRADIATION Northern edge, tracers, radiation condition.
POWER_LAW Power-law shape time-averaging barotropic filter.
PRSGRD31 Standard density Jacobian formulation (Song, 1998).
PROFILE Time profiling activated .
K_GSCHEME Third-order upstream advection of TKE fields.
RHO_SURF Include difference between rho0 and surface density.
!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_M3RADIATION Southern edge, 3D momentum, radiation condition.
SOUTH_TRADIATION Southern edge, tracers, radiation condition.
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_C4ADVECTION Fourth-order centered differences 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.

Thanks！

[stone]

Hi,
I am confused about this trouble just as you mentioned, and still can't work out.
I think the imbalance of ssh with ubar vbar may be the candidate,maybe due to depth disagreement with OPTS bathymetry. Try to adjust accordingly.

[johnluick]

Do you have any data that you can check your OTPS elevations and currents against?

Ultimately if you decide the OTPS amplitudes are too small at the open boundary you may have to construct a larger grid out to deep water to run your tidal model on. Then, take the open boundary conditions for the smaller grid from the larger grid. Alternately, I think someone else there at OUC would have a good tidal model already to take the OBCs from.

I would set the quadratic drag coefficient back to your original value and leave it alone.

I agree that using etopo5 is not likely to result in 1/3 reduction in amplitudes. But it will probably affect the phase (once you get the amplitudes right you will want to check phase.) You should use a better bathymetric database. Again, I am pretty sure someone at OUC has already produced a digital database of the area.

John

[cdw]

First question would be whether the stratification is causing you problems. Also foes a theoreticsl model like Myt own (I am quite red in the face as I type this (Winant(2007)) reproduce anything like the results if yo consider an extreme simplification of Bohai (rectangle open at one end)?

[CBian]

Stone, John, cdw, thanks for your kindly suggestions!

My classmate told me that the quadratic drag coefficent is 0.0025 in the deep sea but is 0.001 near the coast. And I checked the realistic ocean data in my domain, that's true. So I should use a changable drag coefficent in my domain. I believe this could figure out the amplitude problem.

Still, I doubt about the figure d. In my opinion,
(1) I input tide harmonic constant (tidal forcing) in the southern boundary (tide ampliltude: bule line in fig. d)
(2) After running the ROMS, I can get the zeta in the southern boundary
(3) Analysis the zeta result, I can get the tide harmonic constant (tide amplitude: red line in fig. d)

Normally, the bule and red line should same, but they looks different. Why

[lizhicheng]

I also meet the qusetion, M2 amplitude is 25 % less than the real amplitude in the shallow water, but K1 amplitude is 20 % more than the real amplitude in the shallow water.
the quadratic drag was set up different number in the different region.
I am really confused.