tidal model blow up with initial stratification

[rongzr]

I set up a tidal model with summer climatological TS field. The model run 20days and blow up. I checked the model output and found that the sea level very near the open boundary goes to extremely large in several time steps. I use the following tidal flag:
#define SSH_TIDES
#define RAMP_TIDES
#define FSOBC_REDUCED
!#define UV_TIDES
!#define ADD_FSOBC
!#define ADD_M2OBC

/* Open boundary conditions */
#define EAST_FSCHAPMAN
#define EAST_M2FLATHER
#define EAST_M3RADIATION
#define EAST_TRADIATION

I further added a sponge layer at the open boundary, the horizontal viscosity goes to 10 times large in 10 grid. The model blow up after one day, also the sea level very near the boundary goes to extremely large in several steps. The difference is that model blow up at different grid cells.

This tidal model can run smoothly with uniform TS field.
Anybody have any idea for this??

[kate]

What are you using for bottom drag? The quadratic bottom drag can make things blow up (until we get Sasha to cough up his fix for it).

[rongzr]

Hi, Kate, thanks for your reply.
I use the logarithmic bottom friction(UV_LOGDRAG) with Zob=1mm.
The question is that model runs smoothly with uniform TS field but blows up with initial stratification. Do you think it might due to some other reason, e.g. baroclinic instability or what else?

[kate]

Try linear bottom drag to see if the model runs. Or put in a station somewhere and look at the bottom velocities. Do they flip-flop with ever-growing amplitude?

[rongzr]

The bottom velocity near the open boundary looks reasonable compared to the uniform case. Values lie between -0.05m/s ~ 0.05m/s and goes to extremely large when blowing up.

I'm testing the linear bottom friction case.

[rongzr]

Same thins occurs for the linear case.
Any idea? Thanks!

[wilkin]

#define FSOBC_REDUCED may contribute to your problem. If at all possible you should specify tidal velocity harmonics to match your SSH and activate #define UV_TIDES

Since you have a climatological T/S, I suggest you use this information at the boundaries by building the boundary conditions file and adding #define EAST_TNUDGING with the appropriate TNUDG and OBCFAC set in ocean.in

[rongzr]

Hi, John, Thanks for your reply. That might be one reason. I will test it. The reason why i didn't define UV_TIDES is that it underestimated the tidal amplitude,while with SSH_TIDES and FSOBC_REDUCED together i could get the tidal level right! I don't quit understand why SSH_TIDES and UV_TIDES together underestimate it. Or due to other reason,e.g. Flather bounday?
I used the TS_NUDGING and SST nudging(not shown here).

[xcwang]

As John mentioned, you need to have UV_TIDES defined. One thing you might want to check
is the value of ubar and vbar of your tidal forcing. I assume that the tidal ubar, vbar and
sea level is from OSU, please remember to adjust the ubar, vbar from OSU using your ROMS bathymetry
so that vertical integrated total transports in ROMS and OSU are the same, which makes your tidal boundary condition consistent.

Two cents.

Xiaochun Wang

[rongzr]

Thanks. Xiaochun!
I didn't adjust the ubar,ubar. That might be the reason why the model underestimated the tidal level when define UV_TIDES because i cut the depth to 1000m. I will check that. That's really helpful.