# strange disturbance in SSH in the first few seconds

Hi all, I am facing a strange problem with my CROCO setup and was wondering if anyone can help. Details below:

Horizontal resolution = 200 meters with varying bathymetry ranging from 20 m shallow to 2500 m deep. theta_s = 5.0, theta_b = 0.1, Tcline = 50.0 Haidvogel and Haney numbers are rx0 = 0.0123958344, rx1 = 0.2893939053. I think these are well within the stable range.

Here is the bathymetry contour plot with masked islands.

I am running this setup with a time step of 20 sec and barotropic time step of 30 sec. The preprocessor options are the following:

      OBC_EAST
OBC_WEST
OBC_NORTH
OBC_SOUTH
CURVGRID
SPHERICAL
WET_DRY
NEW_S_COORD
SOLVE3D
UV_COR
SALINITY
NONLIN_EOS
UV_VIS2
UV_VIS_SMAGO
TS_DIF4
TS_MIX_S
SPONGE
LMD_MIXING
LMD_SKPP
LMD_BKPP
LMD_RIMIX
LMD_CONVEC
LMD_NONLOCAL
QCORRECTION
SFLX_CORR
CLIMATOLOGY
ZCLIMATOLOGY
M2CLIMATOLOGY
M3CLIMATOLOGY
TCLIMATOLOGY
ANA_BSFLUX
ANA_BTFLUX
OBC_M2CHARACT
OBC_M3ORLANSKI
OBC_TORLANSKI
DIAGNOSTICS_UV
UV_MIX_S
SPLIT_EOS
M2FILTER_POWER
VAR_RHO_2D
HZR
VIS_COEF_3D
TS_MIX_ISO
TS_MIX_IMP
TS_MIX_ISO_FILT
DIF_COEF_3D
NTRA_T3DMIX
SPONGE_GRID
SPONGE_DIF2
SPONGE_VIS2
LMD_SKPP2005
LMD_BKPP2005
LIMIT_BSTRESS
NF_CLOBBER


The initial condition is set with no temperature/salinity gradient, and 0 velocities everywhere in the domain. However, within the first few seconds the outputs show a strange buildup in the zeta (sea surface), and the velocities. This buildup accumulates over the first few seconds, and blows up at the 6th second due to unrealistically high velocity. Here is a vertical transect plot of the velocity at the 6th second when it blows.

Right at that point I looked into the depths of the sigma layers. Here is the sigma layer plot at the 6th second. At the point of instability, the surface height shows a strange disturbance, and the sigma layers are also getting distorted because of it. I couldn't figure out what's causing this problem.

Then, I then looked into an earlier time step. Here is the velocity at the same transect at 5 seconds.

I think this disturbance is mostly because of the batrotropic component which comes from the sea surface height. Here is the barotropic velocity contour plot at 5 sec:

Does anybody know whats causing this? Any suggestions?

I have tried running a larger grid setup with 3 km resolution and that works perfectly with no problems with SSH. However, with finer resolution I am facing these issues. These strange disturbances in SSH are affecting the velocity, blowing up the simulation. Is there any specific modification that I should do for getting the SSH correct in a fine resolution run?

If you want to look at more details, the preprocessor cppdefs.h and input .in files are attached.

Thank you.

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