Tidal Forcing

Description of Tidal Forcing in ROMS

Tidal Forcing Files in ROMS

Once the appropriate cpp options have been set (e.g. SSH_TIDES, UV_TIDES, RAMP_TIDES), a netcdf file of tidal constituents must be generated.

1. Download the desired tidal constituent database and associated software. There are many tidal constituent databases available for download and the choice of databases depends on the desired constituents and location of the ROMS grid. Two possibilities which have broad geographic range and generally the dominant constituents are the OSU Tidal Data Inversion Software(OTIS) and the ADCIRC tidal database. Remember to download the associated software with each data base as there ire typically routines which facilitate the extraction and interpolation of the database to the ROMS grid.
2. Interpolate the tidal constituent database to the desired ROMS grid. See the above comment. While it is possible to write code (e.g. MATLAB, FORTRAN) to perform this task, it is typically easier to use the provided packages.
3. Verify all open boundary grid cells contain valid data. During the interpolation process, depending on the land mask for the ROMS grid and the tidal database grid, it is possible to have grid cells near land points which the ROMS grid may define as water, but the tidal grid defines as land. Should this occur a 180 degree phase shift along the open boundary near land points is possible. As ROMS is tidally forced on the open boundary, this could be problematic.
4. The phase of the u/ v/ zeta components of the tidal constituents should be shifted to the appropriate reference time(t=to).It is typical for tidal constituent databases to be stored with the phase shifted by the equilibrium tidal argument. Consequently the reference time for the tide is not the desired time, as set in ocean.in using the variable TIDE_START. In addition, nodal corrections need to be made due to long period tides.
5. Convert the amplitude/phase information to tidal ellipse parameters, if necessary. ROMS requires tidal information to be stored as ellipse parameters for use.
6. Export tidal ellipse parameters to a ROMS netcdf forcing file.

Two examples of this process are described below.

OSU Tidal Prediction Software Example

http://www.coas.oregonstate.edu/research/po/research/tide/

ADCIRC Tidal Database Example

http://www.unc.edu/ims/ccats/tides/tides.htm