tidal phase mismatch between ADCP data and ROMS output

[sonaljit.m]

Hi all,
My ROMS setup uses tidal inputs from TPXO9 global tidal atlas, climatology from HYCOM and wind forcing from NAVGEM. All data are being used from 2015, and interpolated from their respective grids to my ROMS regional grid. When comparing the velocity outputs with ADCP measurements, I see a mismatch in the tidal phase of the ROMS currents and the ADCP currents. Here is a plot showing the phase mismatch. The ROMS outputs are obtained at the same depth (5 m from the surface) and location where the ADCP measurements were taken (near the coastline). Also, the timing is the same (Feb 1st 00:00) for both the ADCP and CROCO data.

The top panel shows u and v velocities. In the bottom panel, I bandpassed the 12 hr signal to separate out the semi-diurnal components. The phase mismatch seems to be roughly 180 deg (pi rad) for u, and 90 deg (pi/2 rad) for v. Likewise, I am also observing a similar phase mismatch in the diurnal tidal signal.

I prepared the tidal forcing from TPXO9 as per instructions in wikiroms. But the simulation outputs still show this phase difference. Does anyone know how do I correct this? I can manually estimate the phase difference between the measured and simulated signals, and add it to the tidal forcing input. But I am not sure if that would be the right way to do it.

Can anyone help? Thanks.
Sonaljit.

[wilkin]

Did you try computing the tidal harmonics from the data directly and comparing them to TPXO? This to take ROMS out of the loop and be sure you've got the correct interpretation of phase for both data sets.

[sonaljit.m]

Hi John, thanks for the suggestion.
There seems to be mismatch. I calculated tidal phases from the NOAA gauge and TPXO outputs at the same location (south of St Thomas) and compared them.
For the M2 tide, the phases are 24.5 (NOAA) and 17.427 (TPXO) (in degrees). For the S2 - 8.13 (NOAA) vs 21.91 (TPXO).
Likewise, the other tidal signals (N2, K2 etc) are showing a mismatch.

This problem is new to me. What should be done in this case? Should I manually alter the tidal forcing by adding the phase difference to the input?

Thanks.

[wilkin]

I am very surprised that TPXO currents would be so out of agreement with an open ocean current meter time series. Are you sure you have the correct time base for both, and that you are interpreting phase correctly?

[jivica]

Looks to me that you made and used wrong tidal forcing file.
There could be a few things, from time definitions to wrong tidal periods (saving in wrong order) in the file.
To nail down the error I would pick a few points from your tidal forcing file and plot "synthesized" sea level (simple stuff, without vuf etc) and do the same from your "roms output" - solution. This should be few lines in matlab/python.
If you have sea level quite well matching then go to another level of digging (this will confirm you have barotropic tidal signal correct, which is robust).
Use all the data from the used forcing file (periods for example could be saved in wrong order), do not assume anything and be sure you use correctly DSTART and TIDE_START and TIME_REF in your .in file (and that you used the same during the create of tidal forcing file).

Good luck!
Ivica