[MITgcm-support] Problem with tides in the OBCS

[Jody Klymak]

On 2 Jun 2017, at 4:52, Bahman Alavi wrote:

> I'm trying to input Tide on my open boundaries; the first question is: 
> Is
> this a good idea to enabling "useOBCStide"?

OK, no offense to whoever wrote `obcs_add_tides.F`, but I don’t really 
see how it is supposed to work except if the Coriolis parameter f=0.  
Tidal boundary conditions at (for instance) the western boundary are 
only specified in the east-west velocity, `OBWu`.  Unfortunately, it 
doesn’t also specify `OBWv`, and therefore assumes the north-south 
velocity is zero.  So while a tidal signal will almost certainly result, 
I don’t see how it will produce the correct tidal signal as it will 
constantly drive a rectilinear flow through the western boundary, when 
really it should have a tidal ellipse in both u and v.  Maybe I 
haven’t fully though through the physics of this BC, and forcing v=0 
doesn’t matter, but intuitively it seems wrong.

Given this, I’d not use this code, or I’d modify it to also accept 
along-boundary velocities.

Conversely, you can force tides using the following in `data.obcs`:

```
  useOBCSprescribe=.TRUE.,
  OBEuFile = 'Ue.bin',
  OBWuFile = 'Uw.bin',
  OBEtFile = 'Te.bin',
  OBWtFile = 'Tw.bin',
```

(where I have *not* specified `OBEvFile`, but you should unless f=0) and 
in `data`

```
# Forcing for boundary condition files
  periodicExternalForcing=.TRUE.,
# 1/12th M2 tidal period...
  externForcingPeriod = 3720.,
# one tidal period (12 records need to be in OBEuFile, etc)
  externForcingCycle = 44640.,
```

You can see examples of how to generate the files at 
https://github.com/jklymak/MITgcmExampleSteadyGauss

> The second question; I extracted "amplitude"s & "phase"s of my 
> boundary
> points. At first I was getting errors, i thought maybe my units for 
> am/ph
> are wrong. I've searched and finally found the units in file
> "obcs_add_tide.F" which is somehow weird. For amplitude: "m/s" and for
> phase "s". Is this correct? and how should i change to these units?

They want the “phase” specified as a dt = phase*T/2/pi, where phase 
is the tidal phase in radians, and T is the tidal period.

Cheers,   Jody


> Thanks in advance
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> MITgcm-support at mitgcm.org
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