[MITgcm-support] Problem with tides in the OBCS

[Dimitris Menemenlis]

Mea culpa.  Lazy programming.
For time being I will add warning as I have no time to fix and check right now.

We have used useOBCStide for regional studies, and to first order it works OK,
i.e., it generates tidal ellipses as expected and it preserves volume in regional
domain, since flow perpendicular to boundaries is correctly specified.  I think that
the flow parallel to boundaries will only impact the flow inside the regional domain
via horizontal viscosity term, so probably a second order impact in most set-ups.

Dimitris Menemenlis

> On Jun 2, 2017, at 12:43 PM, Jody Klymak <jklymak at uvic.ca> wrote:
> 
> 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 <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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