[MITgcm-support] Open boundaries and sponge layers

[Stanislav Martyanov]

Hello, dear colleagues!

My question is about handling the open boundaries in MITgcm using sponge
layers (but, as they are implemented in MITgcm, I believe it would be more
correct to call them 'nudging layers').

I am using daily-mean velocity fields from the GLORYS12 reanalysis to
prescribe open boundary conditions. Tidal forcing is also applied at the
open boundaries using amplitudes and phases for several tidal harmonics
(taken from another, regional tidal model).

The question is: Can nudging ('sponge' in MITgcm terminology) layers be
used in my model in this scenario?

As I understand, they would pull the internal solution (the calculated
velocities) towards the prescribed daily-mean velocity, thereby smoothing
out (or fully annihilating?) the tidal oscillations near the open
boundaries.

In other words, by relaxing the solution towards the daily-mean velocity
fields from the GLORYS12 reanalysis, the tides might simply disappear from
the internal solution near the boundary; consequently, tidal waves would
not propagate from the open boundaries into the model domain.

Is my understanding of the model's architecture correct for this particular
case?
If so, what should be done to account for both types of forcing along the
open boundaries while still using MITgcm's nudging layers to suppress
artificial reflections from the open boundary? As I know, if sponge layers
are not used, it is commonly believed that the Dirichlet condition for flow
velocity would inevitably lead to partial reflections at the boundaries...


Best regards,
Stanislav Martyanov
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