[MITgcm-support] Open boundaries and sponge layers

[Stanislav Martyanov]

Hello Jody,

Thank you for your reply!

Unfortunately, I do not have hourly nudging fields to use. In fact, all I
have is daily-mean fields from the GLORYS reanalysis (which includes
velocities). And I set them as boundary values in a Dirichlet fashion
(MITgcm's 'prescribe' option). And these values are simultaneously the
nudging fields itself when the sponge option is active.

If I had hourly velocity fields, there wouldn't be a problem then :-)

Meanwhile, MITgcm's sponge layers use these boundary values and nudge the
inner solution towards them inside some bands along the open boundaries.
Thus, I can not figure out how to keep tidal forcing while using daily-mean
velocities (which do not contain tidal oscillations) and still using
MITgcm's sponge layers..

>> If you don’t like the forcing fields being so large,
>> you could modify the code to add the tidal velocities easily enough.
The size wouldn't be a problem, but I simply do not have hourly data.

And the code already deals quite well with tidal velocities in
corresponding subroutines; it efficiently takes them out from the volume
balance procedure, for example. If I'm not mistaken, you suggest to tune
the sponge procedure itself to somehow make it keep tides? But at this
moment the total current velocities already include the tidal component, I
believe.. Anyway, it is something for me to think about..

Or just switch the sponge layers off entirely.. Speaking of which, what is
an accumulated community's experience - do people always use
MITgcm's sponge in their regional applications?

Regards,
Stanislav


сб, 19 сент. 2026 г. в 01:58, Jody Klymak <jklymak at uvic.ca>:

> Hi Stanislav
>
> If I had nudging fields for the boundary conditions, I’d simply prescribe
> them every hour instead of every day, and add the tidal and daily ocean
> velocities together to get the forcing.  Yes your nudging fields are 24
> times larger, but…
>
> If you don’t like the forcing fields being so large, you could modify the
> code to add the tidal velocities easily enough.
>
> Cheers,   Jody
>
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