[MITgcm-support] Rivers imposed as OBCs - problem with fully-open boundary domains (i.e., islands)

[Stefano Querin]

Dear MITgcm-ers,

We are running high-resolution regional simulations in marginal seas of the Mediterranean (and sub-basins).
At resolutions ranging from 1/128° to 1/768° (some examples, respectively, here <https://medeaf.ogs.it/adriatic> and here <https://medeaf.ogs.it/got>), we insert the river inputs as lateral open boundary conditions, since the momentum contribution is significant at those scales (and we can also easily control the thermohaline and biogeochemical inputs by prescribing OB files).
The problem arises when we deal with domains fully surrounded by (water) open boundaries (e.g., regional focus on Sicily or Sardinia).
The attached picture shows an idealized case study (island in the open sea), where we want to impose a river originating from the northernmost end of the channel and flowing southward. If we impose a single land point at the northern edge of the model domain (i_g=21 and j_g=41, plot on the right), we can set an OB point at i_g=21 and j_g=21 and the simulation runs smoothly. Conversely, the configuration on the left cannot work (as far as we know...).
We also tried to use the "insideOBmaskFile" as specified in the documentation for the more complex example <https://mitgcm.readthedocs.io/en/latest/phys_pkgs/obcs.html#a-more-complex-example>, but we are a bit stuck...

Can you confirm that there is no way to prescribe "inner" OBCs (the river input in i_g=21 and j_g=21) in a domain like the one in the picture (left), while keeping the nesting model values all around the (nested) domain (also in i_g=21 and j_g=41)?
In that case, we will insert the same runoff as surface flux by using the exf package. Most likely, we will add the freshwater flux in i_g=21 and j_g=21, to let the water gain some momentum along the channel, otherwise, we will remove the channel and simply put the runoff in i_g=21 and j_g=12.

Thank you!

Cheers,

Stefano and Fabio

P.S.: is it possible that there is an oversight in the documentation (Figure 8.6)? The correct OB configuration in data.obcs should be like that, do you agree?

OB_Iwest  = 1*0,1*5,1*2,141*0,
OB_Jsouth = 2*3,3*2,115*0,

and not

OB_Iwest  = 1*0,1*5,142*0,
OB_Jsouth = 2*3,3*2,115*0,

(or the OB routines automatically set i=2, j=3 as the OB position -red line- ?)

--
Stefano Querin, PhD
Sezione di Oceanografia
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS
via Beirut n. 2
34151 Trieste - Italia
Tel. +39 040 2140623
Skype: stefano_querin
www.ogs.it <http://www.ogs.it/>

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