[MITgcm-support] Using OBCS in a 1D model setup

[Martin Losch]

Nick,

the default of the MITgcm is to have (double) period boundaries, ie. with your configuration (Nx=Ny=1), you have an infinite ocean that is homogenous in the horizontal. Closed boundaries are not possible with this configuration (you’d have to put a land point at your only horizontal grid point).

OBCS: The open boundaries are always at grid point that belong to the computational domain, i.e. they have to be a i = 1 (or larger) and at i = Nx (or smaller), and similarly for j. In your case Nx=Ny=1, so your single horizontal grid point belongs to 4 boundaries. You can imagine that this is not sensible. A 3x3 domain should work, in the sense that the central grid point will not be directly prescribed (although the normal velocities will), but I am not sure what you want to achieve with this configuration that you cannot with a single column (without OBCS). Even for the 3x3 domain the centrail point will be totally determined by the "open boundaries”, because there will be little dynamics that can develop.

I think it would be simpler and more straightforward to use RBCS to restore your fields to your data, but then you also have only very little dynamic development.

Martin


> On 11 Mar 2015, at 17:53, Nicholas Rogan <nrogan at geomar.de> wrote:
> 
> Hi all,
> 
> I have recently been using a 1D model setup to explore the effects of different aerosol depositions on a water column ~ 500m deep.
> 
> Previously the model was set up with closed boundaries and was not a closed system (removal out of the base of the water column is gradually draining the conserved tracers).
> 
> For a more robust model I wanted to add open boundaries on all of the faces of my single column, for which I have diagnosed time varying daily T, S, U, V and six ptracers from a Global 3D model field. 
> 
> I am now trying to implement these velocity fields and tracer concentrations in order to have the diagnosed fluxes at the boundaries of my model.
> 
> The model output from my run with closed boundaries and run with open boundaries is drastically different and I’m not convinced it is solely down to the change in dynamics. 
> I’d greatly appreciate if anyone could have a quick look at my input data files and identify an obvious error, or tell me if using OBCS with a model with a horizontal grid size of 1 x 1 is possible? 
> I’ve thought that I might have to change to a 3 x 3 grid and have the central grid point as the single water column? 
> Or maybe the daily OBCS fields period/cycles do not match with the half hour time step?
> 
> I’ve provided my SIZE.h, data, data.exf and data.obcs files.
> 
> Look forward to hearing from someone!
> 
> Nick