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<p>Hi MITgcmers,</p>
<p>My question is vague and might not even be related to the MITgcm
itself, but I am kinda lost right now, not knowing where to look.</p>
<p>My setup is the following : <br>
</p>
<ul>
<li>2D (X-Z) plane, so only one Y coordinate, with a "thinWall" on
all YG=-dY points.</li>
<li>~9k points in X (with differing Δx on the sides, 15 m on most
of the domain)</li>
<li>70 x 1m in Z. The bathymetry is one of an estuary so ~15 m
deep in the west to 70 m in the east, with some bumps, but
smoothed so dH/dx is always very small.</li>
<li>I used the "old" formulation for the curvilinear grid as so to
pass a Δy that varies along x, in order to represent varying
estuary/river channel width, but staying in 2D. For this I was
helped by this very forum a few weeks ago (thanks Martin!).<br>
</li>
<li>Non-hydrostatic, Δt = 1 s, f=β=0</li>
<li>Implicit free surface<br>
</li>
<li>OBCS used for prescribing T and S on both sides, U in the east
(downstream), and Orlanski used for U and W upstream (west).
This means I haven't specify anything for W in the east, AFAIU
it is set to 0 by OBCS at the boundary.</li>
<li>The downstream forcing of U includes:</li>
<ul>
<li> a tide (large amplitude, period of 12.4h, quasi-constant
above and going to zero at the bottom)</li>
<li> PLUS a constant flow (small amplitude, concentrated in the
first 15 m)</li>
</ul>
</ul>
<p>The forcing is meant to represent both tides and river outflow,
all forced on the same boundary, letting Orlanski deal with the
residuals upstream (my domain is too small to encompass all the
estuary, so there should be some (smaller) tides upstream).</p>
<p>Problem: The tide-averaged flow is not conserved. Pseudo code:</p>
<pre>taF = (UVEL.weighted(hFacW) * dyG).sum('Z').mean(time over each tide period) # is the tidally-averaged flow (m³/s) for each column
</pre>
<p>I would then expect d taF / dx to be zero, but it isn't. After
many tidal periods, I still have a larger "tidally-averaged
inflow" upstream than downstream. And when looking a the free
surface I see that ETAN is drifting, again averaged over the tidal
period. dη / dt = C > 0, where C looks constant after the
first few tidal periods.</p>
<p><br>
</p>
<p>So after this long context (thanks for reading all!), my
questions:<br>
</p>
<p>Has anyone here ever used the MITgcm in this 2D X-Z kind of
configuration, am I missing some crucial parts that could explain
the drift?</p>
<p>Is this a problem that could originate from the use of a
linearized free surface? I am not able to activate the non-linear
version of the algorithm because of my need to use Orlanski's
conditions. But I could into this deeper, if it was clear that
non-linear surface could solve the problem.</p>
<p>Thanks a lot!</p>
<p>--<br>
Pascal Bourgault<br>
</p>
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