<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi Jordi,<div class=""><br class=""></div><div class="">Thanks for the insights.</div><div class=""><br class=""></div><div class="">You could also try to:</div><div class=""><br class=""></div><div class="">1. check your phase and (especially) amplitude OB files: maybe the tidal model/database and/or the scripts/tools you are using (TMD or similar...) to extract phase and amplitude (from the tidal model/database) could have some issues;</div><div class=""><br class=""></div><div class="">2. [perhaps more effective...] the "unbalanced" SSH time series (red) shows a (increasing) trend: you could try an "offline" balancing of your OBCS velocity files (before running MITgcm), with a longer timescale (a few days) in order to maintain the tidal variability, but to avoid longer-term SSH drifts (the interpolation of boundary velocities from the nesting to the nested model -almost- necessarily creates even small unbalances along the OBs -e.g., due to the different grids- that, after a few hours/days, add up and cause possible unwanted drifts...).</div><div class="">I don't know if this can work... but you could try...</div><div class=""><br class=""></div><div class="">Cheers,</div><div class=""><br class=""></div><div class="">Stefano</div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 10 Jul 2025, at 13:21:03, Jordi Iglesias <<a href="mailto:jiglesias@icm.csic.es" class="">jiglesias@icm.csic.es</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
<div style="padding-bottom: 1px;" class="">
Hello,<br class="">
<br class="">
Thanks everyone for the advice. I’ve repeated the simulation, this
time with useOBCSbalance = .FALSE.<br class="">
In the attached figure:<br class="">
The top panel shows the comparison between the MEDSEA SSH (black),
the MITgcm output with useOBCSbalance = .TRUE. (green), and with
useOBCSbalance = .FALSE. (red).<br class="">
The bottom panel shows the atmospheric pressure used to force the
model over the domain, as it also influences the SSH.<br class="">
Additionally, I’ve included another image with a screenshot from a
mareograph near Tarragona lon=1.21° E, Lat=41.08° N (unfortunately,
I don’t have access to the raw data, only the screenshot). But, it
may help to get an idea of the observed nearshore SSH.<br class="">
<br class="">
The model behaviour has improved, disabling the balance gives the
model more freedom to follow the low frequency variation, which are
qualitatively similar to MEDSEA. However, there are still
differences in the mean SSH values.<br class="">
<br class="">
Do you have any further suggestions I could try to improve the
result?<br class="">
<br class="">
Best regards,<br class=""><p class="">Jordi</p><p class=""><br class="">
</p>
<div class="moz-cite-prefix">On 9/7/25 17:11, Dimitris Menemenlis
wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:02DD7326-8DF1-4E1D-8E98-0C2230582A3B@gmail.com" class="">
<meta http-equiv="content-type" content="text/html; charset=UTF-8" class="">
<div class="">So Martin’s guess is correct. Because of:</div>
<div class=""> useOBCSbalance=.TRUE.,</div>
<div class="">you are suppressing all volume transport at the boundaries
except for tidal transports, which you prescribe on top of that.</div>
<div class=""><br class="">
</div>
<div class="">D.</div>
<div class=""><br class="">
</div>
<div class="">
<blockquote type="cite" class="">
<div class="">On Jul 9, 2025, at 8:03 AM, Jordi Iglesias
<a class="moz-txt-link-rfc2396E" href="mailto:jiglesias@icm.csic.es"><jiglesias@icm.csic.es></a> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
<div style="padding-bottom: 1px;" class=""> Hi everyone,<br class="">
<br class=""><p class="">Thanks a lot for your comments and help. Following your
recommendations, I’m attaching two figures and the
namelists. </p><p class="">Fig1.jpg shows the MEDSEA parent model, with my domain
marked by a red rectangle, as well as the three points
where I plotted the SSH from the parent model.</p>
Fig2.jpg contains three subplots. The top panel
corresponds to the SouthWest of Sicily, where I compare
the SSH at that point with and without tides from the
MEDSEA model. I also include the MEDSEA SSH averaged over
my domain (black line) shared in all plots and the MITgcm
output (my result) shown as a black dashed line. I repeat
this comparison in the south of my domain (cyan) and in
Gibraltar (red).<br class="">
<br class="">
Note: The Y-axis scales are not shared between subplots,
but in all cases, it's clear that the parent model shows a
low frequency SSH signal that I’m currently unable to
reproduce in MITgcm.<br class="">
As you mentioned, probably I should disable the balanced
flow option. Additionally, I’m sharing the namelists data
and data.obcs, if you have any advice or suggestions, I’d
be happy to try them.<br class="">
<br class="">
Thanks again for your help!<br class="">
Best regards,<br class=""><p class="">Jordi</p><p class=""><br class="">
</p>
<div class="moz-cite-prefix">On 8/7/25 17:24, Martin Losch
wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:45C12C69-2106-44E7-9A07-44252775F1BC@awi.de" class="">
<meta http-equiv="content-type" content="text/html; charset=UTF-8" class="">
Hi Jordi,
<div class=""><br class="">
</div>
<div class="">it would help if you could provide your namelist
files. And maybe the customised *_OPTIONS.h files, if
they are very different from the standard ones.</div>
<div class="">Do you use open boundary conditions? Balanced flow
is maybe an issue?</div>
<div class=""><br class="">
</div>
<div class="">Martin</div>
<div class="">
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 8. Jul 2025, at 16:57, Dimitris Menemenlis
<a class="moz-txt-link-rfc2396E" href="mailto:dmenemenlis@gmail.com" moz-do-not-send="true"><dmenemenlis@gmail.com></a>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" class="">
<div style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi
Jodi, perhaps a good place to start would be
to understand cause of the larger oscillations
in MEDSEA and ROMS, which might then help you
diagnose what is missing in your MITgcm
configuration. For example, if the ~10-day
oscillation in MEDSEA and ROMS is primarily
caused by transport through Gibraltar Strait,
then you could look at MITgcm’s transport
through Gibraltar Strait to see whether it has
been correctly prescribed.
<div class=""><br class="">
</div>
<div class="">D.<br id="lineBreakAtBeginningOfMessage" class="">
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On Jul 8, 2025, at 6:13 AM, Jordi
Iglesias <a class="moz-txt-link-rfc2396E" href="mailto:jiglesias@icm.csic.es" moz-do-not-send="true"><jiglesias@icm.csic.es></a>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div style="padding-bottom: 1px;" class=""><p class="">Dear MITgcm community,<br class="">
<br class="">
Over the past few months, I’ve
been struggling to achieve
realistic sea surface height
("Eta") results in my MITgcm
simulations. I’ve tried several
configurations,including
non-hydrostatic,
quasi-non-hydrostatic, and
nonlinear free surface options,
but the SSH output often differs
significantly from the parent
model (the Copernicus
Mediterranean analysis, MEDSEA).
I’ve attached an example and also
shared a link to illustrate the
issue. I have tides configured,
and the ssh is initialized from
MEDSEA data. However, when I
compare the SSH output with the
parent model (or with a ROMS
simulation, for example), it
appears to oscillate around a
constant value that doesn’t evolve
much over time, or changes are
very small.<br class="">
<br class="">
A summary of my configuration:
Horizontal resolution: 1.6 km, NW
Mediterranean Sea, Atmospheric
forcing: ECMWF, initial ssh set
with pSurfInitFile,
SphericalPolarGrid,
quasiHydrostatic = .TRUE.,
nonlinFreeSurf = 3, rigidLid =
.FALSE., implicitFreeSurface =
.TRUE., <br class="">
</p><p class="">If any additional parameters
would help diagnose the issue, I’d
be happy to provide them.<br class="">
<br class="">
The temperature, salinity, and
velocity fields are generally in
good agreement with ROMS and
MEDSEA, but I’d really like to
improve the SSH performance. Has
anyone encountered a similar issue
or found strategies to improve SSH
in MITgcm?<br class="">
<br class="">
Thanks in advance for your help!<br class="">
<br class="">
Best regards,<br class="">
Jordi<br class="">
<br class="">
<a class="moz-txt-link-freetext" href="https://saco.csic.es/s/c2fSgN5RWPRcDSQ" moz-do-not-send="true">https://saco.csic.es/s/c2fSgN5RWPRcDSQ</a><br class="">
</p><p class=""><br class="">
</p>
</div>
<lt-container class=""></lt-container> <span id="cid:D8924F77-9B1B-4633-B3DF-F20E164CDA90" class=""><Screenshot from
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<span id="cid:85626D64-FA85-4913-AC51-9AC50E9FC1CE" class=""><fig1.jpg></span><span id="cid:53F558FA-2DC3-49C3-8A56-AC2980468BCE" class=""><data.txt></span><span id="cid:03985B92-7490-4852-9947-4900C04CE5B5" class=""><data.obcs></span><span id="cid:94849AC6-7104-4BFA-9E47-D096094D2691" class=""><fig2.jpg></span>_______________________________________________<br class="">
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<span id="cid:DCE36915-B8EA-41EE-AD0A-54CA0D03CE42"><fig_imp_nobalance.png></span><span id="cid:B9F5963C-466D-42B6-898C-6C28E6E2DE09"><tgn_ssh_mareograph.png></span>_______________________________________________<br class="">MITgcm-support mailing list<br class=""><a href="mailto:MITgcm-support@mitgcm.org" class="">MITgcm-support@mitgcm.org</a><br class="">http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support<br class=""></div></blockquote></div><br class=""></div></body></html>