[MITgcm-support] Linear internal waves evolution under a free surface and a rigid lid.

[Fancer Lancer]

Dear MITgcm,

*First of all I want to wish everyone a happy new year, let all your plans
come successfully true, models converge, researches turn out well!*

And here is my question.
I am trying to run a few setups of quasi-monochromatic internal waves in a
two-layers environment with very sharp transition layer. The target of the
experiments is to research a transformation process of linear internal
waves over a sharp bottom obstacle. The linearity of the waves means very
small amplitudes of the disturbances comparing with a depth. So to speak we
calculate a small amplitude quasi-sinusoidal internal waves on a tiny
pycnocline over a sharp bottom obstacle. Technically there is no pycnocline
at all, but the transition between two layers is sharp.
That setup converges only if I specify the free surface, in case of the
rigid lid the system suddenly blows up.The monitor output shows a small
fluctuation of rhsMax in the middle of calculations:
 cg2d: Sum(rhs),rhsMax =  -1.00649220260016E-04  2.80476672625558E-08
 cg3d: Sum(rhs),rhsMax =  -4.36678712900714E-03  2.73727850080651E-11
 cg2d: Sum(rhs),rhsMax =  -1.00473220760270E-04  2.80986212383509E-08
 *cg3d: Sum(rhs),rhsMax =  -1.02540856095068E-06  1.16576772289106E-07*
 cg2d: Sum(rhs),rhsMax =  -9.74440421204186E-05  2.81449166440040E-08
 cg3d: Sum(rhs),rhsMax =  -4.22056718404259E-03  2.75142884748998E-11
 cg2d: Sum(rhs),rhsMax =  -1.00470277023852E-04  2.81813454193756E-08
 cg3d: Sum(rhs),rhsMax =  -4.35243360164479E-03  2.75448882646779E-11
...
And after several time-steps the model blows up:
 cg3d: Sum(rhs),rhsMax =  -2.95322925916202E-15  1.00918335046307E+15
 STOP GAD_IMPLICIT_R: error when solving 3-Diag problem
That doesn't happen if the free surface is specified. I didn't find any
problem with CFL, but a surface elevation turns to an enormous value during
the calculations:
(PID.TID 0000.0001) %MON dynstat_eta_max              =
 -1.6182199742936E+01
(PID.TID 0000.0001) %MON dynstat_eta_min              =
 -1.6182199808065E+01
...
(PID.TID 0000.0001) %MON dynstat_eta_max              =
 -1.2229692791603E+01
(PID.TID 0000.0001) %MON dynstat_eta_min              =
 -1.2229692856491E+01
...
...
(PID.TID 0000.0001) %MON dynstat_eta_max              =
 -4.3612306317841E+04
(PID.TID 0000.0001) %MON dynstat_eta_min              =
 -4.3612307712367E+04

I don't know how it affects my calculations, since the surface is blocked
by a rigid lid.
Could you please tell me what is wrong here and give some guide how to fix
the problem?
(data file is in attachment)

Sincerely,
-Sergey
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20131231/df2f0396/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: data
Type: application/octet-stream
Size: 2305 bytes
Desc: not available
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20131231/df2f0396/attachment.obj>