[MITgcm-support] grid-scale noise in 1/4-deg model

[Alistair Adcroft]

Ichiro,

For the left column runs (bi-harmonic) were you using the maximum allowed
viscosity?
It's worth finding the upper limit on numerical viscosity before the model
is unstable. My preferred recipe is to use 0.1 of this value - this damps
the grid scale noise one e-folding scale every 10 steps and damps the 2-dx
wave only every 160 steps. I think the relevent expression is something
like:
   A_4 * dt * cos(phi_max)^m * pi^4 [ 1 / dx(phi_max)^2 + 1/dy(phi_max)^2
]^2 < 1
assuming the expression is maximized at the northern edge of the domain. The
choice of "m" can change that.

Do you have bottom friction turned on?

A.
--
Dr Alistair Adcroft            http://www.mit.edu/~adcroft
MIT Climate Modeling Initiative        tel: (617) 253-5938
EAPS 54-1523,  77 Massachusetts Ave,  Cambridge,  MA,  USA

-----Original Message-----
From: mitgcm-support-bounces at mitgcm.org
[mailto:mitgcm-support-bounces at mitgcm.org] On Behalf Of Ichiro Fukumori
Sent: Monday, November 24, 2003 9:41 PM
To: cnh at mit.edu; adcroft at ocean.mit.edu; support at mitgcm.org; Armin Koehl;
Detlef Stammer
Cc: tlee at pacific.jpl.nasa.gov; bnc at pacific.jpl.nasa.gov
Subject: [MITgcm-support] grid-scale noise in 1/4-deg model


Dear Colleagues,

We are experiencing grid-scale noise in our global 1/4-deg model simulation.
It does not appear to be a lack of horizontal friction. Would you have any
idea what may be the problem?  My suspicion is that this may have something
to do with some vertical process.  Places with prominent noise are adjacent
to coastal boundaries (where there's
upwelling) and places with deep mixed layers.

Attached are 2 plots comparing model output of two runs at two different
locations.  Each page shows a 101x101 grid point area.  Plot n03d_1.ps is
surface layer along the west coast of Africa (centered 13.8E, 19.4S) and the
other (n03d_2.ps) is Kuroshio extension area (centered 170.1E, 28.9N) at
350m depth.  The three rows are, from the top, zonal velocity (u, m/s),
meridional velocity (v, m/s), and temperature (T, degC).  The left column is
a run that employs biharmonic friction (1.e10 m^4/s) and is at day 123 of
1992.  The right column used Laplacian friction (200 m^2/s) and is at day 69
of 1992.

In n03d_1.ps, the noise appears along the coast (land is shown in gray),
especially in u & v of the right column (Laplacian friction run).

In n03d_2.ps, the noise is severe in u & v in the left column in the
northern half, just south of the Kuroshio.

All results are 3-day averages.
Could these noise be due to an absence of the CD scheme?  What else could
cause such noise?  We would appreciate any suggestions you may have.

Thanks, Ichiro


===============================================================
Ichiro Fukumori               | Jet Propulsion Laboratory
e-mail: fukumori at jpl.nasa.gov | Mail Stop 300-323
phone: +1 (818) 354-6965      | 4800 Oak Grove Drive
fax: +1 (818) 393-6720        | Pasadena, California 91109, U.S.A.
===============================================================