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

[Martin Losch]

Hello everyone,

the KPP-related grid-scale noise problems I had have been resolved: 
There was a problem in find_alpha/find_beta (thermal expansion, saline 
contraction coefficients) with the "new" equation of state MDJWF 
(McDougall etal 2003). alpha and beta were wrong by orders of magnitude 
which caused KPP to mix the entire watercolumn almost everywhere with 
subsequent crash of the model. The problem is fixed and I use the 
default KPP-parameters.

But I have a humble suggestion for the grid scale noise:
boundary layer width (Munk layer) = \pi(A_h/\beta)^{1/3} = 
3.1427*(200/2e-11)^{1/3} = 3.1427*20km=60km,
for biharmonic friction I have 
pi(B_h/\beta)^{1/3}=3.1427(1e10/2e-11)^{1/5} = 3.1427*14km = 43km;
and you resolution is approximately 30 km, so even if you take into 
account the additional factor pi (which not everyone does) you barely 
resolve the boundary layer. You seem to have one along the African 
coast.

I don't know if that's relevant for the areas that you have plotted, 
but I would think that your horizontal friction is indeed too small, so 
that the boundary layers are to thin to be resolved by your grid. I use 
viscAh=1e11/diffK4T=1e11 and in the vertical viscAr=1e-3/diffKrT=1e-5 
for  a channel with 25km horizontal resolution, to make it stable with 
the default advection scheme (central differences = #2). The CD-scheme 
adds additional viscosity, so that could fix it, if I am right.

Hope that helps,
Martin

On Tuesday, November 25, 2003, at 05:48  AM, Dimitris Menemenlis wrote:

> Martin, the following problem we are having with our 1/4-deg global
> set-up reminds me that at some point you were also having trouble with
> grid scale noise.  Did you end up tracking the problem down?  Was it
> KPP-related?  Would it be possible to get a copy of your data and
> KPP_OPTIONS.h file to see what parameters you ended up using.
> Cheers, Dimitris
>
>
> -----Forwarded Message-----
> From: Ichiro Fukumori <fukumori at jpl.nasa.gov>
> To: cnh at MIT.EDU, adcroft at ocean.mit.edu, support at mitgcm.org, Armin 
> Koehl <koehl at ucsd.edu>, Detlef Stammer <stammer at ifm.uni-hamburg.de>
> Cc: tlee at pacific.jpl.nasa.gov, bnc at pacific.jpl.nasa.gov, Dimitris 
> Menemenlis <menemenlis at jpl.nasa.gov>
> Subject: grid-scale noise in 1/4-deg model
> Date: Mon, 24 Nov 2003 18:41:08 -0800
>
> 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.
> ===============================================================
> -- 
> Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
> Jet Propulsion Lab, California Insitute of Technology
> MS 300-323, 4800 Oak Grove Dr, Pasadena CA 91109-8099
> tel: 818-354-1656;  fax: 818-393-6720
> <n03d_2.ps><n03d_1.ps>