[MITgcm-support] Re: viscosity questions
Martin.Losch at awi.de
Martin.Losch at awi.de
Sat Nov 18 00:21:50 EST 2006
Hi Baylor,
thank you for the comments. They are very helpful!
I'll look into the timestepping issue, although with the current time step I get courant numbers on the order of 0.02. Is there
anything similar for the temperature equation that I can check?
I have been slowly approaching the second point you make, ie., the noise will go away for longer integrations, although I
use the painful way of experience to reach this wisdom, where you use the noble way of knowledge (and as usual, I tried
the third, the simplest one, of copying other peoples work, but that didn't work yet (o: ). I will wait a little and have a look at
the solutions after a some spin up. I do observe that the model sometimes approaches numerical instability and then
recovers and gets smoother again.
Why doesn't that happen with advScheme=2? I may not have been clear about this, but it only works with 2, if I use high
horizontal diffusivities (1e6 for the 900m resolution case). So yes, much higher background diffusivities (not viscosities).
I'll look into your proposed solutions, probably the second one first.
Martin
Martin Losch
Alfred Wegener Institute
Postfach 120161, 27515 Bremerhaven, Germany;
Tel./Fax: ++49(0471)4831-1872/1797
----- Original Message -----
From: Baylor Fox-Kemper <baylor at MIT.EDU>
Date: Friday, November 17, 2006 6:21 pm
Subject: Re: [MITgcm-support] Re: viscosity questions
> Hi Martin,
> A few more comments.
> J-M and I talked last night, and we had a suspicion that maybe
> your timestep was too big. With nonlinear viscosities, you can
> make
> a model that is linearly unstable nonlinearly stable, but your temp
>
> equation might still have residual noise.
> Also, I often see noise like that using pure 33 and smagorinsky
> during spin-up in my calculations (with no background diffusivity
> or
> viscosity). Of course, the 'diffusivity' in a upwind scheme is a
> function of velocity (e.g., 1rst upwind kappa2 \propto dxf |U|, I
> think 3rd order upwind goes something like kappa4 \propto dxf^3
> |d^2U/
> dx^2|, but I am not sure at this second), and the viscosity is also
> a
> function of velocity (in Smag, nu\propto dx^2 |u_i,j| ). So, in
> your
> model, the deep flow is not directly forced, and so it doesn't have
>
> any velocity or velocity gradients at first, so the diffusivity of
> 33
> and the viscosity of Smag are not in play. I find that in my
> simulations, once things 'get humming', and there is velocity
> throughout the domain, these noisy regions go away. In any case,
> they cannot generate a large spurious circulation, because they are
>
> nonlinearly stabilized if U picks up.
> This is a fundamental 'feature' of the smagorinsky/upwind
> approach. In both cases, it is assumed that the flow field is
> everywhere 'active'. There is a lot of research on LES methods for
>
> fixing this problem when there is no background flow, e.g., dynamic
>
> smagorinsky coefficients, which is a topic I'm working on but don't
>
> have it in the MITgcm yet.
> Now, I am wondering why the same doesn't happen for
> advectionscheme=2. Are you using bigger background diffusivity/
> viscosity then?
> So, two solutions: 1) use larger background diffusivity and
> viscosity which will kill the noise in the spinup but probably
> make
> the later stages too viscous, or 2) ignore the spinup and see what
> happens after the velocity field is established. I suppose there
> is
> a third hybrid option, which is use more diffusivity/viscosity
> during
> spin-up, then do a pickup file and drop them after the model has a
> U
> field.
> Cheers,
> -Baylor
>
> On Nov 17, 2006, at 5:03 AM, <Martin.Losch at awi.de> wrote:
>
> > Jean-Michel, Dimitris,
> >
> > thanks again for your comments. I don't think that KPP is the
> > problem as I get noise also without it. With advScheme 33, I
> > tried different diffusivities (smaller than those used with
> > advScheme 2) and also no explicit diffusivity, but none seemed to
> > work properly.
> >
> > I have tried to use a combination of 33 and 2 as Jean-Michel
> > suggests, but I had to turn off multidimadvection (=.false.) for
> > that, so that the solution looks very different, but the noise is
>
> > not gone.
> >
> > For now, I have 3 series of plots to look at, if anyone is
> > interested. For these plots I have reduced the domain (gulf of elat
> > at the northern end of the red sea) to get faster results. I
> > started from rest and ran the model for 30 days and forcing with
> > homogeneous wind stress (wind from the south). "data" is
> attached,
> > no other packages used (except for diagnostics).
> > horizontal resolution is 900m. advection 33 with no explicit
> > diffusivity.
> > Shown are temperatures at layers 1,5,10 with velocity vectors,
> > nothing is scaled for plotting. On the left is a run with
> > viscC2Smag=1 and on the right with viscC4Smag=1. In particular,
> > level 5 has a lot of noise that persists for 30 days. The
> > noise in level 10 appears to fade away towards the end of the
> > intergration period. I do not show W, because that's noisy
> > all the way, nothing to learn. horizontal velocities are more or
> > less smooth (they are affected by the noise in the density
> > field, of course, but the viscosity seems to damp this away along
>
> > the boundaries, where viscAh/4 is high, not so much in
> > the interior where viscAh/4 is low. The plots of Ah/4 are not
> smooth).>
> > here are the plots (approx 2MB each):
> > http://mitgcm.org/~mlosch/eilat_temp1.ps.gz
> > http://mitgcm.org/~mlosch/eilat_temp5.ps.gz
> > http://mitgcm.org/~mlosch/eilat_temp10.ps.gz
> >
> > In the full domain the noise is even more pronounced.
> >
> > M.
> >
> > Martin Losch
> > Alfred Wegener Institute
> > Postfach 120161, 27515 Bremerhaven, Germany;
> > Tel./Fax: ++49(0471)4831-1872/1797
> >
> >
> >
> > ----- Original Message -----
> > From: Jean-Michel Campin <jmc at ocean.mit.edu>
> > Date: Friday, November 17, 2006 2:31 am
> > Subject: Re: [MITgcm-support] Re: viscosity questions
> >
> >> Hi Martin,
> >>
> >> Looks like an interesting problem !
> >> No obvious noise on uVel,vVel. What about wVel ?
> >> Also, the noise on T,S, is it a steady noise or oscillating/fast-
> >> growing ?
> >>
> >> I did not use very often this option, but you can try to
> >> keep the 33 advection scheme for the horizontal, and switch back
> >> to 2nd order in the vertical (at least for a test):
> >> tempAdvScheme=33,
> >> tempVertAdvScheme=2,
> >>
> >> Jean-Michel
> >>
> >> On Thu, Nov 16, 2006 at 11:34:32PM +0200, Martin.Losch at awi.de
> wrote:>>> Hi,
> >>> thanks for the KPP suggestion. However, I never had any problems
> >> with KPP, and this time again, KPP is not the culprit. I
> >>> turned it of (use cAdjFreq=-1) and the problem persists.
> >>>
> >>> Again:
> >>> with temp/saltAdvScheme = 2 and diffK4T/S = 1.e6, I can suppress
> >> the noise, but
> >>> with temp/saltAdvScheme = 33 and diffK4T/S = 1.e6, there is a lot
> >> of noise (also with diffK4T/S smaller or 0),
> >>> staggerTimeStep = .true.. That's what's really puzzling me. I
> >> though that the advection scheme 33 (DST3FL) is extremely
> >>> stable and should remove all grid scale noise (in my previous
> >> experience it does exactly that). Why not now?
> >>>
> >>> Martin
> >>>
> >>> Martin Losch
> >>> Alfred Wegener Institute
> >>> Postfach 120161, 27515 Bremerhaven, Germany;
> >>> Tel./Fax: ++49(0471)4831-1872/1797
> >>>
> >>>
> >>>
> >>> ----- Original Message -----
> >>> From: Dimitris Menemenlis <menemenlis at sbcglobal.net>
> >>> Date: Thursday, November 16, 2006 9:09 pm
> >>> Subject: Re: [MITgcm-support] Re: viscosity questions
> >>>
> >>>> KPP does have a computational grid-scale mode in T/S,
> >> especially
> >>>> visible at
> >>>> Equator, in my experience. That's why those horizontal 121
> >> filters
> >>>> were added.
> >>>> The recommended filters are on by default in KPP_OPTIONS.h.
> >> But
> >>>> worth trying
> >>>> with KPP off. D.
> >>>> _______________________________________________
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