[MITgcm-support] [EXTERNAL] topography with OBCS

Tue Jun 23 03:26:00 EDT 2020

Thanks a lot for that info Martin! I have no idea why the current configuration works either, but I'll keep going with it since the boundary seems to look fine. But I'll definitely have a look into Stevens BCs as well.
Thanks again!
Andreas

________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
Sent: Tuesday, 23 June 2020 5:17 PM
To: MITgcm Support <mitgcm-support at mitgcm.org>
Subject: Re: [MITgcm-support] [EXTERNAL] topography with OBCS

Hi Andreas,

I would use OBCS to prescribe U,V,T,S on the boundary. This usually leads to some noise along the boundary (especially in W), because typically the quartett is not in geostrophic balance. One way of getting around that is to use the Stevens BCs (because vertical shear of velocity is more consistent with model dynamics, see documentation), another one is relaxation in a sponge layer. In my experience, a sponge layer just moves the problem away from the open boundary to the edge of the sponge layer, if you are not very careful. The advantage of RBCS over the sponge layer of OBCS is that you have more flexibility in prescribing the relaxation timescale.

The OBCS sponge layer by itself should not be the problem, but maybe your inner relaxation timescale is too short (10 days)? If the same configuration works with RBCS, then stick with it, but I have no idea, why that should be any different.

Martin

> On 23. Jun 2020, at 09:04, Andreas Klocker <andreas.klocker at utas.edu.au> wrote:
>
> Hi Matt,
> I now turned all of the sponge in the OBCS package off and instead I use the RBCS package to restore T/S over a sponge. As far as I can tell this seems to work...
> Can anyone tell me though what the best way to deal with open boundaries is? Is it best to have U/V prescribed via OBCS and T/S restored in a sponge layer using RBCS? Or is there a better way?
> cheers,
> Andreas
>
>
> You are sponging 40 points in. Is that the problem - is it restoring to 0 because of that?
>
> Try with
> OBCSsponge_Salt=.FALSE.,
> OBCSsponge_Theta=.FALSE.,
>
> And see if that helps?
>
> Matt
>
>
>
>
> From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Dimitris Menemenlis <dmenemenlis at gmail.com>
> Sent: Monday, 22 June 2020 12:36 AM
> To: MITgcm Support <mitgcm-support at mitgcm.org>
> Subject: Re: [MITgcm-support] [EXTERNAL] topography with OBCS
>
> Hi Andreas, I had assumed that your initial tests were an attempt to reproduce a small region from a larger-domain simulation. If you don’t extract the U/V/T/S OBCS fields from the correct locations in the larger-domain simulation, you might end up with a mask mismatch issue. But based on your discussion with Martin, this seems not to be the case.
>
> If your simulations are easy to repeat, you could you try turning off
> useOBCSsponge = .FALSE.,
> useOBCSbalance = .FALSE.,
> and then turning these back on one at a time in order to isolate the bug.
> I don’t think that the bug lives in useOBCSprescribe for 3D variables.
>
> Dimitris
>
> > On Jun 21, 2020, at 2:57 AM, Andreas Klocker <andreas.klocker at utas.edu.au> wrote:
> >
> > Dimitris,
> > What do you mean by extracting my boundary conditions at the correct location?
> > cheers,
> > Andreas
> >
> > From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Dimitris Menemenlis <dmenemenlis at gmail.com>
> > Sent: Friday, 19 June 2020 1:12 PM
> > To: MITgcm Support <mitgcm-support at mitgcm.org>
> > Subject: Re: [MITgcm-support] [EXTERNAL] topography with OBCS
> >
> > Hi Andreas, are you sure that you are extracting your boundary conditions at the correct location?
> > You could be off by one grid cell, which could cause this problem. Dimitris
> >
> >
> >> On Jun 18, 2020, at 6:21 PM, Andreas Klocker <andreas.klocker at utas.edu.au> wrote:
> >>
> >> Hi,
> >>
> >> I am trying to set up a regional model configuration with OBCS. For some reason the model seems to introduce salinities where I actually have bathymetry (see attached figure; bathymetry left, salinities right). Salinities this low definitely do not exist in the files containing the bounday conditions. Is it necessary to set the open boundary exactly to where I have ocean, or should the bathymetry file take care of this by telling the model there should be nothing happening in case of zero water depth?
> >>
> >> Here's my data.obcs file:
> >>
> >> # Open-boundaries
> >> &OBCS_PARM01
> >>
> >> OB_Iwest = 600*1,
> >> OB_Ieast = 600*-1,
> >> OB_Jsouth = 2600*1,
> >> OB_Jnorth = 2600*-1,
> >>
> >> useOBCSsponge = .TRUE.,
> >> useOBCSbalance = .TRUE.,
> >> useOBCSprescribe = .TRUE.,
> >>
> >> useOrlanskiWest = .FALSE.,
> >> useOrlanskiEast = .FALSE.,
> >> useOrlanskiSouth = .FALSE.,
> >> useOrlanskiNorth = .FALSE.,
> >>
> >> OBWuFile = 'OWU',
> >> OBWvFile = 'OWV',
> >> OBWtFile = 'OWT',
> >> OBWsFile = 'OWS',
> >>
> >> OBEuFile = 'OEU',
> >> OBEvFile = 'OEV',
> >> OBEtFile = 'OET',
> >> OBEsFile = 'OES',
> >>
> >> OBSuFile = 'OSU',
> >> OBSvFile = 'OSV',
> >> OBStFile = 'OST',
> >> OBSsFile = 'OSS',
> >>
> >> OBNuFile = 'ONU',
> >> OBNvFile = 'ONV',
> >> OBNtFile = 'ONT',
> >> OBNsFile = 'ONS',
> >>
> >> # OBCS_monitorFreq= 86400.,
> >> # OBCS_monSelect = 1,
> >> &
> >>
> >> # Orlanski parameters
> >> &OBCS_PARM02
> >> # Cmax=0.45,
> >> # cVelTimeScale=1000.,
> >> &
> >>
> >> &OBCS_PARM03
> >> Urelaxobcsinner=864000.,
> >> Urelaxobcsbound=86400.,
> >> Vrelaxobcsinner=864000.,
> >> Vrelaxobcsbound=86400.,
> >> spongeThickness=40.,
> >> &
> >>
> >> Thanks in advance!
> >> Andreas
> >>
> >>
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