[MITgcm-support] negative salt values

[Martin Losch]

Hi Alan,

when you add a lot of fresh water without the proper boundary conditions, you are bound to get negative salinities. I believe, that these are the important parameters, that turn on the "true" fresh water flux boundary conditions together with the non-linear free surface and the z* coordinates:

>  implicitFreeSurface=.TRUE.,
>  exactConserv=.TRUE.,
>  select_rStar=2,
>  nonlinFreeSurf=4,
>  hFacInf=0.2,
>  hFacSup=2.0,
>  useRealFreshWaterFlux=.TRUE.,

Are these your parameters?

In the case of large horizontal gradients, there's always the possiblity of over/undershoots due to insufficient advection schemes. For your experiment you should use something with flux limiter, e.g. 33 or 77 (even then, you might get very, very small undershoots, according to my experience, in theory this should not happen).

Martin
On Feb 12, 2010, at 2:22 AM, Alan Condron wrote:

> Hi all
> 
> I have been doing some experiments simulating large glacial flood events. To simulate such events I'm adding 5 Sv of water as RUNOFF, into 5 model grid boxes in hudson bay .
> When I attempt to do this in the global_ocean.cs30x15 setup, I find that I can create negative salinity values where I add the flood, and also away from this region as the flood event advects in the model. I also get negative values in a tracer I tag the flood waters with.
> 
> I was surprised I could get negative salinity values. And I wonder if its simply the case of setting a flag in the set-up to avoid this?
> 
> Thanks
> Alan
> 
> Dr Alan Condron
> 112 O'Neill
> P.O. Box 755220
> University of Alaska Fairbanks
> Fairbanks, AK 99775-7220
> Phone: (907) 474-7707
> 
> 
> 
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