[MITgcm-support] negative salt values

[acondron at sfos.uaf.edu]

Hi Dimitris
The original advection scheme was 2 in the global_ocean.cs32x15 set-up,
and the one causing causing the negative values. I have switched to 7 now,
and everything is looking good. You are right advection scheme 7 is also
the one I use in my global cs510 setup.
cheers,
Alan



> Alan, for the CS510 configuration, we found that advection scheme 33 was
> too diffusive
> and that it quickly erodes thermocline and halocline, especially in Arctic
> Ocean.
> We typically use advection scheme 7.
>
> What advection scheme was causing you to get negative salinity?
>
> Dimitris
>
> Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
> Jet Propulsion Lab, California Institute of Technology
> MS 300-323, 4800 Oak Grove Dr, Pasadena CA 91109-8099, USA
> tel: 818-354-1656;  cell: 818-625-6498;  fax: 818-393-6720
>
> On Feb 13, 2010, at 12:45 PM, acondron at sfos.uaf.edu wrote:
>
>> Hi Martin
>>
>> I re-ran my simulation with advection scheme 33, and this has resolved
>> the
>> issue. Many thanks for the quick response and assistance.
>>
>> best,
>> Alan
>>
>>
>>> 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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>>>
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>>
>>
>>
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