[MITgcm-support] Re : Water at the bottom of NA below freezingpoint

Tue Mar 11 21:08:41 EDT 2008

Hi

I agree with Martin on this. I don't believe this is due to the  
advection scheme, partial cells, etc. I have run various uncoupled
simulations at different resolutions, including something very  
similar to the ECCO-GODAE 1x1, 23 levels, without running into
this particular problem.

Gus is using an atmospheric mixed layer model which also has (at  
least in my version) a thermodynamic sea-ice model. Perhaps,
this should be turned off for the initial testing. Another aspect I  
would look into is the geometry. I had problems at one point coupling
this model to another OGCM (in my pre-MITgcm days!!) because it was  
tricky to pass it the correct grid areas, etc. Perhaps heat is not
being conserved because of this.

Samar

On Mar 11, 2008, at 7:44 PM, <Martin.Losch at awi.de> wrote:

> Hi all,
>
> I just cannot believe, that the advection (scheme) alone can  
> produce, what you are observing. Advection schemes can be poor and  
> can produce overshoots/undershoots where there are steep tracer  
> gradients. In the abyssal ocean you generally do not have these  
> steep gradients. Also I have never observed the ocean filling with  
> cold water in my uncoupled runs (with sea ice!!), and if I have  
> observe temps below freezing in the past, I could *always* trace  
> this problem back to numerical stability  issues, eg. the timestep  
> was too large, etc.
>
> I run passive tracers with very steep gradients (0 in one box and 1  
> in the next initially), and I get overshoots on the order of 1e-6  
> for the flux limited schemes at most. Certainly not enough to  
> explain your cold temperatures.
>
> Again, my suspicion is, that very cold water is produced probably  
> at the surface (heat flux), which is then immediately convected to  
> lower depths, at the surface being replaced by warmer water. The  
> new warm surface waters will again be cooled (e.g. atmospheric  
> temperatures may be too cold, problems with humidity?) and convect,  
> etc. That way you can easily fill the bottom of the ocean with cold  
> water. Inaccurate advection schemes are much less efficient for  
> doing that (Jean-Michel, say something). In coupled ice-ocean  
> simulations I had situations where I made mistakes with units etc  
> of prescribed atmospheric fields (temperature and salinity), that  
> lead to dramatic ice growth, but never to supercool temperatures in  
> the deep ocean. Diagnose you heat fluxes (eg., diagnostic oceFlx,  
> but there are others that describe the full temperature tendency)  
> and see if it is balanced over longer periods (year to years), I  
> bet it isn't.
>
> I don't see why turning off partial cells should help in this  
> situation, either. Generally, solutions near topography are  
> smoother with partial cells, the only disadvantage I see is that  
> sometimes the cfl-criterion for w is more restrictive with partial  
> cells, but I have not observed that in coarse resolution runs, yet.
>
> In order to rule out the advection schemes as culprits I suggest  
> that you try turning off the advection of tracers altogether. That  
> will give you a completely different circulation in the ocean as  
> only diffusion will distribute tracers, but for a test it should be  
> fine. To do that set
> tempadvection=.false.,
> saltadvection=.false.,
> in data, PARM01
> If you still get the low temperatures, then you know that it's not  
> the advection schemes. If not, I'll shut up (probably).
>
> Gus, what I forgot to mention in my previous email: instead of  
> "useOldFreezing" I recommend using "allowFreezing". With this flag  
> you only reset the surface temperature to -1.9, this acts as a  
> minimal sea-ice model and no supercool temperatures can enter the  
> ocean. Given, I am right about the advection schemes, then there  
> shouldn't be any water below freezing in the ocean.
>
> Scott, I don't think 22layers in 4x4 or 2x2 degress is a problem. I  
> do exactly that (actuatlly 23layers with 10m near surface), and the  
> old ecco solutions are 2x2 with 23 layers (10m near the surface).
>
> Martin Losch
> Alfred Wegener Institute
> Postfach 120161, 27515 Bremerhaven, Germany;
> Tel./Fax: ++49(0471)4831-1872/1797
>
>
>
> ----- Original Message -----
> From: Jeff Scott <jscott at halo.mit.edu>
> Date: Tuesday, March 11, 2008 4:52 pm
> Subject: Re: [MITgcm-support] Re : Water at the bottom of NA below	 
> freezingpoint
>
>>
>>
>>>
>>> I have run many different (uncoupled) configurations where the
>> heat flux is
>>> prescribed at the surface (or computed from atmospheric
>> parameters and bulk
>>> formulae and then prescribed) and I have never seen this type of
>> behavoir,
>>> even in 1000-9000 year runs. My suspicion is that your system is
>> loosing heat
>>> somewhere, probably in the coupling. I suggest that you diagnose
>> your net
>>> heat flux at the surface and then you'll see how big the
>> imbalance is. Is it
>>> possible that your atmosphere is loosing the heat and transfering
>> that
>>> imbalance to the ocean? I know for sure that if the MITgcm (ocean
>> model)
>>> conserves heat very accurately and global mean temperature drifts
>> are only
>>> possible, if you have a non-zero net heat input.
>>>
>>> I would try to find out whether your cold temperatures come from
>> the surface
>>> (I bet they do) and are "convected" downwards.
>>>
>>
>> Gus,
>>
>> By usual course the atmosphere is going to be losing heat through
>> radiative processes... but I think Martin is suggesting there might
>> be a
>> problem/bug with atmosphere or the coupling here and the system
>> cannot
>> come to balance. That is certainly one possibility.
>>
>> But I think it is also certainly possible that the problem is with
>> the
>> ocean. What is your vertical diffusivity? If it is low (.2e-4 or
>> especially if less), I had exactly the same happen -- specifically,
>> problems with advection-dominated flow causes overshoots and
>> undershoots,
>> the later being handled by convection and/or GM to fill the ocean
>> with
>> cold temps, which often keep getting colder the longer the run.
>>
>> (this general problem was very clear when I was using standard MOM
>> in a
>> single hemisphere, coarse-res box configuration with very low
>> diffusivity:
>> the whole deep tropical ocean was convecting near the bottom, and
>> the deep
>> ocean temp kept dropping)
>>
>> Curiously, I also had most problems with my similar 4x4x22
>> (coupled)
>> configuration (I had KPP on, with vertical layers starting at 10m
>> at the
>> top); rather than having the (spurious) deep cold temps confined in
>> the
>> NA, the problem occurred near the ACC and it filled the whole deep
>> ocean
>> with water below the freezing point. I do not have a problem using
>> 2.5x2x22 in the SH, but the spurious cold temps were back again in
>> the NA.
>> Very odd... but I thought perhaps that 22 layers/10m was too fine
>> given
>> the super-coarse horizontal resolution (although I have no real
>> basis to
>> make this statement).
>>
>>
>> My advice - definitely do not use partial cells. If you can stand a
>> little
>> diffusion in your system (I agree with Christopher Wolfe's earlier
>> post
>> about SOM giving +1e-5 or more) and can be a little more patient,
>> use SOM,
>> as this should pretty much solve your problem (assuming this is the
>> problem). For other reasons, I'd turn off the CD scheme too, but as
>> Martin
>> said it might take some retuning of the viscosities.
>>
>> And if by chance you are interested in playing with my setup or
>> topography, and/or the coupled atm2d package, I'd be happy to help.
>>
>> Cheers,
>> Jeff
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