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

[Martin Losch]

Jeff,

I think your problem is very different from Gus's problem (and  
probably Dror's) and we should be care not to mix different issues,  
that why I answer in a separate email.

I think you have me convinced that in your case, the advection  
schemes may have something to do with the sub-freezing temperatures.
All the comparisons between advection schemes that I have seen show that
1. SOM has very small but finite overshoots
2. SOM has very small diffusivity
3. Flux limited schemes (such as 77 or 33) have no overshoots  
(although in my experience they can have very small ones at very  
steep gradients)

So your observations contradict some of my experiences. I don't know  
what to say. You time steps seem OK to (although I would just use  
1200sec for deltaTmom as long as nothing blows up. You could try  
decreasing the deltaTtracer to  3600sec just to try once. It will  
certainly make the solution more accurate.

Partial cells: If you have very thin cells, that can be a problem I  
think.

I'd have to see the solutions etc in more detail to say more, but  
that's probably not feasable via email.

Martin

On 13 Mar 2008, at 15:38, Jeff Scott wrote:

>
> Hi Martin and others,
>
> Continuing this discussion...
>
> I lay out my case for an advection-related problem as follows:
>
> It is not just me that seems to a problem with anomalously cold  
> temps, but Gus, Dror, and Christopher have chimed in, seeming to  
> offer agreement of some sort. That being said, it is not entirely  
> clear that we are all dealing with the same problem - I'm a little  
> puzzled by Gus' comment that he is "losing half the heat content".  
> I'm not sure exactly how that was determined, but sounds like a  
> massive loss, whereas I'm focusing on gridpoints that are colder  
> than they should be (although this does spread in some cases).
>
> I don't think there is much in common between our setups, other  
> than being coarse res; none of them are running my specific coupled  
> configuration. In my setup, some of the problem gridpoints are  
> fully covered with seaice (THSICE) all or at least part of the  
> year. If there were a problem with THSICE and/or the coupling, why  
> would the problem be so localized?
>
> But I think the biggest argument I can offer is that when I use  
> SOM, the problem largely goes away: minimum temps are generally  
> much closer to the -2 C, roughly the coldest possible icefreeze  
> temp. As I mentioned, I found SOM to add some diffusion to the  
> system (ala what Christopher noted), but it also added some  
> "wiggle" to the velocity field, particularly noticeable on a MOC  
> contour plot.
>
> Here is the minimum temps in two configurations, by vertical level,  
> separated into NH and SH:
>
> 4x4x22
> Depth   Min SH  Min NH
> -----   ------  ------
> 5	 -1.8	 -1.7
> 18	 -2.3	 -1.7
> 35	 -2.8	 -1.7
> 60	 -3.2	 -1.7
> 95	 -3.2	 -1.7
> 142	 -2.4	 -1.3
> 205	 -1.9	 -0.4
> 285	 -1.6	 0.0
> 385	 -2.2	 0.0
> 508	 -1.4	 0.0
> 655	 -1.4	 0.0
> 830	 -1.5	 -2.9
> 1038	 -1.5	 -0.7
> 1282	 -1.7	 0.0
> 1570	 -1.6	 0.0
> 1905	 -1.7	 0.0
> 2292	 -1.7	 0.0
> 2738	 -2.0	 -0.0
> 3245	 -1.7	 -0.8
> 3820	 -1.8	 -1.0
> 4468	 -1.8	 -1.2
> 5192	 -1.7	 -0.9
>
>
> 2.5x2x22
> Depth   Min SH  Min NH
> -----   ------  ------
> 5	 -1.8	 -1.8
> 18	 -1.8	 -1.8
> 35	 -1.7	 -1.8
> 60	 -1.7	 -1.7
> 95	 -1.4	 -1.6
> 142	 -0.6	 -1.5
> 205	 0.0	 -3.9
> 285	 0.0	 -4.0
> 385	 0.0	 -1.6
> 508	 0.0	 -1.8
> 655	 0.0	 -3.3
> 830	 0.0	 -1.9
> 1038	 0.0	 -1.8
> 1282	 0.0	 -1.7
> 1570	 0.0	 -1.8
> 1905	 0.0	 -1.8
> 2292	 0.0	 -1.9
> 2738	 0.0	 -1.9
> 3245	 0.0	 -1.9
> 3820	 0.0	 -1.8
> 4468	 0.0	 0.0
> 5192	 0.0	 0.0
>
>
> (min of 0.0 is really 0.0 or above)
>
>
> In the 4x4x22 case, the show-stopper is the problem in the top 100  
> meters - these "bad" temps occur only at one point, just east of  
> the stub of land jutting N of Antartica into the Drake Passage. It  
> would seem this is a bit of a stagnation point.  But note there is  
> also a "bad" point at 830m in the NH. I did look at the SH bad  
> point at every time step; at no time is the surface below the  
> freezing point, at least as dumped in the end of the k-loop in  
> thermodynamics.F (the the coldest point seems always to be at 65 or  
> 95m depth). FWIW, the anomalous cold spot increases during summer  
> and fall, decreases during winter and spring, at least for the year  
> I examined.
>
> In the 2.5x2x22 case, the SH is fine everywhere, but there are  
> isolated cold points at 285 and 655m in the NA. But unlike the  
> 4x4x22 case, these don't seem to spread and influence the whole  
> deep ocean, and the model is not filling up with sub-zero temps.  
> (ergo I tossed the 4x4x22 setup until I could solve this problem).
>
> I also have a 4x4x15 case w/o KPP, and it exhibits the problem at  
> one point in the NA, although the SH is again fine.
>
> In common here are strong flows near rough, jaggedy topography.  
> Perhaps there is some issue in how I'm making the topography? (I do  
> fill in "holes", but otherwise I not trying to smooth it out).  
> Perhaps I should simply try to smooth out topography where the  
> problems occur.
>
> My experience with the partial cells was simply that: I spent a  
> week playing with them, and this problem became much, much worse  
> (and I think the MOC also became more jaggedy too), so it was a  
> pretty obvious conclusion that this was exacerbating my problems. I  
> offer no reasons why...
>
> Other issues/ideas: perhaps decreasing the time step would help...  
> although this is a rather costly solution. Moreover, playing with  
> the timestep can be difficult, as you then have to worry about the  
> ratio of deltaTmom and deltaTtracer, among other things that might  
> be tempted to blow up (in addition to technical issues between  
> atmos, ocean, and coupling timesteps). But perhaps my issues with  
> partial cells help point the finger toward the time step? (My time  
> steps are 14400/480 for 2.5x2, 28800/900 for 4x4)
>
> FWIW, I did turn off temp and salt advection, and the cold anomaly  
> diffuses away into the background on the order of months.
>
>
> Jeff