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

Tue Mar 11 19:44:37 EDT 2008

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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