[MITgcm-support] Re : Water at the bottom of NA below freezingpoint
Gus Correa
gus at ldeo.columbia.edu
Thu Mar 13 17:53:51 EDT 2008
Hello Martin, Jeff, and Samar
Thank you very much for your suggestions.
I am not disputing what is causing the cooling, I just want to fix the
problem.
Your help to do this is much appreciated.
Also, it is not necessarily the same problem that Dror and Jeff
reported, I agree,
despite some similarities.
I'll be away this coming week, but I am very interested in your suggestions.
In case you reply to this message, I'll get back to you when I return.
1) I ran again the coupled ocean-mixed layer atmosphere for 50 years,
changing the advection scheme to 3rd. order upwind, using a staggered
time step,
turning off the shaved cells, and replacing useOldFreezing by
allowFreezing=.TRUE.
I still get the cold temperatures.
As Martin guessed, the cold water sinks from the surface.
This happens around Antarctica
(there is cold water at all depths there) and spreads across the ocean
bottom,
regardless of the new advection scheme and the removal of shaved cells.
So, I may need to go back and find what is wrong on the surface
(something that
I've been doing for quite a while).
One possibility is that my surface forcing shortwave dataset is bad
(it is ERBE surface radiation budget, but I already used NCEP
Reanalysis, and ERA40,
and the problems are about the same.)
Another possibility is that the bulk formulas for longwave,
sensible and latent heat are not very good (but they are more or less
standard).
We use Trenberth wind speed on the bulk formulas, and I hope this is an
ok dataset.
I also used other wind products but the problems weren't fixed.
Another possibility is that there is a bug on my code.
I already searched extensively for bugs, found and fixed several, but of
course there may be a bug.
2) I am trying to avoid any artificial heat source.
I just learned from Martin that one of these is "useOldFreezing",
which should be replaced by allowFreezing=.TRUE., which I will adopt
from now on.
However, I presume another artificial heat source is surface temperature
relaxation to
climatological data (i.e. a non-zero value for tauThetaClimRelax).
I don't use surface temperature relaxation in the coupled runs.
I just ran the (non-coupled) test case global_ocean.90x40x15 with and
without surface
temperature relaxation, and the difference in the resulting temperature
field is very large on the
first three or four levels.
I wonder if any of you have done successful coupled runs without surface
temperature relaxation.
(Our goal for the coupled runs is certainly to avoid surface temperature
relaxation.)
3) I would much appreciate if you can advise on what KPP and GM
parameters I should use.
I am using what I could find on a few test cases and some suggestions of
others,
but I am afraid I may be choosing bad values, and producing wrong
vertical and horizontal mixing.
Aside from the atmosphere code itself, this could be yet another cause
of problems, right?
I don't know if the KPP and GM values below are reasonable for my
4deg X 4deg X 24 levels (10m spacing on the top 100m) grid.
A) In the "data" namelist:
viscAr=1.E-3
viscAh=5.E5
diffKhT=0.0
diffKrT=3.E-5
diffKhS=0.0
diffKrS=3.E-5
implicitDiffusion=.TRUE.,
implicitViscosity=.TRUE.,
B) In the data.kpp namelist (this namelist is actually empty)
&KPP_PARM01
&
C) In the data.gmredi namelist:
GM_background_K = 1.e+3,
GM_taper_scheme = 'gkw91',
GM_maxSlope = 1.e-2,
GM_Kmin_horiz = 50.,
GM_Scrit = 4.e-3,
GM_Sd = 1.e-3,
4) Samar's suggestions/questions:
A) I still didn't implement sea ice. I am trying to solve the current
problem before I venture into more coding,
and to keep it simple, as you suggested.
B) My mixed layer atmosphere version uses exactly the same T grid of MITgcm.
The grid cell areas are the same.
So I don't need to do spatial interpolation of atmospheric fluxes that
are passed to the ocean.
I assume Matlab does a decent job interpolating standard datasets to the
T grid, and this is what I use off line.
5) Jeff's question:
I determined the ocean heat content by integrating it spatially and in
time over a long 1000 year run.
It decreases from ~ 3x10**25 J to ~ 1.5x10**25 J during this period.
Then I saw the inordinate amount of cold water on the ocean bottom.
Since you and Dror reported spurious cold bottom water also,
perhaps caused by the advection scheme or bottom topography,
I thought these two might be the cause of my cold water as well.
However, other than this coincidence, our surface diagnostics were
showing imbalances in global mean Qnet,
suggesting that the problem was on the surface.
Thank you.
Gus Correa
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
>
>
>
>
>
> On Wed, 12 Mar 2008, 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).
>>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support
Samar Khatiwala wrote:
> 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
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>>
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