[MITgcm-support] Temperature drift in tutorial ( tutorial_global_oce_latlon: Global Ocean Simulation at 4◦ Resolution).

[Krishnakumar Rajagopalan]

Hi All,
I have an update on the temperature drift in tutorial. 
In the write up of the Global Ocean Simulation at 4 deg resolution (in the .pdf 
manual), 

the horizontal diffusion coefficient for salt and theta, diffKh  is set at  
1.E3. 

But, the tutorial is configured to use the GMREDI package, thus diffKh has to be 
zero. Note that the \input\data  of this tutorial
is correct with diffKh=0 in the input file.
I think it would be good if the manual (.pdf document) is  revised. I followed 
the manual and did many runs with diffKh=1e3 and GMREDI package, only realizing
the mistake after reading  the writeup on \tutorial_cfc_offline where it is 
spelled out that
when GMREDI package is used, diffKh must be zero.

I think when we set diffKh to some value, and also use GMREDI package, the 
diffusive flux,
df gets calculated once outside the GMREDI_XTRANSPORT subroutine, before being 
incremented
within GMREDI_XTRANSPORT. A configuration check to tell users to ensure diffKh=0 
when
using GMREDI scheme would be great.

 The attached figure (a) shows two cases where GMREDI package is used. In case 
1, diffkH
for salt and theta is set as 1e3. In case 2 diffKh=0. Thus case 2 is the correct 
setting.
The results show that the overshoot of theta is larger in case 1.
Danabasoglu (Ocean Modelling 7 (2004); 323-341; Fig1) showed that the mean theta
increases to about 4.25 degrees when the tracer and momentum time step are 
different and stays constant
thereafter. However, in my case (case 2), the mean theta is seen to reduce after 
reacing a maximum of about
4.15 deg. Any ideas on why this is happening is most welcome. ( I am yet to run 
cases where the
tracer and momentum time steps are equal)
 

Thanks in advance

Krishnakumar




________________________________
From: Krishnakumar Rajagopalan <krishna_raj_2010 at yahoo.com>
To: mitgcm-support at mitgcm.org
Sent: Tue, March 15, 2011 7:03:32 PM
Subject: Re: [MITgcm-support] Temperature drift in tutorial ( 
tutorial_global_oce_latlon: Global Ocean Simulation at 4◦ Resolution).


Hi Martin,

Thanks a lot for your reply and pointing to the reference.

Best regards

Krishnakumar




________________________________
From: Martin Losch <Martin.Losch at awi.de>
To: mitgcm-support at mitgcm.org
Sent: Mon, March 14, 2011 9:31:22 PM
Subject: Re: [MITgcm-support] Temperature drift in tutorial ( 
tutorial_global_oce_latlon: Global Ocean Simulation at 4◦ Resolution).

Hi Krishnakumar,

This behavior is expected: The tutorial_global_oce_latlon is forced by a hea 
flux that is globally balanced (should lead to no net temperature drift), AND a 
restoring term. This restoring term ( -(lev_sst.bin - theta)/tauThetaClimRelax ) 
causes the drift until the model has reached an equilibrium with this forcing 
(the same is true for salinity: fresh water flux plus restoring to sss). This 
can take very long, see e.g.
Gokhan Danabasoglu , Ocean Modelling 7 (2004) 323–341, A comparison of global 
ocean general circulation model solutions obtained with synchronous and 
accelerated integration methods
where the spin up takes up to 10,000 years. That's just the way it is.

If you want to get rid of the mean temperature trend then you have get rid of 
the restoring terms, but that will most likely give you a strange circulation.

Martin


On Mar 14, 2011, at 8:01 PM, Krishnakumar Rajagopalan wrote:

> Hi All,
> 
> 
> 
> We ran the tutorial (tutorial_global_oce_latlon) for a thousand years and find 
>that there is a drift in the temperature.  See the attached figure which shows, 
>for example,  the temperature at longitude=158deg and latitude= 2deg, for some 
>depths;  the temperature at 3010m fails to reach a steady state even after 
>1000yrs. Although the annual integral of Qnet equals zero, the annual integral 
>of heat flux flowing into the computational domain due the relaxation term is 
>not. Could this be a reason for the drift in temperature? Is there any 
>setting/method to reduce this (drift in temperature).  Our study also shows that 
>the temperature drift persists even on a higher resolution grid.
> 
> 
> 
> Thanks in advance
> 
> 
> 
> Krishnakumar
> 
> 
> 
> (University of Hawaii at Manoa)
> 
> 
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