[MITgcm-support] nondivergent flow advecting a tracer

[钱钰坤]

Hi all,


I am using a series of geostrophic velocity data (derived from AVISO sea surface height)
to advect a passive tracer in an offline mode.  Following the paper by Abernathey and Marshall
(2013, JGR), first I made a nondivergent correction to the original geostrophic velocity data
through Helmholtz Decomposition.  Then I used the nondivergent velocity to advect a tracer.
However, after a few timesteps the tracer field is unstable and generates very large values and
eventually contaminated the results.


I think there are two things need to be verified.


First, the correction is done correctly.  Actually
I followed the guidance by Ryan Abernathey at:
    https://github.com/rabernat/mitgcm_2D_global/blob/master/notebooks/correction.ipynb
I run the offline model first using original geostrophic velocity to dump the diagnostics of streamfunction
(psi) and velocity potential (phi).  Then I use the xmitgcm utility to calculate the nondivergent velocity as:
    u_psi = -(numpy.roll(psi, -1, axis=-2) - psi) / dyG / drF
    v_psi =  (numpy.roll(psi, -1, axis=-1) - psi) / dxG / drF

This code above should be modified somewhat if one uses the latest xmitgcm released a few days ago.
I did not do any mask since I think the MITgcm model will mask the velocity when run in offline mode (
I provided a bathymetry data to the model generated based on AVISO SSH land-sea mask).


Second, the offline model parameters (delta-t related) are correctly set.  Although I've read some emails
on this issue, I'm still not so sure how to set these parameters.  The following are my choices:
    data:      deltaTtracer = 86400,
                  deltaTClock = 86400,
    data.off:  deltaToffline = 3600,      (smaller dt for accuracy)
                  offlineForcingPeriod = 86400,    (AVISO data is daily mean)
                  offlineForcingCycle = 0,     (no need to reuse the velocity)
When using the above params, the offline model asks for velocity data whose names are: u.0.data, u.24.data,
u.48.data...  So I rename the velocity data into this convention.


Now I am really confused why the tracer overflows after only 4-5 timesteps (days).  Is there anything wrong
with the above two procedures?


------------------
 Best regards 
 
Yu-Kun Qian (钱钰坤) 
Center for Monsoon and Environment Research 
 Department of Atmospheric Sciences
School of Environmental Science and Engineering 
 Sun Yat-sen University 
No. 135 Xingang West Road, Haizhu District 
Guangzhou, 510275, P.R. China 
Tel; 020-84115227 
Email: qianyk at mail3.sysu.edu.cn
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