[MITgcm-support] nondivergent flow advecting a tracer

[钱钰坤]

Hi Edward,


Thanks very much for your reminding.  After searching for the 'cfl' stuff in STDOUT, I found indeed
that the CFL condition is violated by using deltaTtracer=86400.  So I need to reduce this parameter.


I am now confused that how does this parameter (and deltaTClock) in data file relate to my daily
velocity data and deltaToffline=3600 set in data.off?  Can this parameter be adjusted at will without
any constraint in my OffLine Run?

------------------
 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      


 
 
 
------------------ 原始邮件 ------------------
发件人: "Edward W Doddridge"<ewd at mit.edu>;
发送时间: 2019年5月30日(星期四) 晚上10:06
收件人: "mitgcm-support at mitgcm.org"<mitgcm-support at mitgcm.org>; 

主题: Re: [MITgcm-support] nondivergent flow advecting a tracer

 

 Hi Yu-Kun, 
 
 I suspect that your timestep violates the cfl stability criterion. You should check that deltaTtracer*velocity/grid_spacing is less than 1. Ideally you’d like it to be much less than 1.
 
 
 Cheers,
 Ed
 
   
 
 Edward  Doddridge
 Postdoctoral researcher
  Earth,  Atmospheric and Planetary Sciences
  Massachusetts  Institute of Technology
   
 
  www.doddridge.me
 
 
 
  On 30 May 2019, at 09:48, 钱钰坤 <qianyk at mail3.sysu.edu.cn> wrote:
 
  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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