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 Post subject: ROMS 2-way nesting
PostPosted: Tue Jun 26, 2018 4:31 pm 
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Joined: Tue Feb 12, 2013 2:54 pm
Posts: 20
Location: Bangor University
I'm having some issues with a refined nesting application blow up and wonder if anyone could advise on a working method of grid creation using Matlab? Particularly regarding the correct procedure for setting the land mask in a refinement case? I've tried a few different methods each with low levels of success..

Some background:
I have an outer model with an inner nested grid and there are land points in both grid.

So far I have followed the following steps:

1. Create high resolution outer grid using the ROMS_TOOLS make_grid.m script and edited the mask: (dl of hres outer == inner grid dl).

2. Run fine2coarse.m with a refinement factor of 5 to reduce the resolution of the outer grid which creates my parent grid.

- I've then gone back into the file to check the bathymetry smoothing and mask.

3. Run coarse2fine.m with a refinement factor of 5 for a subset of the parent grid to create the child grid.

4. Run c_contact.m and contact.m to make contact file.

The above steps are something I've come up with from various posts on this forum, but it's possible that I'm getting muddled. Can someone point me in the right direction please?

If it helps the blow up is occurring after 2 timesteps and the error message is as follows:

2 2014-07-20 00:00:08.00 NaN NaN NaN NaN 02
(000,000,00) 0.000000E+00 0.000000E+00 0.000000E+00 NaN
Found Error: 01 Line: 298 Source: ROMS/Nonlinear/main3d.F
Found Error: 01 Line: 298 Source: ROMS/Drivers/nl_ocean.h

Blowing-up: Saving latest model state into RESTART file


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ROMS-blowup.txt [62 KiB]
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 Post subject: Re: ROMS 2-way nesting
PostPosted: Wed Jun 27, 2018 9:43 am 
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Joined: Mon Apr 28, 2003 5:44 pm
Posts: 490
Location: Rutgers University
Does the parent grid run on its own (without any nesting)?

You have open boundary conditions (Chapman/Flather/gradient) for velocities but closed for tracers. That might give you stability problems at the boundary.

You also have explicit horizontal mixing on tracers in s-coordinates (MIX_S_TS) rather than geopotential coordinates (MIX_GEO_TS) which could give appreciable vertical mixing if you have steep bathymetry.

I also notice one odd thing with the grid 02 metrics:

Code:
Metrics information for Grid 02:
 ===============================
...
 Minimum Z-grid spacing, DZmin =  0.00000000E+00 m


You have a DZ of zero somewhere. Not sure how you managed that.

You should check your vertical coordinates are sensible. The vertical coordinate stretching values you use of theta_s = 0 and theta_b = 0 are unconventional.

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 Post subject: Re: ROMS 2-way nesting
PostPosted: Fri Jun 29, 2018 3:01 pm 
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Joined: Tue Feb 12, 2013 2:54 pm
Posts: 20
Location: Bangor University
Thank you for your response -

Yes, the parent grid runs fine without nesting, I've switched to MIX_GEO_TS but I'm still getting a blow up. I'm not sure why I'm getting a DZ of 0.0 in the nested grid, I've tried a run with constant bathymetry everywhere and I get the same problem. I did discover some strange values in my masks which I've now ammended but it's still blowing up at the same time step. Now, regardless of whether the model has constant or variable bathymetry, the blow up is occurring at the eastern boundary of the child grid, which is an ocean only boundary. I'll play around with the sigma layers and keep fiddling with a few other things.


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 Post subject: Re: ROMS 2-way nesting
PostPosted: Sun Jul 15, 2018 6:35 am 
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Joined: Thu Sep 29, 2016 4:03 pm
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Location: Nanjing University of Information Sci. &Technology
i met the similar problem. parent grid's resolution is 1/24°, and child resolution is 1/72°. Could you show you why it didn't work and its solution?
thx!


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 Post subject: Re: ROMS 2-way nesting
PostPosted: Thu Aug 16, 2018 7:21 pm 
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Joined: Tue Feb 12, 2013 2:54 pm
Posts: 20
Location: Bangor University
The previous problem has been solved - I had neglected to include x_rho and y_rho etc in parent and child grid files.

I now have a new problem - 18 hours into the model run I get a blow up. I've restarted and looked into the problem in more detail but I'm still at a bit of a loss. I seem to get erroneous temperature and salinity values - my intention at this stage was to keep these values constant, but there is still an instability occuring. The same blow up occurs if I use a flat bathymetry and a generated bathymetry.

Any thoughts on why temp/salinity instabilities may occur? The parent grid runs to completion.


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roms_nest_bu.txt [50.35 KiB]
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 Post subject: Re: ROMS 2-way nesting
PostPosted: Tue Jul 02, 2019 7:41 am 
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Joined: Fri Dec 15, 2017 6:07 pm
Posts: 16
Dear osua72
Was you able to resolve the blowup? Could you please state out what was wrong with the setup.


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 Post subject: Re: ROMS 2-way nesting
PostPosted: Tue Jul 02, 2019 7:45 am 
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Joined: Tue Feb 12, 2013 2:54 pm
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Location: Bangor University
Unfortunately, I've not been able to solve the problem. I suspect it's due to boundary placement and bathymetry causing instabilities but as yet I'm still unable to run the model past spin up without it blowing up.


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 Post subject: Re: ROMS 2-way nesting
PostPosted: Tue Jul 02, 2019 8:12 am 
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Joined: Fri Dec 15, 2017 6:07 pm
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Dear osua72
I have also met up with such instable values in salinity during my two way nesting and for one way it is almost fine. Would it be possible for you to check the configuration with one way nesting defined instead of two way?


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