[MITgcm-support] Re: MITgcm files

[Renske Gelderloos]

Thanks, it works!

Renske

Martin Losch wrote:
> Renske,
>
> the model horizontal domain is (1:nx,1:ny), but if you want to 
> evaluate horizontal gradients along the boundaries you need at least 
> one more grid point to do so. (otherwised you'd have to include code 
> such as "if (1<i<nx) do something in the interior, else do something 
> different at the boundary"), the second grid point you need for second 
> derivatives for the laplacian (harmonic) viscosity and diffusivity. 
> Olx/y>2 are needed for special advection schemes, etc.
> So, yes the overlap ensures the exchange between tiles, but it's also 
> necessary on only one tile in order to get the computational stencil 
> along the domain boundaries right. Plus I don't know what happens with 
> the "exchange" routines (which are always called regardless of domain 
> decomposition, just to get potentially cyclic boundary conditions 
> right) if they exchange stuff of length zero
>
> I am pretty sure that your model crashes because of CFL-violations: 
> Either your time step is too large for u*deltaT/deltax < (1,1/2, ...) 
> or your viscosity is too large for the time step. Try smaller 
> timesteps, for your resolution of 7.5km I recommend something like 
> deltaT = 600s for a start.
>
> Martin
>
> On 26 May 2008, at 15:27, Renske Gelderloos wrote:
>
>> Hi Martin,
>>
>> Thanks for the advice.  The part about the overlap in SIZE.h  I don't 
>> really understand though.  Isn't that the amount of grid cells that 
>> overlap between different tiles? So if you would use only one tile, 
>> it doesn't make much sense to me to prescribe overlap. Or is it not 
>> allowed to use only one tile? Furthermore, on my computer the model 
>> crashes with nonzero overlap.. ("solution is heading out of bounds 
>> (...), mon_solution: stopping calculation, mon_solution: stopped due 
>> to extreme values of solution"). So what is the idea behind the tiles 
>> and the overlap?
>>
>> Renske
>>
>> Martin Losch wrote:
>>> Hi Renske,
>>>
>>> I am returning your files with my modficiations, but all of these 
>>> are related to general stability of the code. I could not reproduce 
>>> the problem that you described, namely that a non-zero heat flux 
>>> does not change the surface temperature. For testing, try a 
>>> non-uniform heat flux that introduces a horizontal pressure gradient 
>>> and immediate flow. With a uniform heat flux and uniform initial 
>>> conditions (in the horizontal) and a stable water column you should 
>>> never get any flow. (as a side remark: I did get non-zero velocities 
>>> after the second timestep, which should not happen, but that's not 
>>> your problem. Is that a problem with step topography, and vertical 
>>> diffusion? very worrying).
>>>
>>> a couple of problems:
>>> SIZE.h: you set the overlaps to 0, that will probably not work ( and 
>>> did not work for me, the model blows up immediately, I am surprised 
>>> that we don't have a catch for that, maybe it is allowed?). I 
>>> changed them to 4 (but 2 should be enough).
>>>
>>> data:
>>> - deltaT = 86400 is much too large a time step and will also lead to 
>>> the model blowing up immediately (once there is motion), but you 
>>> probably knew that
>>> - I put sref = 15*0.0, (although some compiler will probably take 
>>> sref=0.0 as well)
>>> - diffKzT = 20., is awfully large if implicitVerticalDiffusion is 
>>> not turned on (default). I set it to 1.e-5 for now.
>>>
>>> topography:
>>> - I usually try to set a minimum depth which should be at least the 
>>> thickness of the top-most grid cell, can be set in "data" by hFacMinDz
>>>
>>>
>>>
>>>
>>> On 23 May 2008, at 17:50, Gelderloos, Renske (KNMI) wrote:
>>>
>>>> Dear Martin,
>>>>
>>>> Here are all the files you requested. I would really appreciate it 
>>>> if you would want to take a look at it, I'm pretty stuck... Hope 
>>>> you can help me!
>>>>
>>>> Renske
>>>>
>>>> ________________________________
>>>>
>>>> From: Gelderloos, Renske (KNMI)
>>>> Sent: Fri 5/23/2008 2:09 PM
>>>> To: Gelderloos, Renske (KNMI)
>>>> Subject: MITgcm files
>>>>
>>>>
>>>>
>>>> <data><data.mnc><data.pkg><eedata><Qnetuniform.bin><topogOBCmatlab.bin><packages.conf><SIZE.h><LabradorOBC.m><heatuniform.m> 
>>>>
>>> ------------------------------------------------------------------------ 
>>>
>>>
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