Network RAM revisited

Tim Wait waitt at saic.com
Thu May 29 20:42:25 EDT 2003


>> For traditional hydrostatic models on rectilinear meshes, maybe.
>> Non-hydrostatic models on adaptive unstructred meshes are a whole
>> 'nother story. Of course, that means implementing different physics
>> for cells less than about 5-10 km. But certainly doable.
>>
> So by 'adaptive unstructured' you mean the size of the grid cell in the 
> region varies depending on how much data you have?
> lots of data -> smaller cells. So how does one deal with the edges of 
> the cells? Sounds a little bit like the spatial decomposition approach 
> in molecular dynamics.

No, the resolution is refined over areas of interest. For met, this would be
areas with significant pressure gradients, vertical motion etc. The drawback
to higher resolution in atmospheric modelling is that the timesteps are based
on the edge length (to keep it stable) so you get a big computational hit.
The advantage to using adaptive unstructured grids is that you can have
low resolution over regions that don't have any significant processes
occuring, yet refine over areas that do - or retain high res over areas
over a particular region of interest... ie flow over a specific area.
Another big plus with atmospheric modelling is that you can easilly
do global runs with local high res and avoid boundary condition discontinuities.
See http://vortex.atgteam.com for more info. The gloabl grid on there
at present is from before we had global adaptation working in parallel.
I'll try to get a global daptation case up there tomorrow done for
the OK tornado outbreak a few weeks ago. Mostly dated stuff there, and
it's on a 10 Mb network segment, so be patient. The hurricane floyd run
will give you an idea of what I'm talking about. Higher resolution ->
better track, but don't waste cycles on areas that don't affect the solution
significantly.

The problem with traditional models is that they used fixed rectilinear grids.
They do nesting to get the resolution up, but that wastes a lot of cycles where
nothing is happening. Sure, you can make a nice 1 km nest over a city (Josip)
but that isn't going to help much if the supercell or whatever was formed
outside of that region and advects in.

Tim

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