While this is not done in real time, this is a particle system I have been
working on for some time. It contains 4 distinct partcile systems and over
3000 particles per frame.
While this is not an application but simply graphics, I wanted to share it
with the rest, as it was quite an undertaking. I wrote the shader in
Renderman's script and wrote the tornado script in 3dsmax's script. I can
select a path and apply my script. The tornado will start on one side of
the path and dissipate on the other. The funnel is fully animated and
twists on its own via a noise modifier.
I have yet to add the debris element, but that is next, allowing random
meshes to be used as debris within the tornado.