Interesting application. I made a few tweaks to the parameters in your definition to get it to accumulate.

This got me thinking about particle emitters again, since it is obviously limiting us here that all the particles which are going to fall have to exist above the scene from the start, and most of them end up flying far away.

Creating particle emitters in a script should be easy enough, but seems it would be difficult to make it work as a standard goal component, because of the way the indexing works - if we want to apply other goals to the emitted particles, that is hard to set up, since they do not exist at the start of the simulation.

One solution that occurred to me would be to have a goal which collects all the particles which go beyond some threshold, and then re-emits them from some source location. That way all the forces like gravity and wind would still be effective on the 'new' particles.

Ramus & Daniel

Is it possible to count the shocks between the particles and the breps?
If so, it could be used for people to escape in urban or confined places

i found a nice plugin called eve rain.

do check it out, it gives you the flow trains on the surface / mesh too.