I'm not sure of any tutorials online, but that subject is covered in the help file, in Tutorial 4, under the subject Area Emitter Masks.
Here's an emitter that does the raindrop explosions. You may want to lower the Life of the Droplets particle type down to make it more realistic.
http://pi.shockingtales.com/ipf/rainStrikes01.zip
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Elvis Deane!
The particleIllusion FAQ
particleIllusion Resources and tutorial CD
Astounding Adventures