Mike made a video tutorial to do just what you are looking for here:
Also, check the Maya help docs for:
Particle Instancer and Collision Events.
Use the particle instancer to replace your "white particles" with a "raindrop-looking" geometry shape, and setup a collision event with your particleShape to your ground geometry, so when your raindops hit the floor, they will die, and emit new particle-streaks to simulate a small "splash"
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS