Particle collisions are not supported at the moment. Though, you can achieve this for terrains. You can take the height information of the terrain and its position and calculate for the current x/y position of your particle the predictable collision position on the z-axis.
More generally, you can achieve for all weather effects like snow, rain or else something like this if you calculate for each x/y position the collision z position from high above. This can be achieved through simply using a trace or through rendering a depth map and use the depth information to retrieve the z collision position.
There are a lot of other approaches, each one more or less efficient for each situation and environment.
