ahh, a few errors in my post (edited now) x-particles easy, TP is possible. In that example (it’s way easier when there is an example of what you are trying for)
– as you say there is no falling rain there, the rain just spawns on the surface and I suspect there is a separate emitter for the drops that leave the surface…
– I think the trails in the example wander too much for a car at speed (for whatever that’s worth)
– the trail of spawned particles from each “parent particle” (the blue ones) looks fine as particles but as metaballs would be really tough to trail off in size and how do you texture particles to look like rain (I don’t know a way but it would be great to know one…)
– it’s a lot easier to do this in x-particles so I don’t have great idea how to do it other than it would likely require TP Move Over Surface (unless you are practiced enough in TP/xpresso to do it without the presets.