I was doodeling in After Effects and came up with a pritty good Rain system. 100% Particular. With a little optical flares to make it pritty.

The setup is actualy realy simple.

The scene i made up from 2 solid layers called Ground & Particular.
The ground plane should be 3d and rotated to face up (Like the gound in your scene.

Particular settings:
Emitter Type: Box'
scale it so it fits the area you want to "Rain" on, and position in out of frame (So you dont see the rain drops spawning)

The rain drops motion is a mix between velocity and gravity (Play around to find the sweet spot). also i turn off direction spread, otherwise it looks like snow.

Under Particles/Sec set the number to 100-1500 depending on the amount of rain you want.

Under the "Physics" tab choose "Bounce" and under the "Bouce" tab specify the Floor layer (Your 3d solid)
Also set Collision Event to "Kill"

Under Aux System:
Emit: At Bouce Event
Probability: 100%
Particle/Collision: 10 (This can be varied to find a sweetspot between realism and rendertime)

Set velocity to around 150 (May vary depending on your scene)

Opacity over life: Sloaps down from "full" to "none".

Color from Main: 100%

Lastly turn on motion blur.

That is it, now it is just a matter of playing around with the numbers a bit to find an overall sweet spot.

Side note:
This is a pritty good solution but there may be a much simpler way to get a similar effect. Try looking at the filters called CC Rain and CC Drizzle. These are much more render time efficient and may do the trick for the majority of your shots.

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