Engine has particle system, so, in theory, by meddling with game objects, you can create an entity which spawns rain particles in the area around it.
where is the particles system? only particles i know of are blood and sparks.
although now that i think of it, it might be possible with finite state machine to create an invisible enemy with a custom blood sprite as rain but it would be super jank.
other thing i was thinking might work is create a shader and activate it by script.