Hey -
I will try to address the issues as you listed the above:
The shadows from clouds and the shadow flickering are both a result of fixed bounds issues. I will try to address them more directly in your other issue, but you can set the fixed bounds to an large amount and this should help some, but shadows will still pop in and out if you are turning the camera to eliminate an emitter.
The “Self Shadowing acne” is a result of too many particles overdrawing on top of one another with too high an opacity value. Adjust you opacity or spawn rate downward and this issue will resolve itself, the test map I posted on your other question shows the acne and how I got read of it.
Bad Performance comes from wanting all your clouds to be self-shadowed and shadowing your entire map, this is costly for particles on such a large scale. I would suggest using a Light Function for the large scale clouds shadowing the ground and only self shadow a base level of spawned clouds and use additional filler clouds with no self shadowing to help fill the sky itself (working with a material change of course).
The Rain and Snow leaking can only be fixed in the large scale by spawning CPU particles and not GPU particles. I know ultimately you are wanting to have a universal weather system but a lot of weather systems in games now are catered to run around certain environments for performance savings, so its not really raining everywhere just where you see and when you go “indoors” it is only raining in a thin sheet just outside the door, which is an old movie and theatre trick as well.
Niagara is something we are working hard to finish, but I still have no further information about it yet. Ray Traced Shadows on terrain is on the list as well.
Thank You
Eric Ketchum