A basic rain impact effect spawning on a sampled Skeletal Mesh, will only spawn on a given mesh directional normal.

For example, if you set your directional normal to 0,0,1 and set the tightness to 0.75, particles should only spawn on faces that are pointing up in the Z axis or close to that given direction.

Contains:

Unreal Engine Version: UEFN, 5.3, 5.4
Effect Type: CPU, GPU
Master Materials: 1
Material Instances: 1
Textures: 1

Parameters:

Impacts
Impacts – Colour, Impacts – Emissive Boost, Impacts – Alpha Max, Impacts – Alpha Min, Impacts – Kill Box Enabled?, Impacts – Kill Box Size, Impacts – Directional Normal, Impacts – Normal Offset, Impacts – Normalized Directional Tightness, Impacts – Size Max, Impacts – Size Min, Impacts – Skeletal Mesh, Impacts – Spawn Rate,
 

Optimization
Impacts – Max Spawn Distance

Component Assets:

Material: VFX Sprite Basic
Texture: Rain Impacts 01 
Module: Kill By Normal

Source Files:

Substance Designer File: Rain Impacts 01