How to create this effect?

You could do this with one of the native particle systems like CC Particle Systems II and use one of the textured options to use your own layer. 

You could also do with with the Shape Layer Operators like this:

1) Have one instance of your particle in a Shape Layer

2) Add the Repeater Operator

3) Add the Wiggle Transform Operator

4) Adjust your Repeater Copies

5) Keyframe your Repeater Position to go on up the y-axis

6) Adjust your Wiggle Transform parameters to get your desired results.

The Shape Layer method will be very quick and the particle system method will give you more control, but possibly much more than you need.