Create your own rain simulation in Cycles with this .blend file by jonhickley.
Here is my setup for Rain in Cycles. These blend files do not contain the dynamic paint files (Too big to share) which are needed for the floor texture. You will need to generate these by going into "Dynamic paint" within the "Physics" tab and then select "Bake Image Sequence" for both the paintmaps and the waves. Everything should be setup so all you need to do is Bake those two seperate image sequences. You possibly might have to re link the image sequences in the floor texture node. If you would like a more detailed explantion on how I set up the Dynamic paint maps follow this tutorial by Andrew Price, I just followed this and adapted it for Cycles.
I hope you find this useful and if you make any improvemnets I would love to see them.