Use the raindrops footage as a model for a bunch of animated masks that would get longer as the raindrops roll down the surface. If they roll down at a consistent speed, you may not have a lot of rotoscoping work to do. Then apply the masks to a solid, which you can then use as a track matte for the graphic.
Then turn off the masks on the raindrop footage, and use one of the blend modes to get the raindrops to appear on the graphic. It may take a couple of duplicated layers and perhaps even using the graphic as a track matte if it contains transparency, but you ought to be able to get the look right.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA