Looks somewhat similar to the roughly 10 year old "blood shader" samplefrom the time when "pixel shader 2.0" was considered the newest coolest thing, except of course raindrops aren't blood, i.e. they're colorless, transparent and refracting, less viscuous. And nowadays, the texture would likely be quite a bit larger (probably screen-sized).
Thank you all for your replies. Actually that blood algorithm is better than it seems. If I modify a bit the physics to take not only into account the gravity but also other forces like the wind and I store the resultant force instead of storing only the gravitational force I think I can get something quite good.