You can't detect the "intensity" of rain or snow in FSX - it's purely a visual effect "cone" around the airplane. This is the same reason you can't do a realistic weather radar with what FSX gives you.

 

I'm really not sure how you could ever realistically simulate the effects of water and ice on the runway by just modulating the brakes or whatever's being implied by those other addons - rain and ice don't just make the plane harder to slow down - there's other forces at play there that you can't simulate without actually lowering the coefficient of friction and all of that.

Grabbing my popcorn...