You can see on the screenshot that it doesn't rain visibly, in fact !
*shrugs* You can switch rain off in the renderer, it's a particle system basically.
@FoxtroCC: both the sound and the droplets on your windows depend on /environment/rain-norm (usually), which seems to always represent the rain intensity at ground level
That's the issue - the 'seems'. It's a property that originates being read off from a METAR report. If it's set, BW will produce rain and it'll be made invisible above the lowest cloud level, regardless of whether this makes any sense of not. Try weather with 3/8 clouds and rain - it'll not rain beneath clouds only, it'll rain beneath blue sky just as well (in fact try 0/8 coverage and rain...)
Note that conceptually the environment/ properties are local - wind-from-heading-deg doesn't refer to 'some wind somewhere' but to the wind you're currently experiencing. Pressure doesn't refer to the pressure-InHg 10.000 ft higher but to pressure at your location. So... conceptually rain-norm actually refers to the rain level at your position just as well.
In fact that's how AW treats it, because it creates a box underneath a raining cloud and tells you it's raining in that region - it stops when you leave the region.
So what you're seeing it that BW uses a purely visual trick to make the particle system rain invisible, but tells the environment manager that it continues to rain (rain-norm or snow -norm are often also read out by aircraft systems if something gets wet which should not or if friction coefficients change with rainwater or spray - so the issue actually is with BW not announcing to the environment system that the rain is no longer falling (as I said), not with rain-norm magically referring to the ground level.