First off, you all are making this WAY too complex! There are rules covering *almost* everything being discussed, and you are all ignoring those rules for sake of physics, which has no real place in D&D!
First off, this is a bombing mission, not a ranged attack mission, whether you're attacking a person or an object is thus irrelevant, as are rules for ranged attacks. You must use only rules for dropped objects for this. Ranged attacks can only go five increments at 10 feet each with improvised weapons, but things can be dropped from an UNLIMITED height.
First things first; DROP THE PHYSICS and FORGET ABOUT WIND. Geez . . . Wind would have no appreciable effect on bricks anyway!
Getting to the point . . . As a bombing run, I would say dropping something straight down on a colossal object (broad side of barn is considered colossal) would AUTOMATICALLY HIT. Remember, though, you MUST be able to see your target, which would be impossible from 400 feet in anything but ideal light conditions. (DMG, pg. 60) Next, each brick is a seperate attack, so damage is rated BY THE BRICK. A brick would way about 5 pounds (I'm feeling generous), and assuming you have ideal light conditions, dropping a SINGLE brick from 400 feet (5 increments of falling) would do 5d6 points of damage. Wood has a hardness of 5, so 5 points are taken off from EACH brick. (DMG, pg. 89, for all of the above)
As for hitting a person, that would be nearly impossible with dropping objects, and I would rule as such. If within five ranged attack increments (50 feet), however, I would allow the player to THROW the bricks down at the target using ranged attack rules. Dropping objects, however, I would not allow AT ALL. There is simply no real chance of hitting, at least not with bricks. An anvil, MAYBE, but not bricks. As for throwing bricks, that -4 for non-proficiency, -4 for improvised weapon, and -2 per range increment against the target's normal AC. (IOW, not worth trying.)
If you REALLY want to put a hole in the roof of the tower, do a KAMIKAZE attack and fly straight down into it. If you weight 200 or more pounds after adding in your encumbrance, you do 1d6 points of damage per 10 feet with a maximum of 20d6 points of damage, and remember to take off 5 for wood hardness. (Again, see DMG, pg. 89.) One terrible side-effect of this tactic, however, is that YOU also take 1d6 damage per 10 feet, with the first 1d6 being subdual (DMG, pg. 113), making a possible Jump check DC 15 to reduce the fall by 10 feet, and making a Tumble check for the same. (However, because it's 400 feet, that likely will NOT reduce damage below 20d6. Hope you're insured.)