Depends if there is a ceiling then it would be virtually impossible since to get the arch low enough the arrow would have to have very little velocity thus not be effective against the target.
Assuming its outside, so not impossible I would probably just give at least a +10 AC bonus to the target. Reason being the target is much smaller when viewed from above, and can dodge in all directions. If your firing in a straight line it really only matters if the target moves left or right, with such a high angle any movement forward or backward by the target would lead you to miss. Also the arrow is up in the air for much longer, so unless the target was stationary it would be virtually impossible to hit them, wind speed would also throw the arrow off course.
Perhaps +10 AC (to reflect how hard it is just to get it to land where you expect) and a 50% miss chance (to take account of unknown things such as wind speed at high altitude and the target moving while the arrow is aloft) even if it lands on target.