Studies of Police Officer involved shootings have disclosed that in a SIGNIFICANT number of cases, the officer who discharged his/her firearm were unsure as to the number of rounds that were fired from their weapons. The officers tended to underestimate the number of rounds fired and could NOT (in many cases) reconstruct the various phases of the incident when it came to discharge of rounds. So much for 'knowing' where every round is going to end up (if you knew where your round is aimed, you could remember how many rounds were fired and be able to reconstruct the incident).
As far as a 'warning shot', there ain't no such animal anymore. A round fired into the air has to come down somewhere ... usually in a crowded neighborhood.
The only requirement is to identify yourself as a policeman.