I am a little perplexed by describing this as novel. Mortar fire by nature is high elevation indirect fire. The value of mortars for slowing besiegers was recognized from the beginning of the war. Lack of mortars (and/or ammo) was considered a factor in the fall of several CSA fortifications to sieges.

Porter's mortar boats fired about as indirectly as one could on the forts below New Orleans, using a screen of trees and pre-determined staging points for the bombardment. While the bombardment did not force capitulation or destroy the defenses, it did demoralize one garrison enough (in combination with the running of the forts) that the garrison mutinied.