If a vessel containing a fluid of density ρ upto height h is accelerated vertically downwards with acceleration ao then the pressure by fluid at the bottom of vessel is: (Atmosphere pressure = P0)
The vessel is non-inertial frame of reference. so upward inertia force acts on water in the vessel.
P - P0 = ρh(g - a0) ⇒ P = P0 + ρh(g - a0)