Defining pressure field function
 

Hey everyone,
I'm trying to simulate water jet break up under the influence of air using VOF. I've simulated it as one region, and the pressure increases from top to bottom under the influence of gravity, so the jet isn't propagating once it leaves the orifice as it's going from a point of low pressure to a point of high pressure.
I was wondering if I should consider modeling it as 2 regions with an interface at the boundary, or if I should define the pressure as a field function.
If field function is a better option, then could someone please help me out with defining it?
It's logic would be:
(101325 Pa)+(density of water*9.81*($$Position[2]) up to the distance of the water tank and orifice length.
Beyond this point, the pressure becomes atmospheric pressure again, and increases as
(101325 Pa)+(density of air*9.81*($$Position[2]) up to the end of the air column.
Any help would be highly appreciated.