zGround in ABLconditions,underground case

grasshopper butterfly
1

I use Butterfly to generate a special case, the simulation domain is combined by the sunken space and the space above it.

P1
P11009×508 6.26 KB
P2
P2857×293 3.43 KB

So it is different from the normal outdoor case(see picture3)
P3
P3894×273 1.83 KB

for example, my open sunken space is 4m below the ground, BF will generate an ABLcondition file with zGround = -4.0 ,I wonder if this value is correct to generate the ABLinlet I want, Do I need to set it to 0?
p4
I know we use atmBoundaryLayerInletVelocity as inlet boundary.https://www.openfoam.com/documentation/guides/latest/doc/guide-bcs-inlet-atm-atmBoundaryLayer.html

I would like to know what zGround means and how it is substituted into the formula to calculate the inlet wind speed

Thanks to anyone who noticed this question