Potential inconsistency between displacement height and reference height from ground

[cenlinhe]

Discussed in #159

^{Originally posted by hjgim November 21, 2024}
Displacement height (ZeroPlaneDispSfc) for a vegetation tile is determined as 65% of the canopy top height using the following expression:

ZeroPlaneDispSfc = 0.65 * HeightCanopyTop

noahmp/src/GroundRoughnessPropertyMod.F90

Line 63 in fa2d209

ZeroPlaneDispSfc = 0.65 * HeightCanopyTop

The reference height from the ground (RefHeightAboveGrd) is then calculated using:

RefHeightAboveGrd = max(ZeroPlaneDispSfc, HeightCanopyTop) + RefHeightAboveSfc

noahmp/src/GroundRoughnessPropertyMod.F90

Line 79 in fa2d209

RefHeightAboveGrd = max(ZeroPlaneDispSfc, HeightCanopyTop) + RefHeightAboveSfc

Here, since ZeroPlaneDispSfc is defined as 65% of HeightCanopyTop, the expression max(ZeroPlaneDispSfc, HeightCanopyTop) will always be equal to HeightCanopyTop when there is no snow layer. Thus, the above formula can be simplified as follows in snow-free conditions:

RefHeightAboveGrd = HeightCanopyTop + RefHeightAboveSfc

Issue for Discussion
As I understand it, if we presume the bottom end of the wind log-profile is at the displacement height, the above formula needs to be modified as follows:

RefHeightAboveGrd = ZeroPlaneDispSfc + RefHeightAboveSfc

Could you please review this potential inconsistency and proposed code modification, and share your feedback?