# Stability Parameter $\zeta$ definition

Let $$L_{obu}$$ denote the Obukhov length, $$z$$ denote the height of measurement and $$h_c$$ denote the canopy(obstacle) height.

I have seen two definitions of the stability parameter:

1. $$\zeta = z/L_{obu}$$

2. define zero-displacement height $$z_d = \frac{2}{3}h_c$$. Then

$$\zeta = (z-z_d)/L_{obu}$$

Questions:

A. Which definition is correct? Can you provide some reference where this is discussed?

B. What are the typical values for $$\zeta$$?