 Roughness length

The extrapolation height at which a logarithmic wall profile vanishes. For a fully rough wall it is set mainly by the geometry; for a smooth wall it scales with $\nu /u_{\ast }$. It is not the thickness of the molecular viscous sublayer or the literal location of a no-slip surface.

Roughness length, often denoted as \( z_0 \), is a parameter used in boundary layer meteorology and fluid dynamics to characterize the effect of surface roughness on wind flow or other fluid flows near a surface. It is defined as the height above the surface at which the wind speed theoretically becomes zero due to friction from the surface's roughness elements (like buildings, trees, hills, etc.).