Roughness length#
Roughness length is an important parameter in the evaluation of vertical wind profile.
where :
\(u_{*}\) is friction velocity.
\(\kappa\) is Von Kármán constant (~0.4).
\(z_0\) is roughness length.
\(L\) is the Monin-Obukhov length.
On standard condition (1) becomes :
Deriving (2) we obtain the log wind profile to derive mean wind speed knowing the roughness length & wind speed at \(z_1\).
Roughness length evaluation#
The surface roughness length over land depends on the surface cover and land use and is often difficult to estimate.
Class index |
Short terrain description |
z0 (m) |
---|---|---|
1 |
Open sea, fetch at least 5 km |
0.0002 |
2 |
Mud flats, snow; no vegetation, no obstacles |
0.005 |
3 |
Open flat terrain; grass, few isolated obstacles |
0.03 |
4 |
Low crops; occasional large obstacles, x/H > 20 |
0.10 |
5 |
High crops; scattered obstacles, 15 < x/H < 20 |
0.25 |
6 |
Parkland, bushes; numerous obstacles, x/H ≈ 10 |
0.5 |
7 |
Regular large obstacle coverage (suburb, forest) |
1.0 |
8 |
City centre with high- and low-rise buildings |
≥ 2 |
Todo
Roughness length user guide under construction.