Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 489

Table 8-3.
Surface Roughness Parameters for Various Types of Terrain

Type of terrain Typical surface roughness length, z o (m)

Urban and suburban areas 3.0 to 0.4

— cities with very tall buildings 3.0

— cities and large towns 1.2

— small towns 0.55

— outskirts of towns 0.40

Woodlands and forest 1.2 to 0.4

Farmland and grassy plains 0.30 to 0.002

— many trees and hedges, and a few
buildings

0.30

— scattered trees and hedges 0.15

— many hedges 0.085

— few trees; summer 0.055

— crops; tall grass 0.050

— isolated trees 0.025

— uncut short grass 0.020

— few trees; winter 0.010

— cut grass 0.007

— snow covered cultivated farmland 0.002

Large expanses of water 0.001 to 0.0001

— coastal areas with off-sea winds 0.001

— calm open sea 0.0001

Flat desert .0001 to 0.0001

Snow-covered flat ground 0.0001

Mud flats; ice 0.00003 to 0.00001

of atmospheric mixing which are characteristic of each. A review of the literature through
1980 on the relation of atmospheric stability to the siting of wind turbines is given in [Frost
and Shieh 1981].
The concept of atmospheric stability is best illustrated by considering the upward
displacement of a small element of air to an altitude with a lower ambient pressure.
Assuming a rapid displacement, there will be no time to lose or gain heat and the element
will expand adiabatically. If the expanded element is less dense than the surrounding air,
it will continue to rise due to buoyancy and will not return to its original location. This
unstable atmospheric state is characterized by significant mixing that tends to decrease
vertical gradients of wind speed. If our expanded element of air has the same density as
the air at its new elevation it will not move further, and the atmosphere is termed neutrally
stable. There is little mixing in a neutrally-stable atmosphere, and vertical gradients in wind
speed tend to remain constant. Finally, if the element is more dense than its new
surroundings it will sink back to its original location. This stable atmospheric state is also
characterized by very little mixing between layers at different altitudes, so vertical gradients
tend to be larger.