Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 67

Height

Neutral

Stable

Unstable

Temperature

Figure 10: Vertical temperature profi le: changes of temperature with height.

The vertical wind profi le in the surface layer can be described by a number of
simplifi ed assumptions.
The profi le depends, next to roughness and orography, on the vertical tem-
perature profi le, which is also referred to as atmospheric stability. Three general
cases can be categorised (Fig. 10). In neutral conditions, the temperature profi le
is adiabatic meaning that there is equilibrium between cooling/heating and
expansion/contraction with no vertical exchange of heat energy. The tempera-
ture decreases with around 1°C per 100 m in this situation. Neutral conditions
are typical for high wind speeds. The vertical wind profi le depends only on
roughness and orography.
In unstable conditions, the temperature decreases with height faster than in the
neutral case. This is typically the case during summer time where the ground is
heated. As a result of the heating the air close to ground starts rising since the air
density in the higher layers is lower (convective conditions). As a consequence, a
vertical exchange of momentum is established leading to a higher level of turbu-
lence. The vertical wind shear is generally small in these situations due to the
heavy mixing.
In stable conditions, which are typical for winter or night time, the air close to
the ground is cooler than the layers above. The higher air density with increasing
height suppresses all vertical exchange of momentum. Thus turbulence is sup-
pressed. The wind shear however can be signifi cant since there is little vertical
exchange. During these conditions large wind direction gradients can occur.

4.2.2 Infl uence of roughness
In neutral conditions and fl at terrain with uniform roughness the vertical profi le
can be described analytically by the power law:

a
⎛⎞
= ⎜ ⎝⎠

uh ()

()

h

1 1

(6)

uh h

2 2