Page 44 - Wind Energy Handbook

18 THE WIND RESOURCE

approximation is not reliable for estimating, say, the probability of a large gust
within a certain period.
The turbulence intensity clearly depends on the roughness of the ground surface
and the height above the surface. However, it also depends on topographical
features such as hills or mountains, especially when they lie upwind, as well as
more local features such as trees or buildings. It also depends on the thermal
behaviour of the atmosphere: for example, if the air near to the ground warms up
on a sunny day, it may become buoyant enough to rise up through the atmosphere,
causing a pattern of convection cells which are experienced as large-scale turbulent
eddies.
Clearly as the height above ground increases, the effects of all these processes
which are driven by interactions at the earth’s surface become weaker. Above a
certain height, the air flow can be considered largely free of surface influences. Here
it can be considered to be driven by large-scale synoptic pressure differences and
the rotation of the earth. This air flow is known as the geostrophic wind. At lower
altitudes, the effect of the earth’s surface can be felt. This part of the atmosphere is
known as the boundary layer. The properties of the boundary layer are important
in understanding the turbulence experienced by wind turbines.

2.6.2 The boundary layer

The principal effects governing the properties of the boundary layer are the strength
of the geostrophic wind, the surface roughness, Coriolis effects due to the earth’s
rotation, and thermal effects.
The influence of thermal effects can be classified into three categories: stable,
unstable and neutral stratification. Unstable stratification occurs when there is a lot
of surface heating, causing warm air near the surface to rise. As it rises, it expands
due to reduced pressure and therefore cools adiabatically. If the cooling is not
sufficient to bring the air into thermal equilibrium with the surrounding air then it
will continue to rise, giving rise to large convection cells. The result is a thick
boundary layer with large-scale turbulent eddies. There is a lot of vertical mixing
and transfer of momentum, resulting in a relatively small change of mean wind
speed with height.
It the adiabatic cooling effect causes the rising air to become colder than its
surroundings, its vertical motion will be suppressed. This is known as stable
stratification. It often occurs on cold nights when the ground surface is cold. In this
situation, turbulence is dominated by friction with the ground, and wind shear (the
increase of mean wind speed with height) can be large.
In the neutral atmosphere, adiabatic cooling of the air as it rises is such that it
remains in thermal equilibrium with its surroundings. This is often the case in
strong winds, when turbulence caused by ground roughness causes sufficient
mixing of the boundary layer. For wind energy applications, neutral stability is
usually the most important situation to consider, particularly when considering the
turbulent wind loads on a turbine, since these are largest in strong winds. Never-
theless, unstable conditions can be important as they can result in sudden gusts
from a low level, and stable conditions can give rise to significant asymmetric