Vertical Profiles Over Flat Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 22

Chapter 3
Vertical Profiles Over Flat Terrain
This chapter is going to introduce the basic laws for the shape of the vertical
profiles of wind speed and turbulence in a flat, horizontally homogeneous atmo-
spheric boundary layer (ABL) over land, because this is the simplest surface type.
See Chap. 4 for orographically structured complex terrain and Chap. 5 for the
marine ABL. The ABL is the lower part of the troposphere and by this the lowest
layer of the atmosphere as a whole. In contrast to the free atmosphere above,
which had been introduced in Chap. 2 , the presence of the underlying Earth's
surface has a measurable influence on the ABL. It is the only part of the atmo-
sphere where frictional forces play an important role, and where the temperature
and atmospheric stability can exhibit diurnal and annual variations. The ABL acts
as a kind of broker that communicates the transport of energy, momentum, and
other substances between the Earth surface and the free atmosphere, i.e. the ABL
is dominated by vertical fluxes of these variables. These fluxes have their largest
amounts directly at the surface and vanish at the top of the ABL. We will con-
centrate here on vertical wind and turbulence profiles, because these are the most
important ABL feature for the generation of energy from the wind. More general
descriptions of the ABL can be found in Stull (1988), Arya ( 1995 ), Garratt ( 1992 )
and other topics. Because the ABL enwraps the whole Earth, it is often also called
planetary boundary layer (PBL).
The wind speed profile laws for the ABL form the basis for vertical interpo-
lation and/or extrapolation from measurement or model layer heights to hub height
or other heights in the rotor plane of a wind turbine. The profile laws also indicate
the vertical wind shear which has to be expected across the rotor plane of a turbine.
The growing hub heights of modern wind turbines require a careful investigation
of the vertical structure of the boundary layer in order to describe the wind profiles
correctly. Hub heights of 80 m and more are usually above the surface layer which
forms about one tenth of the depth of the total boundary layer. Simple power law
or logarithmic profiles are strictly valid in the surface layer only.
The Earth's surface is a place where turbulence is generated, it is usually a sink
for atmospheric momentum, and either a source or a sink for heat and moisture.
Therefore, we find in the ABL less momentum but more turbulence, and different