Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 471

Scales of Motion in the Atmosphere
In order to understand the wind energy resource and its distribution in time and space,
it is helpful to look at the different scales of air flow that occur on our rotating globe.
While the ultimate energy source for atmospheric flow is the sun, wind is not driven
directly by solar radiation but by energies derived through different kinds of conversion.
Thus it is beneficial for the wind turbine engineer to be acquainted with the physical
processes ( i.e. , dynamic, thermodynamic , and radiative ) that take place simultaneously in
the atmosphere and cause the wind vector.
The atmosphere operates on many time and space scales, ranging from seconds and
fractions of a meter to years and thousands of kilometers. Time and space scales of the
atmospheric motions and their importance to wind energy utilization are summarized in
Figure 8-3. The very large or climatic scale includes seasonal and annual fluctuations in
the wind, which are useful for assessing regional wind resources. On a scale comparable
with the weather maps seen in the press or used by airlines there are large-scale synoptic
fluctuations identified by the patterns of isobars moving across a country. These large-scale
fluctuations influence the output of wind power stations, so effective site selections depend
on a knowledge of regional atmospheric motions. Small-scale fluctuations , which are even
more local in size and higher in frequency, are best observed on local anemometer records,
providing data for wind turbine design and micrositing ( i.e. , siting of individual turbines).
Figure 8-3. Temporal and spatial scales of atmospheric motion.
Given a record of a meteorological variable, such as the horizontal wind speed, we can
always decompose it into components representing a time or spatial average and fluctuations
superimposed on the average. After doing this, we view the average value of the variable
as a deterministic quantity and assign statistical characteristics to only the fluctuations.
Wind loadings on the turbine can also be classified into two parallel groupings: those
associated with the mean wind speed, which are conveniently described as quasi-steady or