WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–22
no aiming or blade control.
Thus far, we have talked only about gusts due to thermal turbulence, which has a very
strong diurnal cycle. Gusts are also produced by mechanical turbulence, causedbyhigher
speed winds flowing over rough surfaces. When strong frontal systems pass through a region,
the atmosphere will be mechanically mixed and little or no diurnal variation of wind speed
will be observed. When there is no significant mixing of the atmosphere due to either thermal
or mechanical turbulence, a boundary layer may develop with relatively high speed laminar
flow of air above the boundary and essentially calm conditions below it. Without the effects
of mixing, upper level wind speeds tend to be higher than when mixed with lower level winds.
Thus it is quite possible for nighttime wind speeds to decrease near the ground and increase
a few hundred meters above the ground. This phenomenon is called the nocturnal jet. As
mentioned earlier, most National Weather Service (NWS) anemometers have been located
about 10 m above the ground, so the height and frequency of occurrence of this nocturnal jet
have not been well documented at many sites. Investigation of nocturnal jets is done either
with very tall towers (e.g. 200 m) or with meteorological balloons. Balloon data are not
very precise, as we shall see in more detail in the next chapter, but rather long term records
are available of National Weather Service balloon launchings. When used with appropriate
caution, these data can show some very interesting variations of wind speed with height and
time of day.
Fig. 13 shows the diurnal wind speed pattern at Dodge City, Kansas for the four year
period, 1970-73, as recorded by the National Weather Service with an anemometer at 7 m
above the ground. The average windspeed for this period was 5.66 m/s at this height. The
spring season (March, April, and May) is seen to have the highest winds, with the summer
slightly lower than the fall and winter seasons. The wind speed at 7 m is seen to have its peak
during the middle of the day for all seasons.
Also shown in Fig. 13 are the results from two balloon launchings per day for the same
period for three Kansas locations, Dodge City, Goodland, and Wichita. The terrain and
wind characteristics are quite similar at each location, and averaging reduces the concern
about missing data or local anomalies. The surface wind speed is measured at the time of the
launch. These values lie very close to the Dodge City curves, indicating reasonable consistency
of data. The wind speed pattern indicated by the balloons at 216 m is then seen to be much
different from the surface speeds. The diurnal cycle at 216 m is opposite that at the surface.
The lowest readings occur at noon and the highest readings occur at midnight. The lowest
readings at 216 m are higher than the highest surface winds by perhaps 10-20 %, while the
midnight wind speeds at 216 m are double those on the surface. This means that a wind
turbine located in the nocturnal jet will have good winds in the middle of the day and even
better winds at night. The average wind speed at 216 m for this period was 9.22 m/s, a very
respectable value for wind turbine operation.
Measurements at intermediate heights will indicate some height where there is no diurnal
cycle. A site at which the wind speed averages 7 or 8 m/s is a good site, especially if it is
relatively steady. This again indicates the importance of detailed wind measurements at any
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001