Vertical Profiles Over Flat Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 65

Fig. 3.25 Monthly mean wind profiles over the city of Hannover (Germany) from SODAR
measurements (bold lines) for February 2003 (upper left), April 2003 (upper right), August 2002
(lower left) and November 2002 (lower right). Thin lines are computed from Eq. ( 3.16 ), the
necessary parameters are given in the boxes to the upper left of each frame. Full lines show all
data, dash-dotted lines show daytime data and dashed lines show night-time data
deviation is in the same height as the core of the low-level jet. The ratio of the
standard deviation values near the ground to the friction velocity is a little bit
higher than expected from Eqs. ( 3.9 ) and ( 3.18 ) for flat terrain. According to these
relations and using the u * values used for fitting in Fig. 3.25 , the standard deviation
values should range between about 0.3 m/s for night-time in August and about
0.7 m/s for daytime in April. The differences between daytime and night-time
profiles are small in winter and autumn, although the winter profiles show a large
dependence of the synoptic wind direction. In wintertime, the largest values of this
standard deviation occur with usually stronger westerly winds. Night-time and
daytime profiles differ most in spring and summertime. In these seasons, the
differences between the mean daytime and night-time profiles are much larger than
the differences between the mean profiles for different wind directions.
Figure 3.27 shows monthly mean profiles of the vertical component of the
turbulence intensity observed in Hannover, i.e. the standard deviation depicted in
Fig. 3.26 divided by the average horizontal wind speed shown in Fig. 3.25 .This
quantity therefore inversely depends on the mean wind speed. Turbulence intensity
is highest in summer and spring. In these two seasons, the daytime values are twice
as high as the night-time values. At daytime, turbulence intensity profiles in spring
and summer are more or less constant with height up to 300-400 m above ground.