Wind Energy

4 Mean Wind and Turbulence in the Atmospheric Boundary Layer 23
as representing the marine atmosphere. Easterly winds on the other hand
represent a terrestrial boundary layer.
In Fig. 4.1 we have plotted the annual stability distributions for easterly
and westerly flows. The figures both reflect the tendency to stable stratification
at larger heights and that thermal effects have to be included also for the
winds of relevance for wind energy.
Next we plot in Fig. 4.2, the annual mean wind profiles from East and
West for u100 > 8, 10, 12, 14, 16 ms −1 , and for z between 10 and 160 m. From
West the internal boundary layer growth is clear in the kinks in the profiles
and in agreement with the standard models [5]. Most from East but also from
West the tendency for the upper part of the profiles to increase more than a
logarithmic law is clear and can be modeled to be in accordance with (4.1) to
that of (4.5) for climatologic value of N ∼ 0.01 s −1 , yielding LN ∼ 2–300 m.
Pdf (%)
20
15
10
5
0
−0.2 0
100 m 160 m
0.2
R iB East
West
Pdf (%)
17.5
15
12.5
10
7.5
5
2.5
0
0.4 −0.2 0 0.2
R iB East
West
Fig. 4.1. Annual stability distribution for u100 > 8ms −1 from East (terrestrial) and
West (marine) boundary layers at z=100 and 160 m above terrain. The width of the
PDF corresponds to a Bulk Richardson number, RiB with ∆θ defined between 80–
100 and 100–165 m respectively, of about ± 0.02 for both figures. This corresponds
to |100/L| ∼1.3 over land and 3.4 over water. Note, RiB= (g/T)( ∆θ/∆z)(z/uz) 2
z [m]
150
100
70
50
30
20
15
10
8
10 12 14 16 18
U [m/s]
z [m]
150
100
70
50
30
20
15
0.4
10
20 8 10 12 14 16 18
U [m/s]
Fig. 4.2. Profiles of wind from West (left) and East (right hand side), for 10–160 m
over terrain, see text above for discussion.
20