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328 11 Air Dispersion

Fig. 11.6 Calculated wind 200
speed profile under neutral
180
condition
160
140
Elevation, z (m) 100
120

80
60
40
20
10
0
0 2 4 6 8
Wind speed, u (m/s)

Table 11.3 Coefficients a and b for different Pasquill stability classes
P class A B C D E F
Stability Extremely Moderately Slightly Neutral Slightly Stable
unstable unstable unstable stable
a −0.096 −0.037 −0.002 0 0.004 0.035
b 0.029 0.018 0 −0.018 −0.036

1
¼ a þ b log ðÞ ð11:20Þ
z 0
10
L
where the coefficients a and b are listed in Table 11.3.
With the availability of Obukhov Length, we can calculate the stability indicator
z
s z ¼ ¼ za þ b log ðÞ½ 10 z 0 Š ð11:21Þ
L
A positive value of s z corresponds to stable atmosphere and a negative s z means
unstable condition. With the Obukhov length determined using Eq. (11.20), the
wind speed under stable and unstable conditions can be determined now as follows.
For stable conditions (s z [ 0Þ,

u 1 z 5 z z 0
¼ ln þ ln ð11:22Þ
k k L
u z 0
Readers can find a more complex yet accurate approach in literature (e.g. [8]).

11.2.6.3 Wind Speed Profile in Unstable Atmosphere

The procedure for the calculation of wind speed profile under unstable atmosphere
is more complex; one widely used approximation is the one given by Benoit [2]