Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 98

Fig. 4.14 Daytime ( dash-dotted line ), night-time ( dashed ), and daily mean ( full line )profilesof
turbulence intensity from SODAR measurements in summer 2005 near Paris airport Charles de
Gaulle

In the fits with the analytical profiles, the roughness length was set to z 0 =

1m.

is varying between 0.356 m s − 1 for the night-time profile
and 0.538 m s − 1 for the daytime profile. The analytic and the mean observed wind
profiles agree up to 350m above ground, while the observed wind speeds are on
average 1-2m s − 1 lower than expected according to the logarithmic wind profiles
at higher levels.

The friction velocity, u ∗

Mean Urban Turbulence Profiles

The software of SODAR instruments usually delivers the standard deviation of the
vertical velocity component, which can be used as a rough measure of atmospheric
turbulence. Figure 4.16 shows monthly mean profiles of the vertical component of
the turbulence intensity observed in Hannover from SODAR measurements, i.e. the
variance of the vertical wind component σ w divided by the average horizontal wind

speed. The daytime values of turbulence intensity in this example are twice as high
as the night-time values. At daytime, turbulence intensity profiles are more or less
constant with height up to 300-400 m above ground. In autumn, winter, and gen-
erally at night-time, the profiles show a strong decrease of the turbulence intensity
with height within the lower 150-200 m in accordance with (2.35) together with the
first equation of (2.27) .
The strong difference to turbulence profiles over rural terrain becomes obvious
by comparing Figs. 4.14 and 4.16 . While the turbulence measurements over the