Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 96

Fig. 4.12 Diurnal variation of the horizontal wind speed in three different heights above ground
(40, 100, and 160 m) during an undisturbed cloudless weather situation in summer 2005. The
half-hourly data are from SODAR measurements near the Paris airport Charles de Gaulle
low-level jet blowing at night in heights of a few hundred metres above ground (see
Section 4.5.1 ).
Figure 4.14 merges the turbulence (Fig. 4.13 ) and wind speed (Fig. 4.12 ) infor-
mation together in vertical profiles of turbulence intensity. Turbulence intensity
is formed here as the ratio of the standard deviation of the vertical wind speed
component and the mean horizontal wind speed. This is the only non-dimensional
turbulence measure, which can be derived from SODAR measurements. A strong
difference between daytime and nocturnal conditions is observed. The daytime tur-
bulence intensity is more than twice the night-time turbulence. The other difference
is the vertical slope of the profiles. While the nocturnal profile exhibits a slight
decrease with height, the daytime profile shows a clear increase with height. This
increase is due to the increase in the standard deviation of the vertical wind speed
component, because the mean horizontal wind speed is nearly identical in these
three heights during daytime.
Wind and Turbulence Profiles over Cities
Wind and turbulence profiles over cities differ from those over flat rural terrain due
to the specific vertical structure of the urban boundary layer (UBL) presented in
Chapter 2 . Surface-based vertical soundings in the UBL for longer time periods have
so far been made only in Moscow (Russia, Pekour and Kallistratova 1993 ; Pekour