Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 99

Fig. 4.15 Mean monthly vertical wind profiles ( bold full line :alldata, dashed line : nocturnal data,
dashed-dotted line: daytime data) over a town (Hannover, Germany) for April 2003 from SODAR
measurements. The profiles are compared to the diabatic logarithmic wind profile (upper equation
in (2.27) . Parameters for (2.27) are given in the insert
urban area show constantly high turbulence intensities near the ground in the urban
canopy layer, the rural profiles exhibit a strong diurnal variation close to the ground
as well. This high turbulence level that is maintained during the night over urban
surfaces is mainly due to the large heat storage of the urban surface, which prevents
the formation of strong stable stratification in the urban canopy layer during night-
time. This is in agreement with the findings presented by Uno et al. ( 1988 , 1992 ) and
Dupont et al. ( 1999 ) who also showed that the large roughness of the town surface
and the urban heat island prevents the formation of a cool stably stratified nocturnal
surface layer as it is typical for rural areas for nights with clear skies.
Comparison to Analytical Wind Profile Laws
The wind profiles shown in Fig. 4.15 deviate from the analytical profiles from the
first equation in (2.27) in greater heights. Therefore, in Fig. 4.17 comparison is made
with the analytical curve from the complete equation (2.27) . This fit is much bet-
ter for greater heights. Here the mean monthly nocturnal wind profile observed in
Hannover, Germany, in August 2002 is displayed. Apart from a secondary maxi-
mum due to the frequent occurrence of low-level jets in this month between 200
and 350 m above ground, the full eq. (2.27) gives a good representation of the wind
profile at all heights. As the low-level jet is a non-stationary feature, it is clear that