Environmental Engineering Reference
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Fig. 13.2
Wind profiles measured by sodar at CDG airport. Winds from the sector 160-250
◦
(
thin
full line
) are from built-up areas (airport infrastructure) only, winds from 340-70
◦
(
dotted line
)are
from rural areas only
from sodar measurements is shown in Fig. 13.3a. The nocturnal level of this vari-
ance is considerably higher than over rural terrain.
13.2.3 Stratification of the Urban Boundary Layer
and Mixing-Layer Height
The combined use of acoustic (sodar) and optical (ceilometer, i.e. a mini-lidar)
remote sensing allows analysing the diurnal variation of the vertical structure of the
entire UBL and the MLH over Budapest (Fig. 13.5). The lidar easily reaches heights
of 3000 m and more while the maximum vertical range for the sodar is only 1300 m.
Figure 13.5 shows for four summer days the diurnal variation in the occurrence of
the nocturnal stable boundary layer (SBL), the daytime convective boundary layer
(CBL), and the residual layer (RL) which is left aloft after the formation of a new
stable layer near the ground in the evening. It is this residual layer where for exam-
ple ozone survives during the night until it is mixed downward again on the next
morning when the new CBL grows.
Long-term measurements like in Hanover allow statistical evaluations of MLH
(Fig. 13.6). The MLH is an important parameter which determines and limits the
dilution of freshly emitted pollutants. The mean diurnal variation of MLH shows
a clear annual variation. The variation is largest in spring and summer months and
nearly vanishes in late autumn. Details of the MLH estimation from remote sensing
data are given in Emeis and Türk (2004), Emeis et al. (2007b) and Emeis et al.
(2008). Recommendations from the COST-715 Action with respect to MLH are
documented in Piringer et al. (2007).
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