Geoscience Reference
In-Depth Information
Sect. 3.2
). If a convectively driven boundary layer (CBL) is present, no distinction
is made between the CFL or Prandtl layer and the Ekman layer but they are jointly
addressed as mixed layer. Good overviews of the special features of the UBL can
e.g. be found in Roth (
2000
), Arnfield (
2003
) and Grimmond (
2006
).
Wind and turbulence within the UBL are different from flat terrain. Numerous
field experiments (for an overview see e.g. Grimmond (
2006
)), numerical studies
(see e.g. Batchvarova and Gryning (
2006
)) and several wind tunnel studies
(Counihan
1973
; Farell and Ivengar
1999
; Schatzmann and Leitl
2002
) therefore
have been conducted to investigate the structure of the UBL. Besides of the better
understanding of turbulence within the UBL, a realistic representation of the flow
field within street canyons and above the buildings is essential for the deployment
of suitable urban wind turbines to urban areas (e.g. model simulations for London
with ADMS Urban (CERC
2001
)).
3.7.2 Vertical Profiles of Wind and Turbulence
Basically, wind profiles over urban areas can be described by the profile laws
derived in this chapter above by choosing a large roughness length (usually a metre
or more) and a displacement height [see (
3.6
) and the remarks following this
equation] of about two thirds of the mean building height.
Figure
3.25
gives monthly mean wind profiles over a city for four different
seasons. The April data in Fig.
3.25
shows the phenomenon of cross-over which
has been introduced and explained in
Sect. 3.5
. The cross-over height is roughly
125 m. This is rather high and probably due to the large aerodynamic roughness of
the urban surface that is about 1 m in this case. The August data exhibits the low-
level jet phenomenon in the night-time profile at about 325 m above ground even
in a monthly average (see
Sect. 3.4.1
). Both phenomena are closely related and the
above given rule of the cross-over height being roughly one third of the height of
the low-level jet core is fulfilled as well. The occurrence of these phenomena needs
rapid night-time cooling which does not appear over urban heat islands. Therefore,
it must be assumed that the low-level jet has formed on a regional scale over the
rural environment of this city and has been advected by the mean wind over the
city. This again demonstrates the missing horizontal homogeneity for urban
boundary layers as depicted in Fig.
3.23
.
Figure
3.26
gives an indication on the vertical turbulence profiles over an urban
area by showing vertical profiles of the standard deviation of the vertical velocity
component from the same measurements as those depicted in Fig.
3.25
. Most
profiles show an increase with height even for stable stratification. The daytime
increase can be explained by unstable stratification [see Eq. (
3.18
]. The still
considerable increase of the night-time values with height evident in the lower
100 m are due to unstable stratification, but above this height they are probably
also related to the formation of nocturnal low-level jets (see the upper right and
especially the lower left frame in Fig.
3.26
). The maximum of this standard
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