Analytical Description and Vertical Structure of the Atmospheric Boundary Layer - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 23

is the urban canopy layer (UCL), which reaches up to the mean top height of the
buildings. The next layer is the wake layer in which the influence of single build-
ings on the flow is still notable. This wake layer usually extends to about two to five
times the average building height. These two layers are often jointly addressed as the
urban roughness sublayer (URL, Rotach 1999 ). Strong vertical exchange by forced
vertical motions can occur in this layer. Above the urban roughness layer is the con-
stant flux layer (CFL) or inertial sublayer (IS), over homogeneous terrain usually
addressed as surface layer or Prandtl layer. In the uppermost part of the boundary
layer above the CFL, the wind direction turns into the direction of the geostrophic
wind (often called Ekman layer). 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 mixing layer. Good overviews of the special
features of the UBL can, for example, be found in Roth ( 2000 ), Arnfield ( 2003 ), and
Grimmond ( 2006 ).
Wind and turbulence within the UBL determine the horizontal and vertical dis-
persion and transport of air pollutants in towns and is thus important for the health
of the citizens. Numerous field experiments (for an overview see, e.g. Grimmond
( 2006 )), numerical studies (see, e.g. Batchvarova and Gryning ( 2006 )), and sev-
eral wind tunnel studies (Counihan 1973 , Farell and Iyengar 1999 , Schatzmann and
Leitl 2002 ) therefore have been conducted to investigate the structure of the UBL.
Besides a better understanding of mixing and transport processes within the UBL, a
realistic representation of the flow field within street canyons and above the build-
ings is essential for the application of dispersion models to urban areas (e.g. model
simulations for London with ADMS Urban (CERC 2001 )).
2.4 Forest Boundary Layers
Forest-covered surfaces are a special form of vegetated surfaces. The special fea-
tures of the forest boundary layer decisively depend on the density of the trees. If
trees grow very close together, their crowns form a rough surface, which has much
in common with a rough grass land (Raupach 1979 ) as depicted in Fig. 2.1 .Butif
the trees grow sparser, then the surface has many similarities with an urban surface
and must be described as shown in Fig. 2.7 . In contrast to the urban canopy layer
(Fig. 2.6 ), the forest canopy layer must be subdivided in two layers: the stem layer
and the crown layer. In the stem sublayer the horizontal wind speed may be higher
than in the denser crown sublayer. The main difference between a densely vegetated
forest (Fig. 2.1 ) and a sparsely vegetated forest (Fig. 2.7 ) is that larger air parcels
can enter (sweeps) and leave (ejections) the forest canopy sublayer. This permeabil-
ity of the forest canopy sublayer leads to an anomaly featuring higher turbulence
intensities in the wake sublayer than expected from the mean vertical wind gradi-
ent in this layer. Therefore, the usual flux-gradient relationships are not valid in the
whole roughness sublayer (Högström et al. 1989 ). This anomalous wake layer may
extend to about three to five tree heights.