Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 221

Figure 7.4 Schematic of
the urban boundary layer
(UBL) including its vertical
layers and scales. PBL stands
for planetary boundary
layer, and UCL for urban
canopy layer. (Revised by
Oke and Rotach after a figure
in Oke 1997 ) (Source:
Piringer et al. 2002 : with
permission from Elsevier)

(a) Meso scale

Urban "plume"

Mixing layer

PBL

UBL

Rural BL

(b)

Surface layer

Rural Urban Rural

(b) Local scale (c) Micro scale

Inertial
sublayer

Surface
layer

Roughness
sublayer

Roughness
sublayer

(c)

UCL

UCL

can be divided into the roughness sublayer (RSL) and the inertial sublayer
(ISL). In cities, the presence of large roughness elements (buildings, trees,
etc.) means that the RSL, which includes the UCL, is deep. Within this layer
the turbulent fluxes of momentum, energy, moisture, and pollutants are
height dependent, thus any measurements are representative of micro-scale
processes. To document local-scale (neighborhood) processes, instruments
have to be mounted above the RSL and within the ISL.
The importance of appropriately considering scale is well illustrated by
studies of urban air temperature (Figure 7.5 ). Temperature anomalies asso-
ciated with urban areas, from villages to megacities, have long been studied;
Luke Howard's ( 1833 ) investigation of London is often cited as one of the
first studies of urban climate. These urban temperature effects are of interest
not only to urban climatologists but also to global change researchers trying
to document long-term regional and global temperature trends and thus
wishing to remove any urban ''contamination'' from long-term records, and
those interested in the impact of urban areas on energy consumption, in
particular related to air conditioning demand, and the implications for
enhanced global carbon dioxide concentrations given the use of fossil fuels
for power generation.
When meso-scale (whole city) urban heat islands (UHI) are studied
in detail (Figure 7.5a and b ), it is apparent that rarely does temperature
increase continuously from the outskirts of the city into the central busi-
ness district. Rather, local-scale thermal influences are evident. Parks,