Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 232

variations (Plate 1995 ). Buildings of various heights will tend to slow and mix
the wind (Figure 7.4 ), creating uncomfortable and unexpected variations in wind
speeds for people at ground level. Under these conditions, the roughness sub-
layer created by the city can extend to 2.5 to 3 times the height of the buildings,
especially where a group of buildings of varying heights exists. Coriolis force is
reduced in the city due to the friction, and the convective interactions between
the urban airflow and synoptic airflow can affect weather fronts, clouds, and
precipitation (Oke 1997 ). The variations in the surface morphology mean that
there is no consistently reliable method to incorporate urban surface roughness
into a wind model (Arnfield 2000 ).
When the synoptic wind is weak, the extra thermal effects (artificial heat
sources; release of heat by surfaces, etc.) in an urban environment become much
more important to the wind than roughness and friction. Thus UHI formation
from thermal effects best occurs at night under anticyclonic circulation, cloud
cover less than 2 oktas, wind speed less than 3 m s 1 , and a large negative Q*.
Regional circulations, such as drainage flows from surrounding higher eleva-
tions, and micro-advection processes, become influential (Eliasson 1994 ). The
stability of the atmosphere and the strength of any inversion present define the
depth of the mixing layer, and it thus becomes an important controller. The wind
in the planetary boundary layer may be separated from that in the UBL by an
inversion (Plate 1995 ). Further separations between airflows at UCL and UBL
level can allow local circulations to dominate (such as between the buildings in
Figure 7.4 ). In smaller coastal cities, such as G¨ teborg, such separations can be
critical to UHI development (Eliasson 1994 ).
Depending on the physical size of the city, its population, and its density and
height variation of buildings, one or more UHIs may develop when the wind
speed is small. UHIs also can affect the wind, altering local pressure fields, and
creating changes in wind direction (by up to 208) due to increased convective
mixing from the heat releases (Oke 1995 ). While the general structure of thermal
turbulence characteristics may be similar to the rural environment, the urban
center modifies the stability characteristics of the lower atmosphere and can
significantly increase the convective turbulence.
7.4.2 Meteorological conditions and urban temperatures
The relationship between meteorological conditions and the UHI is generally
well known. The city of Melbourne is used as an illustration. Research results
reported by Morris and Simmonds ( 2000 ) and Morris et al.( 2001 ) provide a
detailed evaluation of the relationship between UHI development and strength
and meteorological conditions (wind speed, synoptic pressure anomaly, cloud
cover) between 1973 and 1991 in Melbourne. Comparing a central business
district site with three rural airport sites, the UHI detected over the 20-year
period averaged 1.13 8C and ranged from 3.16 to þ 6 8C. Negative temperature