Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 231

in the United States. This means the imprint of urban areas on climate is
increasing.
In all urban areas resource use and emissions are increasing with global
consequences. Urban environmental problems, related to air quality, thermal
stress, issues of water demand and quality, all of which are linked directly or
indirectly to urban climate, are emerging as major environmental concerns at
the start of the twenty-first century. The study and understanding of funda-
mental causes of urban climates is critically important to predict, plan for, and
mitigate negative inadvertent effects.
7.4 Wind, cloud cover, and pressure
7.4.1 Wind
The urban environment has significant interactions with airflow. The city modi-
fies the vertical and horizontal wind structure in several ways, creating altera-
tions in the forces and pressures that make up the wind. The variations in the
height of the buildings increase the surface roughness, creating extra turbulence,
and the potential for stronger convection. Wind speed is generally lower in the
city (see Table 7.2 ) due to friction, but under light winds, urban streets and
canyons can act as wind tunnels, increasing local airflow strength. Wind direc-
tion is deflected considerably in the maze of buildings and streets, often making
definition very difficult. The buildings themselves will respond in varying ways
to the stress created by the wind. These alterations make using a 10 m mast to
measure wind inappropriate.
The variation of wind speed with height in the lower troposphere is often
described through wind profiles or roughness calculations. For example, the
exponential wind profile depends on the speed of the wind with height (u(z)), the
airflow at a defined base level (z 0 ), the zero-plane displacement of the airflow
(z d ), the height and wind speed at a defined reference level (z ref , u ref ), and an
exponent ( ) which indicates the roughness of the surface (Plate 1995 ):
u ð z Þ= u ref ¼ð z 0 z d = z ref Þ
Unfortunately, use of wind profiles alone can only provide a coarse indication of
vertical wind structure over a city, because the roughness varies considerably
and aerodynamic responses are irregular depending on the type, size, and shapes
of surface features (urban morphology). Grimmond and Oke ( 1999 ), in a review
of methodologies used to assess the relationship between roughness geometry
and aerodynamics over a city, summarize how difficult it is to obtain represen-
tative results. They concluded, for example, that acceptable estimates of z d and
z 0 are almost non-existent.
When the wind is strong, its interaction with the urban structures is mechan-
ical, and internal friction and roughness are the main reasons for the wind