Geog. 310 Urban Climatology
The Urban Wind Field
The changes brought about by urbanization in the local atmospheric boundary layer have
a notable effect on the low-level wind. This is caused by the Heat Island and the change
in surface roughness.
A. Surface Wind
1. When regional winds are light and skies are clear, a drift or convergence of surface air
towards the urban heat island and a compensating outflow aloft have been recorded
empirically in different cities. This is a logical consequence of the heat island, which
creates an unstable vertical lapse rate of temperature and so induces a rising air current
(see figure of urban dust dome).
2. Centripetal urban air flow (into the city) is usually more pronounced at night than
during the day because the temperature difference (urban-rural) is greater at night.
3. When air flows from country to city, it encounters a great increase in surface
roughness. The wind near the ground therefore slows down except for local
accelerations around individual buildings, and becomes more turbulent. Landsberg
(1981) states that there is a reduction in speed of 20 to 50% at the standard observing
level of 10 m.
Changes in Mean Wind Speeds in Parma, Italy in Three
Consecutive Decades*
Interval January April
July
October Year
1938-1949
0.5
1.8
1.8
1.0
1950-1961
0.5
1.4
1.4
0.7
1962-1973
0.3
1.0
1.3
0.6
*wind speed (m sec-1); adopted from Zanella (1976).
1.3
1.0
0.8
4. When there are deciduous trees in the city, the reduction in wind speed increases in
summer and decreases in winter.
5. A differential day-night wind speed reduction has also been observed in different
cities. Lee (1979) clearly showed that if in the daytime the wind speeds in London
were reduced on an average of 30 percent, the average nighttime reduction was only
20 percent. In some cases, a strong nocturnal convergence results in greater wind
speed in the city than in its surrounding.
6. The flow of air around buildings and in the street canyon is very complex and depends
greatly on the design of the city (see diagram on wind flow around buildings).
B. Vertical Wind
1. One of the common mathematical models describing airflow over a city was proposed
by Davenport in 1968:
U*/VG = (Z/ZG)α,
where:
U* is the wind at height ZG
VG is the gradient wind speed at height ZG
α is dependent on surface roughness
Measurements of the vertical Wind Profile for Different Surface Roughnesses*
Terrain characteristics
Exponent of power law Gradient wind level (m)
Open country, flat
0.16
270
Suburban settlement
0.28
390
Inner cities
0.40
420
*Davenport (1965).
2. The previous formula is good for determining the average wind over a city but does
not help in predicting and calculating turbulence and peak gust winds. Generally the
wind speed increases with height. However city friction reduces the rate of increase
with height, reaching the non-friction level at a much higher level (gradient wind
level).
3. Urban areas have also an affect on the wind direction and turbulence. There is more
turbulence within the urban boundary layer than in the surrounding areas.
4. Narrow streets between tall buildings can have a funneling or Venturi Effect, in which
wind is actually speeded up when forced to flow through a narrowing or bottleneck.
5. Edges of buildings and the geometry and spacing of buildings may cause turbulent
eddies (small whorls) as wind flows around these obstacles.
C. Practical Importance of Knowing the Wind Structure
1. Air pollution control: the important variable is the concentration of pollutants at
ground level, which is a function of wind speed, direction and turbulence.
2. Building heating and ventilation.
3. Wind loading on structures (wind stress).
4. Windbreaks to minimize drifting and blowing snow on expressways and around
buildings, in cold climates.
5. Coastal cities prone to damaging winds, such as hurricanes.
D. Conclusions
The urban windfield is rarely simple. Even when the synoptic weather situation is least
complicated, small differences in local topography will cause irregular air flows. River
valleys, bays, and shore locations on lakes and oceans develop their own secondary
circulations, such as land and sea breezes, and mountain and valley breezes. They
interact with the urban-induced circulation and may, at times, dominate it. That is why
all the broad generalizations that have been made previously are at best guideposts to
the phenomenon.
BIBLIOGRAPHY
Angel, J.E. and Pack, D.H. (1973): Urban influence on a strong daytime air flow as
determined from tetroon flights. Journal of Applied Meteorology (Sept.) 924-936.
-------------------------------- (1966): Tetroon trajectories in an urban atmosphere. J. of
Applied Meteorology (Oct.) 565-572.
Arakawa, H. and Tsutsumi, K. (1967): Strong gusts in the lowest 250-m layer over the
city of Tokyo. J. of Appl. Meteorol. (Oct.) 848-851.
Arya, S. P. (1989) Introduction to Micrometeorology. San Diego: Academic Press.
Druyan, L.M. (1968): A comparison of low-level trajectories in an urban atmosphere. J.
of Appl. Meteorol. (Aug.) 583-589.
Ferrar, T.A. (1976) The Urban Costs of Climate Modification. N.Y.: Wiley Interscience
Publ.
Landsberg, H. (1981) The Urban Climate. N.Y.: Academic Press.
Lettau, H. (1969): Notes on aerodynamic roughness parameter estimation on the basis of
roughness element description. J. of Appl. Meteorol. (Oct.) 828-832.
Munn, R.E. (1968) “Airflow in Urban Areas,” in Urban Climates, Technical Note No.
108. Geneva: WMO, pp 15-39.
Oliveira, A., P., R., D., Bornstein, and J., Soares, 2003: Annual and diurnal wind
patterns in the city of Sao Paulo. Water, Air, and Soil Pollution: Focus 3, 3-15.