Geoscience Reference
In-Depth Information
The gradient wind is usually slowed down in the urban areas, although urban
geometry may cause unwanted acceleration due to channeling along some streets or
the
venturi
effect [CER 95]. Conversely, country and sea breezes may be intensified
due to UHI. Cities are major sources of pollutants whose removal depends on the
dispersion capability of the atmosphere [BAT 06] and on wind speed and direction.
Figure 5.2a shows the streets of the city of Beijing (China) on a calm anticyclonic
day of November 2005, while dense smog covered the city hampering the sun and
causing health problems. The influence of the city on smog formation is still clearer
if one compares Figure 5.2a with Figure 5.2b showing a view of the Great Wall
taken the next day approximately 50 km from the urban area.
Figure 5.2.
a) Dense fog in the streets of the city of Beijing (China) on a
calm anticyclonic day of November 2005. b) View of the Great Wall,
taken the next day, approximately 50 km from the urban area
Following the Metromex experiments and other studies quoted above it was
verified that increases of 5-25% of rainfall took place downwind of the cities (in
relation to the precipitation upwind) and that the rainfall anomalies increased with
the size of the urban area [CHA 74]. The possible mechanisms of the urban impact
in precipitation are the enhanced convergence due to high surface roughness in the
cities and to urban heating, as well as enhanced aerosols in the UBL, functioning as
“cloud condensation nuclei” [SHE 05]; as a consequence urban generated
convective clouds may occur or convective clouds may be enhanced due to urban
effect. As pointed out by Bornstein and Lin [BOR 00] and Shepherd [SHE 05]
“bifurcating or diverting of precipitation systems” may also occur, but “to date there
is no conclusive answer as to what mechanism dominates urban induced
precipitation process or what the relative role, if any, of each mechanism.
Furthermore, how the urban environment modifies these processes is also poorly
understood”.
The deeper knowledge recently acquired on urban climate is due to a great extent
to the enormous progress in monitoring techniques, which have contributed the
development of modeling methods. The results obtained need to be transformed or
adapted to the different end-users. An overview of the urban climate monitoring and
assessment methods is given in the next section.
