Geoscience Reference
In-Depth Information
7.4.3 The urban heat island circulation (UHIC)
When synoptic conditions are calm or very weak, a local urban circulation, the
UHIC, can form, controlled by the difference in temperature between the urban
center and the rural surroundings (Haeger-Eugenssen and Holmer 1999 ). The
higher temperatures in the city induce a pressure differential, and air from the
surrounding rural areas is attracted toward the urban center (Oke 1995 ). This air
becomes modified in speed and direction by the urban morphology. Once it
reaches the UHI center, the increased heating and turbulence causes it to rise.
At the boundary layer inversion, it then spreads outward, back to the country-
side, where it cools and sinks. Variations in the number and strength of convec-
tion cells can add to the mixing and turbulence, creating an intermittent and
variable UHIC.
The UHIC forms most strongly in the daytime because solar heating favors
greater vertical convection (Oke 1995 ). In Mexico City, there is a relationship to
high ozone levels, and the stable synoptic-scale meteorology that creates these.
Overall, there is no significant correlation with change in boundary layer thermal
structure with height, or the difference in temperature between the bottom and
top of the inversion. Shallow surface inversions, however, are much more likely
to create stronger UHIs than deeper inversions, but shallow inversions may
interfere with UHIC development (Jauregui 1997 ). Clouds and rain showers
will reduce the urban-rural temperature contrast, reducing the UHIC.
When UHIs were stronger than 2.5 8C, an urban circulation system was
established over G¨ teborg on winter nights (Eliasson and Holmer 1990 ).
Development also depended on a Q*of 90Wm 2 or less, and wind speeds
< 3ms 1 . A strong inversion (at least 4 8C), located at an average altitude
between 40 and 60 m, separated the regional airflow from that close to the
surface. The UHIC occurred in stages. Early in the evening, there was significant
differential cooling between the city center and the rural areas, with the latter
cooling very rapidly compared to the former. The UHIC started about two hours
after the UHI was established (4 to 6 hours after sunset), bringing a sudden
increase in urban cooling as the circulation transferred sensible heat from the
rural environment. Cooling rates in both the urban and rural environments then
equalized (at about 0.5 K h 1 ) and the UHIC became self-sustaining until morn-
ing. Any change in one component of the circulation/heat differential system
(for example wind speed) required compensatory changes in the other compo-
nents to maintain UHIC equilibrium.
In the eastern part of G¨ teborg, the surface airflow is toward the center of the
city, but in the western part of the city, the urban circulation flows through
toward the ocean. Its spatial extent is determined by the valleys to the north, and
other, more coastal, airflows to the south. The urban circulation does not extend
more than 13 km outside the city center. It can be difficult to distinguish from
topographic airflows. UHIC development at night can bring cooler air into the
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