Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 233

periods suggested the existence of an urban cool island (UCI - warmer rural than
urban temperatures). On a seasonal basis, the average UHI was 1.29 in summer,
1.13 in spring, 1.02 in autumn, and 0.98 in winter. The differences between
spring/summer and autumn/winter were statistically significant. Diurnally there
was little change in the mean UHI between 10:00 p.m and 6:00 a.m. This study
allowed an evaluation of the total range of weather conditions on UHI, rather
than just a focus on extreme conditions.
The dominant frequency of UHI values occurred between 0 and 2 8C (on about
75% of the days). The average pressure anomaly for the 0-1 8C UHI was
statistically significant at 2.12 hPa, representing a low pressure center between
28 and 488 S (SE of Tasmania). For the 1-2 8C UHI, the average pressure
anomaly was a slightly positive (non-significant) 1.73 hPa, associated with the
high pressure center over and just east of Tasmania. Wind speed averaged
3-4m s 1 and cloud cover 4.5 to 5.6 oktas. The influence of cloud cover and
wind speed on UHI development and strength was mixed during these periods,
with cloud cover tending to have a dominant influence under calmer conditions,
and wind speed the dominant influence under clear or slightly cloudy skies.
UCI conditions were dominated by high pressure over eastern Australia,
centered over the Tasman Sea (32 to 488 S). The pressure anomalies averaged
5.06 hPa for UCI < 1.0 8C (significant), and 3.9 hPa for 1 UCI < 0. A cooler
urban environment was associated with the prevention of warmer (prefrontal)
NW airflow, which reached the rural airport sites, but was prevented from
entering the city by the urban area. This situation was associated with higher
than average minimum temperatures. Often, with a change in the synoptic
situation, these periods were immediately replaced by UHI conditions, asso-
ciated with the release of heat storage from the urban area. UCI conditions were
more frequent between March and July, the winter half of the year.
At the other extreme, the 17% of the data set exhibiting a strong UHI ( > 2 8C)
also showed highly significant mean pressure anomalies (3.6 and 4.9 hPa
respectively). The mean location of the pressure center was over Bass Strait,
just east of Melbourne but somewhat north of Tasmania. These conditions are
typical of the expected UHI development from the literature under high pressure
and a stable atmosphere, and the meteorological data supported this conclusion.
Airflow was often warm, and weak ( 1.5 m s 1 ) from the N or NE, with cloud
cover less than 3 oktas, creating optimum UHI conditions. Under these situa-
tions, the UHI averaged 2.7 8C. Wind speed was the more important controller of
UHI development. These periods occurred with greatest frequency between
October and March, the summer half of the year.
A UHI did exist under a cloud cover of 8 oktas and a wind speed of > 5m s 1 ,
averaging about 0.5 8C. The main controller was cloud cover, with diminishing
cloud leading to increases in UHI. Overall, although both the wind speed and
cloud controllers were not strong statistically, cloud cover tended to be more
important. The contributions from each tended to be independent of the other.