Urban Climate News - FAU

Urban Projects 1
Figure 3. Diurnal variation of mean monthly urban-rural differences in air temperature (left) and relative humidity
(right).
a synoptic hour basis is presented in Fig. 4, which shows
that the Akure UHI contour patterns differ from the simple
idealized concentric structure around the city centre
during the dry and wet seasons respectively. Warm regions
extend over the city core to the south during the
cooler parts of the day (at night), while cooler regions
are located in the central vegetated park area and outer
fringes. The influence of different LCZs in the evolution
of the UHI in Akure is of special interest in this study. The
central Alagbaka park, LCZ 16 consisting of close-set 10-
15 year old deciduous Teak trees (Tectona grandis) and
the evergreen open-set FUTA trees, LCZ 15 (Magnifera
indica) create cool island effects of 5-8 o C during both dry
and wet seasons. The cooling effect is of the same magnitude
during the dry season, even though the deciduous
trees are devoid of leaves (DJF), but markedly different
during the wet season (JJA). This is because both
LCZs experience similar radiation regimes during the dry
season as the ovate leaf deciduous close-set trees behave
similar to the lance leaf evergreen open-set tree.
Relationship between sky view factor & heat island intensity
The value of SVF varies with individual canyons because
of subtle differences in the street width from canyon
to canyon. Figure 5 shows the relationship between
UHI intensity and SVF for all built-up areas during the
dry (solid diamonds) and wet (open squares) seasons of
2010, excluding LCZ 3 (concrete), LCZ 5(asphalt), LCZ 5
(grass lawn), LCZ 15 (evergreen trees) and LCZ 16 (deciduous
trees) that have extreme thermal regimes as a
result of differing surface properties in similar LCZs. The
observed relationship is better during the dry (R 2 = 0.62)
than wet (R 2 = 0.25) season. The strong relationship observed
in the dry season (time of maximum UHI) is similar
to those reported by Svensson (2004) and Yamashita et al.
(1986) in Göteborg, Sweden and the cities of Tama river
basin, Japan respectively. The weak relationship observed
during the wet season suggests that the relationship between
the UHI and SVF is not only dependent on the LCZs
but also on prevailing weather conditions such as clouds
and wind speed, as well as varying surface properties.
Conclusion
This study has investigated the impacts of urbanisation
on the local climate of Akure, a hot humid tropical
city in Nigeria, West Africa. The paper has illustrated that
land-use and land-cover changes are closely associated
with climatic responses. In this study, the potential use
of remote sensing data and GIS techniques in investigating
these changes has revealed some interesting findings.
Results of the five classes distinctly produced for
each study year indicated a rapid change in built-up
and arable land as they have increased tremendously
and the dense forest terribly depleted. Results further
revealed the modifying effect of urbanisation on the
city’s climate, resulting in the urban heat island phenomenon
and urban humidity deficits. A clear linear relationship
between the sky view factor and UHI intensity
is obtained, which is an indication that street geometry
and its regional distribution within the city play a fundamental
role in generating differential air temperature
patterns. Findings from this research have identified the
the potential of trees as a cost effective UHI and climate
change impact mitigation tool in Akure.
ISSUE NO. 46 DECEMBER 2012 INTERNATIONAL ASSOCIATION FOR URBAN CLIMATE