URBAN CLIMATE




Urbanisation creates cities that have higher concentrations of man-made structures and materials, and less natural vegetation. This produces a hotter and more polluted urban climate as human activities generate heat and pollutants that are trapped and disperse slower to the surroundings. The imbalance results in a phenomenon known as the Urban Heat Island (UHI) effect, whereby there are marked differences in air temperatures between the city centers and the vegetated rural surroundings, as shown in Figure 1 below.


Figure 1 UHI profile in Singapore (Wong and Chen, 2003).


The satellite image in Figure 2 below shows the air temperature differences between man-made areas and vegetated areas in Singapore.



Figure 2 Satellite infrared image of Singapore’s Heat Island effect (Wong and Chen, 2009).


Measurement campaigns are undertaken to understand the significance of the UHI phenomenon. Figure 3 below shows weather measurements being carried out on the roof to capture the ambient weather conditions, and near pedestrian level in the city centre to capture conditions inside street canyons.



Figure 3 The outdoor weather measurement setup on the roof (left) and close to pedestrian level (right).


Pedestrian level measurements are done to understand the thermal environment surrounding the human body, as shown in Figure 4 below. At that level, the biggest heat contribution is from vehicles. Figure 5 shows the impact of vehicles on the air temperature differences between weekends with less vehicular presence and weekdays with more vehicular presence.



Figure 4 Roadside measurement tripod with meteorological sensors to capture pedestrian level conditions.




Figure 5 Average air temperature recorded at 1.5m above ground during weekdays (blue) and weekends (orange) for peak hour period (5:45PM to 6:15PM) for nine sunny days.


Thermal comfort measurements and surveys are also important to understand the impact of environment conditions on humans. Figure 6 shows the typical setup of conducting surveys.



Figure 6 Thermal comfort surveys done with environment measurements on the ground.


The high reflective surfaces of metal and glass can also cause discomfort due to glare. Figure 7 below shows a setup indoors to measure how glare from opposite structures and buildings affect conditions indoors.



Figure 7 Indoor glare measurement setup.



Figure 8 below shows the profile of air temperatures at pedestrian height at the Kent Ridge campus.




Figure 8 Air temperature contour map at pedestrian height at the Kent Ridge campus, with cooler zones in blue and hotter zones in red


Mitigation Strategies

URBAN VENTILATION

Urban ventilation is a passive strategy whereby buildings are designed to allow prevailing wind directions to flow through the site. Figure 9 below shows the North East prevailing wind direction blowing through the Kent Ridge campus.



Figure 9 Wind velocity contour (top) and vector (bottom) of the Kent Ridge campus.


COOL PAINT

Cool paint on horizontal surfaces is a good strategy to reflect solar radiation back to the atmosphere. Figure 10 shows the application of cool roofs and the comparison of surface temperatures against conventional roofs.



Figure 10 Infrared thermal imaging of conventional and cool roofs.


GREENERY

Greenery is important to minimise solar exposure on man-made surfaces in the urban environment by providing shade and evaporative cooling to lower the immediate air temperature. Figure 11 below shows the impact near a single tree with and without transpiration rate. Volume weighted average air temperature under the tree canopy shows a maximum of 0.8˚C reduction under canopy.



Figure 11 Single tree cooling effect. From the top (Case 1 - no tree scenario), middle (Case 2 - tree without transpiration rate) and bottom (Case 3 - tree with transpiration rate).

Figure 12 below shows the impact of multiple trees in a precinct called Matilda at 2m above ground. The area weighted average air temperature shows a maximum of 1.9˚C reduction.



Figure 12 Multiple trees cooling effect. From the top (Case 4 – precinct with no tree scenario), middle (Case 5 – precinct with trees without transpiration rate) and bottom (Case 6 – precinct with trees with transpiration rate).


REFLLECTED HEAT

In a high density tropical environment like Singapore, reflection of solar radiation via reflective surfaces, such as metal and glass, causes discomfort due to glare and increase in surface temperature of the surroundings. By reflecting solar radiation back to the atmosphere, for instance, using the retro reflective film as shown in Figure 13 below, both heat and discomfort glare can be minimised.



Figure 13 Results of the reflected shortwave (300nm to 2800nm) solar radiation to an upward direction.


DRY MIST

Evaporative cooling via misting sprays can help minimise the air temperature around its immediate surroundings. Computational Fluid Dynamics (CFD) simulations are conducted to understand the impact at urban canyon level. For low density areas (Height-to-Width (H/W) ratio ≤ 1.0), a windward wall injection of water droplets provided more effective air cooling at the pedestrian level than a leeward wall injection as shown in Figure 14 below.



Figure 14 The parametric study of dry mist effect on air temperature at Height-to-Width (H/W) ratio of 0.4 and 1.0.

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