Who would have guessed that a zebra-crossing could go viral without a band walking across?! But this year it happened to pictures of a road in Delhi, India as the asphalt softened under soaring heat-wave conditions that caused hundreds of deaths. The summer of 2015 again brought record-breaking temperatures and heat-waves in different parts of the world. Given the impact of such extreme weather conditions is particularly felt in cities, where the majority of the world’s population lives, it is important to understand the thermal characteristics of urban environments.
The urban surface is composed of a huge number of construction materials arranged in a complex three-dimensional structure (Figure 1). Due to the combination of impervious materials and building morphology, the urban surface has a great capacity to absorb and store energy, so that cities can heat up considerably in response to the incoming solar radiation. In combination with other energy exchange processes, such as anthropogenic heat emissions from human activities (buildings, vehicles, people), the urban surface is one of the key influences on the urban heat island (UHI) effect, whereby cities are warmer than their surroundings. The UHI is amongst the best known phenomena of climate conditions in cities.
Figure 1. The complex urban surface, here central London, is composed of a huge number of construction materials and vegetation, arranged in a spatially varying 3D structure. The urban canopy has a strong influence on the surface temperature patterns within a city.
Urban construction is often dominated by mineral-composite materials such as concrete, brick and asphalt, however, modern urban architecture increasingly uses metal and glass surfaces for outer building facets. While large windows and glass facades may have the advantage of providing extensive access to sunlight to the indoor environment, their high transmissivity can allow the inside of a building to heat up considerably as a response to the solar gain. The high thermal conductivity of metals can also contribute to efficient energy exchanges between a building and its surroundings where insulation is limited. In hot climates or during heatwaves, this can have severe impacts on people living and working in such buildings.
To counteract overheating, indoor temperatures are increasingly regulated using air conditioning and ventilation systems which are generally not considered sustainable methods as such systems may cause negative feedback. They directly contribute to urban warming by producing anthropogenic heat, and potentially to global climate change where greenhouse gases are emitted during the generation of the electricity required to operate them. Metal and glass surface effects further extend beyond the building itself given the high specular reflectance of these materials. Surrounding buildings may receive a large amount of the reflected energy or the thermal comfort of pedestrians passing by may be affected … remember London’s walkie-talkie building (20 Fenchurch Street) melting parts of nearby cars? Maybe one reason it recently got judged UK’s worst building?! Sustainable urban planning aims to reduce energy consumption and heat emissions, while providing a comfortable urban climate to the inhabitants under present and future climate conditions.
One environmental variable that provides valuable insights into thermal conditions across a city is the surface (skin) temperature of the urban canopy (the temperatures of all the surfaces components, i.e. roads, buildings, gardens, etc). It responds to the amount of solar radiation absorbed by the surface and the anthropogenic heat emitted, and serves as an indicator for the amount of energy being stored in the urban fabric which in turn is used to drive turbulent surface exchanges (heating the air and evaporating water). Given the significance of surface temperature to the heating of the lowermost atmosphere in which we live, it is a critical variable in many model parameterisations of energy exchange processes.
Given the complex, three-dimensional structure of the urban canopy, however, surface temperatures can vary significantly not only across a large urban area but even within a facet of one particular building – as a response to material characteristics and shading patterns. So how can such a complex system be characterised? It was the aim of the Fourth Annual Meeting of the EarthTemp network to provide a platform for international experts in the fields of thermal remote sensing and urban climate (Figure 2) to advance the understanding of urban surface temperatures. The three-day meeting was hosted at University of Reading by the Department of Meteorology during 8-10 June 2015. A series of keynote presentations and interactive discussion groups addressed critical aspects for observing and interpreting urban surface temperatures. On the first day of the workshop, the participants also performed some practical observations at the building of the Reading Enterprise Centre (REC) on Whiteknights campus. This building is an example of contemporary architecture, as it combines classical materials such as brick and wood with modern materials such as aluminium and coated glass.
Figure 2. Thermal infrared image of the participants of the Fourth Annual Meeting of the EarthTemp network at University of Reading in June 2015.
Surface temperatures are often observed using imaging devices with a response in the long-wave infrared. Operated on different platforms (i.e. satellite, airplane or ground-based) techniques can capture thermal characteristics at the local scale (~1 km) down to the micro-scale (~ 1 m – 1 cm). The processing of the remotely sensed imagery faces special challenges over urban areas, mainly due to the three-dimensional structure and the complexity of building materials.
Orientation and shading of a facet dictate access to solar insolation so that surface temperatures vary significantly across the canopy. Due to this thermal anisotropy, the viewing geometry of the remote sensor plays a particularly important role. The latter further determines which part of the canopy is sampled as the three-dimensional structure can never be captured from one direction alone.
Both morphology and material composition influence the emissivity of the urban surface, a variable required to derive surface temperature from the observed radiance signal. Multiple reflections between buildings can increase the absorption due to ‘radiation trapping’, while a large number of materials with different radiative properties may contribute to one pixel in the remotely sensed image. The effect of multiple reflections of thermal radiation is nicely illustrated by a thermal image taken at the REC building during the EarthTemp exercise (Figure 3): thermal radiation emitted by the neighboring wood wall and the cold sky is reflected by the aluminium and glass surfaces that can be identified in the visible image.
Figure 3. Thermal infrared image (left) and visible image (right) of a wood and an aluminium wall at the Reading Enterprise Centre (REC) building observed during the practical exercise of the Fourth Annual Meeting of the EarthTemp network at University of Reading, 8 June 2015.
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