The Effect of Building Facades on Outdoor Microclimate—Reflectance Recovery from Terrestrial Multispectral Images Using a Robust Empirical Line Method

Abstract

Climate change and the urban heat island effect pose significant health, energy and economic risks. Urban heat mitigation research promotes the use of reflective surfaces to counteract the negative effects of extreme heat. Surface reflectance is a key parameter for understanding, modeling and modifying the urban surface energy balance to cool cities and improve outdoor thermal comfort. The majority of urban surface studies address the impacts of horizontal surface properties at the material and precinct scales. However, there is a gap in research focusing on individual building facades. This paper analyses the results of a novel application of the empirical line method to calibrate a terrestrial low-cost multispectral sensor to recover spectral reflectance from urban vertical surfaces. The high correlation between measured and predicted mean reflectance values per waveband (0.940 (Red) < r s > 0.967 (NIR)) confirmed a near-perfect positive agreement between pairs of samples of ranked scores. The measured and predicted distributions exhibited no statistically significant difference at the 95% confidence level. Accuracy measures indicate absolute errors within previously reported limits and support the utility of a single-target spectral reflectance recovery method for urban heat mitigation studies focusing on individual building facades.Climate 2018, 6, 56 2 of 26 UHIs develop in most cities regardless of climate [10] and have been observed globally in over 400 urban areas [4]. The magnitude of the screen height air temperature difference between urban and non-urban locations, or between different Local Climate Zones [10], is quantified by the "UHI intensity" which is most pronounced during calm, clear summer nights [2]. The average maximum UHI intensity for 87 European cities has been reported to be almost 6.2 K within a range of 2.8 K to 12 K [11]. Similar magnitude UHI intensities were reported for 101 Asian and Australian cities [12].The superimposition of inadvertent urban heating (i.e., UHIs) and global warming, including more frequent, longer and intense heat waves [13][14][15] elevates the risk of heat-related mortality and illness in cities [16][17][18] and extreme urban heat has detrimental outdoor comfort, economic and building energy impacts [19][20][21]. The intentional modification of urban surface geometry, cover and fabric to reduce urban heat and decouple heat waves from amplified UHIs [22,23] is now a policy priority for many cities [24,25]. Urban Heat Mitigation-Current Status and Cooling MagnitudeIn summer, solar absorption by urban surfaces is the dominant cause of the UHI effect [5]. Recent efforts to mitigate the formation of urban heat at different spatial scales have focused on changes to urban surface geometry and fabric [26][27][28] with the primary aim of controlling the absorption of solar radiation and increasing moisture availability [22,29]. However, due to the significant urban surface and air temperature reduction potential of reflective technologies [19,30] heat mitigation solely...

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Introducing Urban Overheating—Progress on Mitigation Science and Engineering Applications

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