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

Jonathan Fox,Alan Peters,Paul Osmond

doi:10.3390/cli6030056

Jonathan Fox, Alan Peters + Show 1 more

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https://doi.org/10.3390/cli6030056

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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) < rs > 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.

Highlights

Anthropogenic alterations to the optical, thermal, moisture and aerodynamic properties of city surfaces generate distinct urban climates, typically characterized by the urban heat island (UHI)effect [1]
The albedo of a surface is defined as its hemispherical and wavelength-integrated reflectance [50] and broadband albedo is the ratio of reflected to incident solar radiation (250–3000 nm), or the fraction of incident sunlight reflected by the surface quantified from 0 to 1 [61]
This paper described a novel application of the Emprical Line (EL) method for radiometric calibration of a relatively low-cost MS sensor applied to close-range images of vertical urban surface materials

Summary

Introduction

Anthropogenic alterations to the optical, thermal, moisture and aerodynamic properties of city surfaces generate distinct urban climates, typically characterized by the urban heat island (UHI)effect [1]. Anthropogenic alterations to the optical, thermal, moisture and aerodynamic properties of city surfaces generate distinct urban climates, typically characterized by the urban heat island (UHI). The UHI effect refers to hotter air (and surface) temperatures observed in cities compared to non-urban surroundings [2]. UHI spatial and temporal characteristics are influenced by synoptic weather conditions [3,4] but UHI formation is attributed to differences in urban surface structure (3-D geometry), cover (land use and permeability), fabric (optical and thermal properties of materials) and metabolism (human activity) compared to non-urban surroundings [5,6]. 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].

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