Study of the Seasonal Effect of Building Shadows on Urban Land Surface Temperatures Based on Remote Sensing Data

Abstract

Building shadows (BSs) frequently occur in urban areas, and their area and distribution display strong seasonal variations that significantly influence the urban land surface temperature (LST). However, it remains unclear how BSs affect the LST at the city scale because it is difficult to extract the shaded area at the subpixel scale and to connect such areas with the LST at the pixel scale. In this study, we combined the sun angle, building height, building footprint and building occlusion to extract the seasonal spatial distribution of BSs in the central area of Beijing. The effect of BSs on the LST was analyzed using LST retrieved from Landsat-8 thermal infrared sensor data. First, the relationship between the LST patch fragmentation and proportion of BSs in the sample areas was modeled without vegetation. Then, we quantitatively studied the mitigated intensity of the LST in pure impervious surfaces (IS) and vegetation pixels covered by BSs; next, we analyzed the LST sensitivity of these pixels to BSs. The results showed that the existence of BSs influences the fragmentation of the low LST patches strongly from summer to winter. On the other hand, pure IS pixels totally covered by BSs experienced a greater cooling effect, with 3.16 K on 10 July, and the lowest cooling occurred between 14 and 25 December, with a mean of 1.24 K. Without considering the relationship in winter, the LST is nonlinearly correlated to the building shadows ratio (BSR) in pixels, and an approximate 10% increase in the BSR resulted in decreases in the LST of approximately 0.33 K (mean of 16 April and 10 May), 0.37 K (10 July) and 0.24 K (28 September) for pure IS pixels, and 0.18 K, 0.20 K and 0.15 K, respectively, for pure vegetation pixels. Further analysis indicates that the LST of pure IS pixels is more sensitive to BSs than that of vegetation because the self-regulation mechanism of vegetation reduces the cooling effect of BSs. These findings can help urban planners understand the cooling characteristics of BSs and design suitable urban forms to resist urban heat islands (UHIs).

Assessing the impact of urban geometry on surface urban heat island using complete and nadir temperatures

“…The dependence of SUHI on the urban land cover has been studied often (Weng et al, 2004;Chen et al, 2006;Yuan and Bauer, 2007;Li et al, 2016). Recently, the dependence of SUHI on the geometry of the urban built-up space has also been addressed (Yu et al, 2019). The impact of shadows determined by urban geometry has been studied by Yu et al (2019) and results showed that in Beijing the building shadow reduced by 3.16 K the temperature of the urban impervious surface in July.…”

“…Recently, the dependence of SUHI on the geometry of the urban built-up space has also been addressed (Yu et al, 2019). The impact of shadows determined by urban geometry has been studied by Yu et al (2019) and results showed that in Beijing the building shadow reduced by 3.16 K the temperature of the urban impervious surface in July. More generally, the effect of urban morphology on radiometric surface temperature has been studied by Huang and Wang (2019) and the results showed that the urban geometric parameters have complex effects on the 2D and 3D pattern in urban radiometric surface temperature provided by Landsat thermal images.…”

The impact of building height on urban thermal environment in summer: A case study of Chinese megacities

“…In summer, building shade can cool urban temperature effectively by reducing total solar radiation in sunlight [ 38 , 39 ]. Furthermore, nearby buildings could partly be relieved from the hot sunlight owing to building shadows during the day.…”

Températures de surface et mesures mobiles confrontées aux zones climatiques locales : exemples des agglomérations de Tokyo et de Lyon

“…Les différences constatées entre les agglomérations de Lyon et de Tokyo proviendraient des morphologies urbaines propres aux deux mégapoles. En effet, à Tokyo, la hauteur et la densité du bâti étant nettement plus élevées que sur Lyon, les bâtiments ont donc une plus grande propension à intercepter les rayons solaires et ainsi, à créer un rafraîchissement en projetant leur ombre (Zhang et al, 2017 ;Sun et al, 2019 ;Yu et al, 2019). Cela a déjà été notifié dans différentes études, surtout dans les pays du sud-est asiatique (Lin et al, 2010 ;Chan, 2012 ;Martinelli et al, 2015).…”

“…Ce phénomène n'est pas un objet d'étude récent, les premières publications remontant aux années 1990 et 2000 (Oke, 1992 ;Lam, 2000). Il est cependant toujours très difficile d'évaluer quantitativement l'impact sur le bilan énergétique à l'échelle micro-locale (Lin et al, 2017 ;Yu et al, 2019). Dans un premier temps, à Tokyo, la classe des sols nus ou sablonneux se localisent préférentiellement dans les lits majeurs des cours d'eau.…”