Driving forces of UHI changes in China's major cities from the perspective of land surface energy balance

From city to countryside: Unraveling the long-term complex effects of urbanization on vegetation growth in China

The urban environment is the "natural laboratory" of the global ecosystem, and it has complicated effects on vegetation growth, including direct effects (land use transformations) and indirect effects (climatic environment changes). However, the long-term responses of vegetation to urbanization and its associated controlling factors across different spatial scales, from pixels to regions, remains unknown. Here, we unraveled the dual influence of urbanization on vegetation growth and its potential drivers along the urban development gradients in China with satellite observations of leaf area index (LAI) during 2000-2020. The results showed that 65.68 % of pixels in whole China exhibited an increasing trend in vegetation growth, with prominent greening (as indicated by LAI increases) in rural background areas (0.198/10a), significant greening in urban core areas (0.0343/10a), and significant browning in suburban areas (-0.0391/10a). As the process of urbanization intensified, the relationship between urbanization and vegetation growth became increasingly complex, transitioning from linear to non-linear interaction. The overall direct effects of Chinese cities were negative and increased annually. Meanwhile, the positive indirect effects of urban environments on vegetation growth initially declined and then recovered. Cities with high urbanization level (urbanization rate, ULp >70 %) had higher indirect effects (0.24 %) and growth offsets (0.98 %) than that with moderate (ULp = 60 %-70 %) and low urbanization levels (ULp <60 %) (0.18 %, 0.10 %). In economically developed cities, land use changes from construction to vegetation, influenced by urban policies and management strategies, positively impacted urban greening. Overall, more urbanized cities (ULp >70 %) experienced vegetation growth enhancement due to more intense land use changes, whereas less urbanized cities (ULp <70 %) showed the opposite trends. Understanding the direct and indirect effects of urbanization on vegetation growth is crucial for devising effective urban planning and environmental conservation policies. It can help guide future urbanization processes and minimize adverse effects on the natural environment.

Anthropogenic heat variation during the COVID-19 pandemic control measures in four Chinese megacities

Anthropogenic heat (AH) is an important input for the urban thermal environment. While reduction in AH during the Coronavirus disease 2019 (COVID-19) pandemic may have weakened urban heat islands (UHI), quantitative assessments on this are lacking. Here, a new AH estimation method based on a remote sensing surface energy balance (RS-SEB) without hysteresis from heat storage was proposed to clarify the effects of COVID-19 control measures on AH. To weaken the impact of shadows, a simple and novel calibration method was developed to estimate the SEB in multiple regions and periods. To overcome the hysteresis of AH caused by heat storage, RS-SEB was combined with an inventory-based model and thermal stability analysis framework. The resulting AH was consistent with the latest global AH dataset and had a much higher spatial resolution, providing objective and refined features of human activities during the pandemic. Our study of four Chinese megacities (Wuhan, Shanghai, Beijing, and Guangzhou) indicated that COVID-19 control measures severely restricted human activities and notably reduced AH. The reduction was up to 50% in Wuhan during the lockdown in February 2020 and gradually decreased after the lockdown was eased in April 2020, similar to that in Shanghai during the Level 1 pandemic response. In contrast, AH was less reduced in Guangzhou during the same period and increased in Beijing owing to extended central heating use in winter. AH decreased more in urban centers and the change in AH varied in terms of urban land use between cities and periods. Although UHI changes during the COVID-19 pandemic cannot be entirely attributed to AH changes, the considerable reduction in AH is an important feature accompanying the weakening of the UHI.

Co-occurrence of urban heat and the COVID-19: Impacts, drivers, methods, and implications for the post-pandemic era

Cities, the main place of human settlements, are under various mega challenges such as climate change, population increase, economic growth, urbanization, and pandemic diseases, and such challenges are mostly interlinked. Urban heat, due to heatwaves and heat islands, is the combined effect of climate change and urbanization. The COVID-19 is found to be a critical intervention of urban heat. However, the interrelationship between COVID-19 and urban heat has not been fully understood, constraining urban planning and design actions for improving the resilience to the dual impacts of heat and the pandemic. To close this research gap, this paper conducted a review on the co-occurrence of urban heat and the COVID-19 pandemic for a better understanding of their synergies, conflicts or trade-offs. The research involves a systematic review of urban temperature anomalies, variations in air pollutant concentrations, unbalanced energy development, and thermal health risks during the pandemic lockdown. In addition, this paper further explored data sources and analytical methods adopted to screen and identify the interventions of COVID-19 to urban heat. Overall, this paper is of significance for understanding the impact of COVID-19 on urban heat and provides a reference for coping with urban heat and the pandemic simultaneously. The world is witnessing the co-existence of heat and the pandemic, even in the post-pandemic era. This study can enlighten city managers, planners, the public, and researchers to collaborate for constructing a robust and resilient urban system for dealing with more than one challenges.

Variations of Urban Thermal Risk with Local Climate Zones

Due to the differences in land cover and natural surroundings within cities, residents in various regions face different thermal risks. Therefore, this study combined multi-source data to analyze the relationship between urban heat risk and local climate zones (LCZ). We found that in downtown Shenyang, the building-type LCZ was mainly found in urban centers, while the natural- type LCZ was mainly found in suburbs. Heat risk was highest in urban centers, gradually decreasing along the suburban direction. The thermal risk indices of the building-type LCZs were significantly higher than those of the natural types. Among the building types of LCZs, LCZ 8 (open middle high-rise) had the highest average thermal risk index (0.48), followed by LCZ 3 (0.46). Among the natural types of LCZs, LCZ E (bare rock and paved) and LCZ F (bare soil and sand) had the highest thermal risk indices, reaching 0.31 and 0.29, respectively. This study evaluated the thermal risk of the Shenyang central urban area from the perspective of LCZs and combined it with high-resolution remote sensing data to provide a reference for thermal risk mitigation in future urban planning.

Industrial heat island mitigation in Angul-Talcher region of India: Evaluation using modified WRF-Single Urban Canopy Model

Linkages of urban and industrial cooling with sustainable development goals and climate change perspectives are well acknowledged, mainly for developing economies in tropical climates. Angul-Talcher region is one of the oldest industrial clusters of India, and the region experiences higher atmospheric heat island intensities with magnitudes of 7 to 9 °C attributed to the Industrial Heat Island (IHI) effect. In the present study, various measures for mitigating heat island effect in the region and assessed their impact using an Improved Weather Research and Forecasting model coupled with the Single-Layer Urban Canopy Model. The improved framework includes the release of industrial emissions at stack height and sector-wise diurnal profiles of anthropogenic heat (AH) released from vehicles, residential, and industry/power. The mitigation measures comprised strategies like alteration in building materials and conversion of landuse-landcover (LULC) of selected grid cells in the model domain to more vegetation or water bodies. It was noted that the cool roofs and walls together reduced IHIs by 0.5 °C, while green roofs and cool pavements achieved a reduction of 0.3 °C and 0.1 °C, respectively. The introduction of water bodies showed maximum reduction in IHIs by 3 to 5 °C during daytime and 1 to 2 °C over mining and industrial stations. During night-time, conversion to mixed forests was more effective (ΔT ≈ 1 °C) than conversion to water bodies. A combination of cool roofs with the introduction of water bodies in the mining areas and mixed forest patches in industry stations was found to be the most effective mitigation strategy for mitigating the industrial heat island effect over the Angul-Talcher region. These mitigation scenarios can/should serve as a theoretical reference for implementing actual mitigation measures, which would require consideration of economic, social, and policy aspects apart from scientific ones for practical application.

Improvement Strategies for Microclimate and Thermal Comfort for Urban Squares: A Case of a Cold Climate Area in China

Urban squares are an important part of a city’s overall spatial environment. However, many urban squares lack rational designs, causing the thermal environment to deteriorate. To ensure sustainable urban development, urban square microclimates should be improved. Given that, this study investigates the effects of three coverages of three landscape elements of urban squares through modeling and simulation using the ENVI-met model validated by field measurements. The correlation between physiological equivalent temperature (PET) and different amounts of landscape elements is investigated using Spearman analysis. This study presents a case study of a typical urban square in a cold climate area. Design strategies in the area are proposed. The results show that the microclimate and thermal comfort of the urban square can be improved by expanding water bodies, modest increasing buildings and optimizing vegetation. Vegetation is the most important landscape element affecting thermal comfort in the urban square. The PET can be reduced by about 1.5 °C by increasing the vegetation cover from 40% to 70%. However, the degree of microclimate regulation by vegetation is disturbed by water bodies and buildings (|ρ| ≥ 0.5). Therefore, to achieve a more comfortable thermal environment, a combination of landscape elements should be considered.