Quantifying the spatial pattern of urban heat islands and the associated cooling effect of blue-green landscapes using multisource remote sensing data

Planning for a cooler metropolitan area: a perspective on the long-term interaction of urban expansion, surface urban heat islands and blue-green spaces' cooling impact

Urbanization is widely acknowledged as a driving force behind the increase in land surface temperature (LST), while blue-green spaces (BGS) are recognized for their cooling effect. However, research on the long-term correlation between the two in highly urbanized areas remains limited. This study aims to fill this research gap by investigating the correlation and changes between urban expansion-induced LST rise and the cooling effect of BGS in the Hangzhou metropolitan area from 2000 to 2020. Our approach combines Geographic Information System (GIS), Remote Sensing (RS), and Google Earth Engine (GEE) cloud platforms, utilizing a random forest land use classification technique in conjunction with the Geographically and temporally weighted regression (GTWR) model. The findings reveal a strong relationship between land expansion and the intensification of the surface urban heat island (SUHI) effect. The spatial heat island effect exhibits an exponential expansion in area, with an interannual LST rise of 0.4 °C. Notably, urban centers exert the highest regional heat contribution, while remote suburbs have the most significant impact on reducing LST. The impact of BGS on LST varies, fluctuating more in areas close to urban centers and less in water-rich areas. This study contributes to a better understanding of the cooling potential of BGS in rapid urbanized Metropolitan, offering valuable insights for sustainable urban planning.

Urban core greening and industrial decentralization lead to contrasting trends in surface urban heat islands in a metropolitan area in China

Surface urban heat islands (SUHI) in urban agglomerations display diverse spatiotemporal patterns, particularly in rapidly urbanizing regions, where these patterns are not well understood. This study examined the spatiotemporal trends of SUHI intensity (SUHII) on the west bank of the Pearl River Estuary, China, from 1990 to 2020, focusing on spatial variations within urban core (UC) and urban expansion (UE) areas and their driving mechanisms. Results show that urban areas expanded rapidly, leading to the formation of a regional heat island, with SUHI intensity varying across the region and the hottest areas shifting from the UC to the UE. In the UE, SUHII increased significantly due to farmland conversion to impervious surfaces and a decline in the Normalized Difference Vegetation Index (NDVI), driven by industrial suburbanization. In contrast, SUHII in the UC exhibited a marked decline, attributable to increased vegetation greening and urban renewal initiatives. These findings reveal long-term trends in SUHI evolution and underscore the significant impact of industrial decentralization and urban renewal on the SUHII dynamics. The study provides critical insights for urban planning and strategies to mitigate SUHI in metropolitan regions.

Impact of newly constructed parks on urban thermal environment: A comparative analysis of 20 parks before-and-after construction

The urban heat island (UHI), exacerbated by urbanization and warming, poses a threat to urban residents' health. While urban parks are key to UHI mitigation, their cooling effects and dynamics of newly constructed parks remain poorly understood. In this study, 20 new parks in Hangzhou were selected to investigate the characteristics of urban land surface temperatures (LST) alterations before-and-after park construction by using the single-window inversion on three years of Landsat 8 satellite imagery. Correlation analysis and Random Forest model were utilized to explore the influence of park attributes and surrounding landscapes on their cooling impact. The results indicate that newly constructed parks significantly reduce urban LST: by 0.31 °C inside the park and 0.64 °C in its surroundings, reaching 0.84 °C and 1.08 °C as parks mature. From 2022 to 2023, average park cooling distance (PCD) increased from 104.40 m to 147.50 m, park cooling area (PCA) expanded from 14.11ha to 17.88ha, park cooling intensity (PCI) rose from 0.64 °C to 1.08 °C, and park cooling efficiency (PCE) increased from 4.66 to 7.30. Correlation analysis and Random Forest model reveal that PCA is mainly affected by park area, while PCD, PCE, and PCI are mainly affected by landscape structure of surrounding building patches. These findings deepen understanding of park cooling dynamics, offering a clearer direction for maximizing park cooling benefits.

Evaluating the benefits of urban green infrastructure: Methods, indicators, and gaps

Green infrastructure (GI) offers a promising solution for mitigating the adverse effects of climate change, but evaluating its effectiveness necessitates a comprehensive understanding of how that has been quantified in the literature. This study aims to review the methods (monitoring, remote sensing, and modelling) employed to assess the effectiveness of GI in urban areas for three ecosystem services: heat mitigation (cooling of air temperature), thermal comfort control, and air quality mitigation. The objectives include evaluating the suitability of these approaches across diverse scales, categorising the essential parameters, and identifying the strengths and limitations inherent in each method. Through a literature review, 126 research papers were selected for detailed analysis. Modelling was the dominant method for heat mitigation (45.6 %), thermal comfort (70 %), and air pollution (51.9 %). The main inputs for assessing these three ecosystem services by GI were: meteorological parameters used in monitoring or modelling, morphological parameters (describing vegetation, surface, and built-up area conditions), specified parameters depending on the evaluated benefit such as landscape metrics (for heat mitigation), personal factors (for thermal comfort), pollutant measures (for air pollution), and other parameters (e.g. building and traffic heat emissions). The application scale of each method was dependent on the instruments, satellite data, and simulation tools utilised. Monitoring methods were employed in studies ranging from street-scale to neighbourhood-scale, remote sensing methods covered city-scale to regional-scale assessments, and modelling studies spanned from street-scale to regional-scale analyses. These diverse methods used to assess the GI benefits each have individual strengths and limitations which need to match the context and objectives of the study.

Enhancing the city-level thermal environment through the strategic utilization of urban green spaces employing geospatial techniques

Smart urban planning needs to have a multicriteria-based approach to prevent the deteriorating local thermal climate. Maximizing the cooling potential using the available grey infrastructure would be the utmost priority of future smart cities. Remote sensing and GIS can be the appropriate tools to develop a climate-resilient urban planning framework. Studies are needed to include different features of vertical and horizontal landscaping to mitigate heat stress and enhance liveability at the city level. With this goal, the current work outlined a holistic approach to efficiently using green spaces with minimal reconstruction. The problem of regional climate threat was evaluated with urban heat island characterization. Moran's I clustering identified nearly 12% of the study area to be under considerable heat stress during summer days. Multiple techniques, such as mapping local climate zones, segment mean shift-based roof extraction, vegetation index computation, solar azimuth-based green wall site selection, etc., were applied to formulate solutions and provide an integrated method for city-level environment enhancement. A considerable area was identified as most suitable for green roof cover, and it was also computed that the transition towards green roof at only these locations may bring down the maximum heat island intensity by 0.74 °C. Additionally, solar zenith, illumination effect, and building height information were combined to create a distinct method where vertical plantation would flourish exceptionally. A rigorous assessment of more than 130 urban green spaces further quantified the relation between landscape geometry and cooling effect to provide optimum green space designs for future urban planning.

Exploring the combined cooling effect of street canyon geometry and the surrounding built environment

Exploring the impact of complex urban morphology on the urban heat island (UHI) effect is essential for sustainable environmental management and enhancing human well-being. This study explored the combined cooling effect of street canyon geometry and the surrounding built environment using a CatBoost model and the Shapley method. The findings indicated that in streets with low building height and density, a high proportion of sky and vegetation and a flatter skyline are conductive to mitigate UHI effect. In streets with high building height and density, a lower proportion of sky and vegetation, and a well-proportioned skyline, can effectively mitigate UHI effect. Regardless of the building density and height around the street, street trees are the optimal choice for greening construction and improvement of large and medium-sized cities in China, given their high controllability and the current urban stock background. Therefore, reasonable control and allocation of street trees can effectively adjust the street canyon geometry, providing suitable cooling strategies for streets with different surrounding built environments. This study proposed a method to mitigate the UHI effect through street canyon geometry, which can be extended to other high-density urban thermal environment studies and guide policymakers on street construction and urban design.

Construction of a cold island network for the urban heat island effect mitigation

The urban heat island (UHI) effect seriously challenges sustainable urban development strategies and livability. Numerous studies have explored the UHI problem from the perspective of isolated blue and green patches, ignoring the overall function of cold island networks. This study aims to explore the construction method of cold island network by integrating scattered cold island resources, rationally guiding urban planning and construction, and providing effective ideas and methods for improving the urban thermal environment. Taking the central city of Fuzhou as an example, the identification of the cold island core source (CICS) was optimized by applying relative land surface temperature (LST), morphological spatial pattern analysis, and landscape connectivity analysis. The combined resistance surface was constructed based on a spatial principal component analysis. Subsequently, the cold island network was constructed by applying circuit theory and identifying the key nodes. The results showed that the central and eastern parts of the study area experienced the most significant UHI effects and there was a tendency for them to cluster. Overall, 48 core sources, 104 corridors, 89 cooling nodes, and 34 heating nodes were identified. The average LST of the CICSs was 28.43 °C, significantly lower than the average LST of the entire study area (31.50 °C), and the 104 cold corridors were classified into three categories according to their importance. Different targeting measures should be adopted for the cooling and heating nodes to maintain the stability of the cold island network and prevent the formation of a heat network. Finally, we suggest a model for urban cold island network construction and explore methods for mitigating issues with UHI to achieve proactive and organized adaptation and mitigation of thermal environmental risks in urban areas, as well as to encourage sustainable urban development.

The Application of Nature-Based Solutions for Urban Heat Island Mitigation in Asia: Progress, Challenges, and Recommendations

PURPOSE OF REVIEW

Unprecedented urbanization in Asia affects the net radiation and energy flux of urban areas in the form of urban heat islands (UHI). The application of nature-based solutions (NbS) via urban green and blue infrastructures is a promising approach to mitigate UHI via urban boundary condition modifications, which affect the energy balance. This narrative review discusses the application of green and blue infrastructures in the Asian context by highlighting its progress, challenges, and recommendations. This review is descriptive in nature and includes perspectives on the discussed topics.

RECENT FINDINGS

Studies on the application of green and blue infrastructures in UHI mitigation are still scant in Asia. Their cooling performance is greatly influenced by their types, size, geometry, surface roughness, spread (threshold distance), temporal scales, topography, pollution levels, prevailing climate, and assessment techniques. Distinct urban characteristics, climatic conditions, environmental risks, lack of awareness and expertise, lack of policy and government incentives, and limited scientific studies are the major challenges in their implementation of UHI mitigation in Asia. Although green and blue infrastructures are associated with urban cooling, more in-depth experimental work and multidisciplinary research collaboration are paramount to exploring its implementation potential in Asia and other countries that share similar urban and environmental characteristics.

Efficacy assessment of green-blue nature-based solutions against environmental heat mitigation

Nature-based solutions (NBS) such as green (vegetation) and blue (waterbodies) infrastructure are being promoted as cost-effective and sustainable strategies for managing the heatwaves risks, but long-term monitoring evidence is needed to support their implementation. This work aims to conduct a comparative assessment of the cooling efficiency of green (woodland and grassland) and blue (waterbody) NBS in contrast to a built-up area. Over a year of continuous fixed monitoring showed that the average daily maximum temperatures at NBS locations were 2-3 °C (up-to 15%) lower than the built-up area. Woodland showed the maximum temperature reduction in almost all seasons, followed by waterbody and grassland. NBS performed the best during the summers, peak sunshine, and heatwave hours (up to ∼ 6 °C cooler than built-up area). Using an e-bike for mobile monitoring, the areas where green-blue NBS were combined showed the highest spatial cooling extent, followed by waterbody, woodland, and grassland areas. The database generated can validate city-scale environmental models and assist city planners to incorporate NBS into urban dwellings based on the opportunity, need and scope, aligning with Sustainable Development Goals 11 (sustainable cities and communities) and 13 (climate action).

The climate adaptive characteristics of urban inside/outside water bodies based on their cooling effect in Poyang and Dongting lake regions, China

Most publications have focused on the cooling effect of urban inside water bodies. However, the climate adaptive characteristics of urban inside/outside water bodies is seldom studied. In this paper, three types of water bodies, i.e., urban inside water bodies, urban outside discrete water bodies and large water bodies are identified according to their relative spatial relationships with built-up areas. The climate adaptive landscape characteristics of water bodies are analyzed based on water bodies' cooling effect (WCE) inside and outside cities in the Poyang Lake and Dongting Lake regions. Seventy-three Landsat TM/OLI/TIRS images acquired from 1989 to 2019 are employed. Landscape scale characteristics of urban inside/outside water bodies are described by area, water depth, perimeter to area ratio (PARA) and distance-weighted area index (DWAI). Three temperature-related parameters are calculated to estimate the WCE in different conditions. Climate adaptive characteristics of water bodies inside/outside cities are determined by correlation and regression analysis. Results show that: 1) The long river shape, depth, orientation and fluidity of urban inside water bodies are benefit to enhance their cooling effect; 2) the distance of urban outside water bodies from built-up areas are positive correlated with their cooling effect; 3) the optimal acreage of large water bodies are >2500 km² and 1111-1287.5 km² for climate adaption of Poyang Lake and Dongting Lake, respectively. Simultaneously, the WCE of urban outside large water bodies is related with human activities and climate conditions. The results of our study provide a significant contribution to blue-space planning in cities, and provide insights into actionable climate adaption planning in inland large lake areas.

Evaluating the benefits of ecosystem-based urban cooling using a dynamic "on-site" method

Ecosystem-based cooling helps residents cope with the urban heat-island problem. In order to improve the accuracy of traditional heat-island measurements based on comparisons between urban and rural areas, we use an "on-site" method developed with only urban data. The essence of this method is a regression analysis of the relationships among different types of green space and blue space, elevation, vegetation dynamics, and temperature. We then simulate the temperature pattern in a scenario where there is no built-up area (Scenario A), and then in another scenario where there are no ecological spaces (Scenario B). The gap between the actual temperature pattern and the simulated temperature pattern of Scenario A is considered the heat-island effect. Conversely, the gap between the actual temperature pattern and that of Scenario B is considered as the effect of ecosystem-based urban cooling. This method was tested using data from two megacities in China (each had a population of over 10 million people). For Beijing, the average heat-island effect was 4.87 °C and effect of the ecosystem cooling service was 9.07 °C. For Shenzhen, the respective values were 0.8 °C and 2.71 °C. The "on-site" (local small size sampling), "dynamic coefficient", and "no-positive-coefficient rule" are the three defining characteristics of this method. The application of this method to model ecosystem-based urban cooling can aid urban planning and management in improving the residential thermal environment.

A Quantitative Study of a Directional Heat Island in Hefei, China Based on Multi-Source Data

Surface urban heat islands (SUHIs) are essential for evaluating urban thermal environments. However, current quantitative studies of SUHIs ignore the thermal radiation directionality (TRD), which directly affects study precision; furthermore, they fail to assess the effects of TRD characteristics at different land-use intensities, on the quantitative studies of SUHIs. To bridge this research gap, this study eliminates the interference of atmospheric attenuation and daily temperature variation factors, in quantifying the TRD based on land surface temperature (LST), from MODIS data and station air temperature data for Hefei (China) from 2010-2020. The influence of TRD on SUHI intensity quantification was evaluated by comparing the TRD under different land-use intensities in Hefei. The results show that: (1) daytime and nighttime directionality can reach up to 4.7 K and 2.6 K, and occur in areas with the highest and medium urban land-use intensity, respectively. (2) There are two significant TRD hotspots for daytime urban surfaces, where the sensor zenith angle is approximately the same as the forenoon solar zenith angle, and where the sensor zenith angle is near its nadir in the afternoon. (3) The TRD can contribute up to 2.0 K to the results of assessing the SUHI intensity based on satellite data, which is approximately 31-44% of the total SUHI in Hefei.

Identifying and characterizing frequency and maximum durations of surface urban heat and cool island across global cities

Surface urban heat island (SUHI) has been widely reported from a local to global scale. However, variations and controls of temporal indicators for SUHI and SUCI (surface urban cool island) remain unclear. This paper firstly reconstructed the seamless daily LST (land surface temperature) based on ATC-SKT (annual temperature cycle-skin temperature) and comprehensively validated for SUHI applications across 1112 global cities. Based on the seamless daily LST, this paper further characterized the spatiotemporal variations of the frequency (SUHIF and SUCIF) and maximum duration (SUHID and SUCID) and investigated the impacts from related factors, inconsideration of the different characteristics of SUHI and SUCI. There are five major findings. (1) The seamless daily LST reconstructed based on ATC-SKT is validated through pixel-based temperature and city-based SUHII accuracy assessments. (2) The selection of the frequency threshold is based on robustness for LST accuracy, approximation to SUHII global average, and mitigation of frequency saturation. (3) The average daytime SUHIF is 214 days/year, with 44 % of cities exhibiting SUHI occurrences for almost every day in summer. The nighttime SUHIF is 175 days/year, with increasing latitudinal variations from equatorial to polar regions. The daytime SUCIF is 41 days/year, with the greatest average frequency of 172 days/year exhibited in arid regions. (4) The average SUHID is 147 days at daytime and 58 days at nighttime, with relatively opposite geographical distributions between day and night. (5) Greater vegetation difference at daytime and greater albedo difference at nighttime result in more occurrences and longer consecutive durations of SUHI, with opposite effects on the temporal indicators of SUCI. Furthermore, the improvements in daily SUHII and impacts from the maximum duration were discussed. This paper aims to identify and highlight the period with significant SUHI and SUCI effects across global cities for further mitigation.