The surface urban heat island (SUHI), which represents the difference of land surface temperature (LST) in urban relativity to neighboring non-urban surfaces, is usually measured using satellite LST ...data. Over the last few decades, advancements of remote sensing along with spatial science have considerably increased the number and quality of SUHI studies that form the major body of the urban heat island (UHI) literature. This paper provides a systematic review of satellite-based SUHI studies, from their origin in 1972 to the present. We find an exponentially increasing trend of SUHI research since 2005, with clear preferences for geographic areas, time of day, seasons, research foci, and platforms/sensors. The most frequently studied region and time period of research are China and summer daytime, respectively. Nearly two-thirds of the studies focus on the SUHI/LST variability at a local scale. The Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM+)/Thermal Infrared Sensor (TIRS) and Terra/Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) are the two most commonly-used satellite sensors and account for about 78% of the total publications. We systematically reviewed the main satellite/sensors, methods, key findings, and challenges of the SUHI research. Previous studies confirm that the large spatial (local to global scales) and temporal (diurnal, seasonal, and inter-annual) variations of SUHI are contributed by a variety of factors such as impervious surface area, vegetation cover, landscape structure, albedo, and climate. However, applications of SUHI research are largely impeded by a series of data and methodological limitations. Lastly, we propose key potential directions and opportunities for future efforts. Besides improving the quality and quantity of LST data, more attention should be focused on understudied regions/cities, methods to examine SUHI intensity, inter-annual variability and long-term trends of SUHI, scaling issues of SUHI, the relationship between surface and subsurface UHIs, and the integration of remote sensing with field observations and numeric modeling.
Urban residents suffer more from heat stress, compared to people living in rural areas, due to the urban heat island (UHI) effect. Mitigation of UHI is thus essential to improving human thermal ...comfort and living environment in urban residential areas. However, little attention has been paid to the integrated effect of UHI mitigation strategies on human thermal comfort, which is influenced by the combination of temperature, humidity, wind, and radiation. This study evaluates the effectiveness of two promising UHI mitigation strategies, cool and green roofs, in improving human thermal comfort during a heatwave in Berlin. Human thermal comfort is represented by the Universal Thermal Climate Index (UTCI), calculated by combining the Weather Research and Forecasting model coupled with the Urban Canopy Model (WRF/UCM) with the RayMan model. The results show that cool roofs outperform green roofs in reducing urban temperatures, especially at night. Besides temperature reduction, both strategies show lower wind speed, lower mean radiant temperature, and higher relative humidity. These combined effects lead to a city-scale decrease in UTCI. Cool roofs reduce more UTCI than green roofs, although they both shorten the duration of strong heat stress from 7 h d−1 to 5 h d−1. A higher albedo and irrigation can strengthen the cooling effect of cool and green roofs, respectively. Our study can deepen the understanding of the mechanism of natural infrastructure in improving human thermal comfort, providing scientific guidance for future city management.
•We evaluated the effectiveness of cool and green roofs in improving human thermal comfort during a heatwave in Berlin.•Cool roofs outperform green roofs in mitigating urban heat island and improving human thermal comfort.•Both strategies shorten the duration of strong heat stress from 7 h d−1 to 5 h d−1.
Accurately characterizing spatiotemporal changes in surface urban heat islands (SUHIs) is a prerequisite for sustainable urban development. Although urban administrative boundaries have typically ...been used for SUHI modeling, they are inaccurate, because urban main built-up areas (UMBAs) are not well characterized. In this study, we developed an UMBA extraction method based on impervious surface distribution density (ISDD), to better differentiate suburban boundaries and ensure the integrity of land cover types. Additionally, we propose a new intensity classification method to analyze SUHI spatial distribution and variation. The UMBA was extracted using LANDSAT-8 data, and the temporal dynamics of SUHI intensity (i.e., daily, monthly, seasonal, and yearly changes) were extracted from MODIS data. A case study for Beijing showed that the mean daytime and nocturnal SUHI intensities vary at multiple time scales. In the daytime, SUHI intensities in Beijing UMBA were mainly level-2 and level-3, with central-south Beijing, a high incidence area, at level-3 in spring and summer. At night, with the rise of SUHI intensity levels, the frequency of SUHI intensity levels increased from the periphery to the center within the same season. ISDD had a marked influence on the frequency of SUHI intensity levels during the daytime, and the frequencies of level-1 to level-4 intensities increased with ISDD. This influence tended to weaken when ISDD exceeded 50%.
•Urban main built-up area was extracted by impervious surface distribution density.•An urban heat island intensity classification method was used to measure intensity.•Impervious surface distribution density has relationship with SUHI intensity level.
•The implementation of UHI mitigation is limited by lack of policy, economic and technical supports.•SPC and UHI mitigation are compatible in technical, financial, institutional and social ...aspects.•SPC can provide a solid financial foundation for taking further the UHI mitigation.•The inclusion of UHI mitigation can consolidate PPP model for more funds .•The inclusion of UHI mitigation can balance the weights of different authorities in SPC construction.
Climate change and its impacts cannot be addressed once for all due to internal complexity. Some implemented strategies may only be capable of dealing with a cluster of problems while leaving many others untouched. The co-benefits approach, however, opens up a ‘window of opportunity’ via achieving multi-goals simultaneously. Based on this, this paper aims to unpack the interrelationship between the struggling urban heat island (UHI) issue and the already ongoing sponge city (SPC) projects in China. Specifically, the co-benefits are investigated from technical, financial, institutional and social perspectives, after which pathways to implementation are presented. In these aspects, the co-benefits approach can bring opportunities for implementing SPC and UHI mitigation. The inclusion of UHI mitigation into SPC construction can enhance public participation and thereby consolidate the public-private partnership model for funds. During the co-benefits approach implementation, the weights of different authorities can be rebalanced to promote institutional transitions. SPC-derived UHI mitigation approach, potentially realising synergies of urban flooding and UHI mitigation, can be a model for countries which have already released low-impact development water management practices, and may also provide references for other projects such as green building, low-carbon eco-city, smart city, forest city and haze treatment for UHI mitigation.
•The average peak temperature drop from mitigation techniques is close to 2K.•The decrease of the average ambient temperature is close to 0.74K.•Almost 31% of the projects resulted in a peak ...temperature drop below 1K and 62% below 2K.
Local and global climate change increases the ambient temperature of cities by several degrees with important consequences on energy consumption, health and the economy. Advanced urban mitigation technologies contribute to decrease the ambient temperature and counterbalance the impact of urban heat islands. The present paper analyses and presents in a comparative way the mitigation potential of the known mitigation technologies using performance data from about 220 real scale urban rehabilitation projects. The average and peak temperature drop of reflective technologies, greenery, evaporative systems, earth to air heat exchangers and their combinations is calculated and presented. The mitigation potential of the main systems like cool roofs, cool pavements, green roofs, urban trees, pools and ponds, sprinklers, fountains, and evaporative towers, is analysed. It is found that the potential of the main mitigation technologies is considerable and can counterbalance UHI effects partly or fully. The average peak temperature drop calculated for all projects is close to 2K, while the corresponding decrease of the average ambient temperature is close to 0.74K. Almost 31% of the analysed projects resulted in a peak temperature drop below 1K, 62% below 2K, 82% below 3K and 90% below 4K.
With rapid urbanization, population growth and anthropogenic activities, an increasing number of major cities across the globe are facing severe urban heat islands (UHI). UHI can cause complex ...impacts on the urban environment and human health, and it may bring more severe effects under heatwave (HW) conditions. In this paper, a holistic review is conducted to articulate the findings of the synergies between UHI and HW and corresponding mitigation measures proposed by the research community. It is worth pointing out that most studies show that urban areas are more vulnerable than rural areas during HWs, but the opposite is also observed in some studies. Changes in urban energy budget and major drivers are discussed and compared to explain such discrepancies. Recent studies also indicate that increasing albedo, vegetation fraction and irrigation can lower the urban temperature during HWs. Research gaps in this topic necessitate more studies concerning vulnerable cities in developing countries. Moreover, multidisciplinary studies considering factors such as UHI, HW, human comfort, pollution dispersion and the efficacy of mitigation measures should be conducted to provide more accurate and explicit guidance to urban planners and policymakers.
Context
Cities have elevated temperatures compared to rural areas, a phenomenon known as the “urban heat island”. Higher temperatures increase the risk of heat-related mortality, which will be ...exacerbated by climate change.
Objectives
To examine the impact of climate change and urban growth on future urban temperatures and the potential for increased heat stress on urban residents.
Methods
We conducted a systematic review of scientific articles from Jan 2000 to May 2016.
Results
The majority (n = 49, = 86%) of studies examined climate change and the urban heat island in isolation, with few (8) considering their combined effect. Urban growth was found to have a large impact on local temperatures, in some cases by up to 5 °C in North-east USA. In some locations climate change increased the heat island, such as Chicago and Beijing, and in others decreased it, such as Paris and Brussels. When the relative impact of both factors was considered, the temperature increase associated with the urban heat island was always higher. Few studies (9) considered heat stress and its consequences for urban populations. Important contributors to urban temperatures, such as variation in urban density and anthropogenic heat release, were often excluded from studies.
Conclusions
We identify a need for an increased research focus on (1) urban growth impact on the urban heat island in climate change studies; (2) heat stress; and, (3) variation in urban density and its impacts on anthropogenic heat. Focussing on only one factor, climate change or urban growth, risks underestimating future urban temperatures and hampering adaptation.
Urban heat island circulation (UHIC), commonly established under calm background conditions, is important for understanding the accumulation of pollutants and heat in a city. In a city cluster in ...which multiple cities exist in proximity, the resulting multiple UHICs can interact. As many city clusters continue to grow in size and in number, particularly in rapidly developing Asia, it is necessary to understand the interactions of multiple UHICs. In this study, the development of a single UHIC, interaction between two identical UHICs, and that of three different UHICs were investigated by water tank modelling experiments. UHIC is characterised as convergent inflow at lower levels, upward flow over the urban area and divergent outflow at upper levels. Stagnant zones were found between two adjacent cities due to competition between their inflows. If the vertical sizes (mixed heights) of two adjacent UHICs are different, the outflow of the smaller UHIC will be lower than that of the larger one and will join the inflow of the larger one; thus, the pollutants will be transported from the smaller to the larger UHIC. Because the outflow of the larger UHIC is higher than that of the smaller one, the outflow of the larger UHIC can shelter the smaller one and thus limit the vertical pollutant dispersion within the smaller one. These findings are useful to explain the haze accumulation phenomenon in major haze episodes such as those occurring in the Beijing-Tianjin-Hebei region in China.
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•UHIC is induced by urban heat island under calm background condition.•Revealed how a single UHIC is developed using water tank modelling.•The city cluster can generate multiple UHICs.•Chain flow does not exist for two cities of equal thermal strength.•A small city between two large cities can be “cleaned” by large city UHICs.
Urbanization is taking place at an unprecedented rate around the world, particularly in China in the past few decades. One of the key impacts of rapid urbanization on the environment is the effect of ...urban heat island (UHI). Understanding the effects of landscape pattern on UHI is crucial for improving the ecology and sustainability of cities. This study investigated how landscape composition and configuration would affect UHI in the Shanghai metropolitan region of China, based on the analysis of land surface temperature (LST) in relation to normalized difference vegetation index (NDVI), vegetation fraction (Fv), and percent impervious surface area (ISA). Two Landsat ETM+ images acquired on March 13 and July 2, 2001 were used to estimate LST, Fv, and percent ISA. Landscape metrics were calculated from a high spatial resolution (2.5
×
2.5
m) land-cover/land-use map. Our results have showed that, although there are significant variations in LST at a given fraction of vegetation or impervious surface on a per-pixel basis, NDVI, Fv, and percent ISA are all good predictors of LST on the regional scale. There is a strong negative linear relationship between LST and positive NDVI over the region. Similar but stronger negative linear relationship exists between LST and Fv. Urban vegetation could mitigate the surface UHI better in summer than in early spring. A strong positive relationship exists between mean LST and percent ISA. The residential land is the biggest contributor to UHI, followed by industrial land. Although industrial land has the highest LST, it has limited contribution to the overall surface UHI due to its small spatial extend in Shanghai. Among the residential land-uses, areas with low- to-middle-rise buildings and low vegetation cover have much high temperatures than areas with high-rise buildings or areas with high vegetation cover. A strong correlation between the mean LST and landscape metrics indicates that urban landscape configuration also influences the surface UHI. These findings are helpful for understanding urban ecology as well as land use planning to minimize the potential environmental impacts of urbanization.
► Strong relationship between LST and vegetation and impervious surface. ► Stronger vegetation mitigation effect on UHI in summer than in early spring. ► Largest residential contribution to UHI. ► Strong influence of Urban landscape configuration on UHI.
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