Urban Cool Research Articles

Urban blue-green space planning based on thermal environment simulation: A case study of Shanghai, China

Abstract Improving the thermal environment of highly urbanized mega cities by optimized blue and green space brings great ecological and social benefits. In this study, applying remote sensing and Computational Fluid Dynamics (CFD) simulation, the Urban Cool Island (UCI) of blue and green space is explored. It indicates that complex outline of faceted water bodies leads to high UCI. Faceted water bodies have stronger UCI than linear ones. The UCI strength ranking of different types of green lands is wedge > radial > punctate > banding. The UCI distribution of green lands are different, which makes them appropriate in specific urban applications. Then, optimized design method of green lands and water bodies to maximize UCI is proposed, including integrating eco-system of green lands and water bodies, limiting riparian development, introducing artificial water within parks, inserting punctate green lands in old districts, setting green bands along roads in direction of prevailing wind and planning wedge green lands on the edge of cities. Finally, according to the optimized design method and the thermal environment simulation of Shanghai, strategy of distribution of blue and green space in urban planning to improve urban thermal environment is formulated, including polishing cool bands beside major and ring roads to increase permeability, inserting cool points into continuous UHI districts, protecting and improving cool surfaces on the edge of cities. The strategy is proved by simulation and provides hints for future planning of urban blue and green spaces.

Effects of Spatial Pattern of Forest Vegetation on Urban Cooling in a Compact Megacity

Urban forests can be an effective contributor to mitigate the urban heat island (UHI) effect. Understanding the factors that influence the cooling intensity of forest vegetation is essential for creating a more effective urban greenspace network to better counteract the urban warming. The aim of this study was to quantify the effects of spatial patterns of forest vegetation on urban cooling, in the Shanghai metropolitan area of China, using correlation analyses and regression models. Cooling intensity values were calculated based on the land surface temperature (LST) derived from remote sensing imagery and spatial patterns of forest vegetation were quantified by eight landscape metrics, using standard and moving-window approaches. The results suggested that 90 m × 90 m was the optimal spatial scale for studying the cooling effect of forest vegetation in Shanghai’s urban area. It also indicated that woodland performed better than grassland in urban cooling and the size, shape, and spatial distribution of woodland patches had significant impacts on the urban thermal environment. Specifically, the increase of size and the degree of compactness of the patch shape can effectively reduce the LST within the woodland. Areas with a higher percentage of vegetation coverage experienced a greater cooling effect. Moreover, when given a fixed amount of vegetation covers, aggregated distribution provided a stronger cooling effect than fragmented distribution and increasing overall shape complexity of woodlands can enhance the cooling effect on surrounding urban areas. This study provides insights for urban planners and landscape designers to create forest adaptive planning strategies to effectively alleviate the UHI effect.

A METHOD TO EVALUATE THE IMPACT OF URBANIZATION ON GROUND TEMPERATURE EVOLUTION AT REGIONAL SCALE

The replacement of natural soil and vegetation by artificial surfaces increases temperatures of the surrounding air and subsurface throughout the year, because of indirect solar heating of urban structures, building heat losses and land use change. This phenomenon is called Urban Heat Island and it can be better perceived during night-time, when the city releases the heat accumulated during the day. During day-time, due to relatively small amounts of solar radiation received by urban surface, especially in high-density cities in arid and semi-arid climates, Urban Cool Island can be identified as well. The present work illustrates a mixed probabilistic-deterministic method to estimate ground temperature at shallow depth, starting from information on geology, hydrogeology, climate, but also urban presence, through correlations with global land cover and population density. A dedicated mapping on regular grid has been produced. Results have been compared with ground and aquifer temperature available in the literature, for some representative cities of Italian Peninsula and Alpine Zone. Preliminary validations are encouraging and can be taken as a starting point for more comprehensive mapping of ground temperature evolution at regional scale.

Dominant control of agriculture and irrigation on urban heat island in India

As is true in many regions, India experiences surface Urban Heat Island (UHI) effect that is well understood, but the causes of the more recently discovered Urban Cool Island (UCI) effect remain poorly constrained. This raises questions about our fundamental understanding of the drivers of rural-urban environmental gradients and hinders development of effective strategies for mitigation and adaptation to projected heat stress increases in rapidly urbanizing India. Here we show that more than 60% of Indian urban areas are observed to experience a day-time UCI. We use satellite observations and the Community Land Model (CLM) to identify the impact of irrigation and prove for the first time that UCI is caused by lack of vegetation and moisture in non-urban areas relative to cities. In contrast, urban areas in extensively irrigated landscapes generally experience the expected positive UHI effect. At night, UHI warming intensifies, occurring across a majority (90%) of India’s urban areas. The magnitude of rural-urban temperature contrasts is largely controlled by agriculture and moisture availability from irrigation, but further analysis of model results indicate an important role for atmospheric aerosols. Thus both land-use decisions and aerosols are important factors governing, modulating, and even reversing the expected urban-rural temperature gradients.

Correlation Analysis of the Urban Heat Island Effect and Its Impact Factors in China

Urban heat islands (UHIs), which are urban areas with higher surface or air temperatures than surrounding rural areas, can further enhance the heat stress already exacerbated by global warming. This poses great challenges to human health and sustainable development. China has been burdened with heavy air pollution in recent years, and the effect of haze pollution on UHIs is still far less well understood. This study investigated the spatial variations of daytime, nighttime, and seasonal surface UHI effects in China during 2003-2013, based on MODIS land surface temperature data, and analyzed the correlations between the UHI effect and its impact factors from both biophysical and biochemical perspectives. Our results show that MODIS-derived annual nighttime UHI effect (3.4 K±0.2 K, mean±1 s.e.) is higher than the annual mean daytime UHI effect (2.1 K±0.3 K). The daytime UHI effect is strongest in summer and weakest in winter but contrasts with the seasonal variation characteristics of nighttime UHI effect. During daytime, UHI effects in humid and semi-humid regions are much more obvious than those in semi-arid/arid regions; during nighttime, the UHI effect in semi-humid and semi-arid/arid regions is much stronger than that in humid regions. A Daytime Urban Cool Island effect exists in semi-arid/arid regions during spring, autumn, and winter seasons. Population, vegetation activity, and irrigation are three factors controlling annual mean daytime UHI effect. The nighttime UHI is significantly influenced by latitude, albedo, precipitation, and aerosol concentrations. We provide evidence for a long-held hypothesis that the biogeochemical effect of urban aerosols is an important contributor to the UHI effect. The important role played by rural background environment in calculating the UHI effect is further discussed in terms of surface aerodynamic roughness and the contrast in albedo between urban and rural areas. Mitigation of haze pollution has a co-benefit of reducing the UHI effect and heat stress for urban dwellers. It is also conducive to reducing negative impacts of regional and global climate change.

Reply to Lofgren, B.M.: Comment on Hicham Bahi, et al. Effects of Urbanization and Seasonal Cycle on the Surface Urban Heat Island Patterns in the Coastal Growing Cities: ACase Study of Casablanca, Morocco. Remote Sens. 2016, 8, 829

First, we would like to thank Lofgren [1] for his comments [2]. The aim of our research is toevaluate the effect and the impact of both urbanization and the season cycle on the variation ofthe Surface Urban Heat Island (SUHI) in the Casablanca region, Morocco. The SUHI experiencesa significant emergence in the urban zone of our study area during winter while it experiencesa reciprocal effect during summer. However, some wintry days can differ from this trend andshow a significant cooling effect in the urban core. In our analysis, we notice that there are someclimate conditions that are more conducive to this reciprocal effect of the SUHI, such as cold/low airtemperature and precipitation. For example, in Figure 17, we show the strength of the link betweenthe spatial distribution of precipitation and the Surface Urban Cool Island (SUCI) pattern and howthe urban areas coinciding with the highest precipitation contribute significantly to the enhancementof the SUCI. On the other hand, we mentioned that winter cool/low air temperature values allowthe control of the SUCI. Once the air temperature exceeds the threshold of approximately the meanwinter temperature, we observe a significant emergence of the SUHI while the SUCI occurs in theother case. Figure 5d accentuates this fact and illustrates a weak SUHI due to the important decrease inair temperature compared to the threshold mentioned above. Some research shows that the presenceof low outdoor temperature keeps moisture in the soil as long as possible [3–5] and then contributes incontrolling the cooling effect. Hence, it was concluded that air temperature controls soil moisture andthe response of soil moisture to air temperature is more sensitive than precipitation [3]. Soil moisture isone of the prime environmental variables related to land surface climatology and is directly connectedwith the process of evapotranspiration and I agree with you that it is controlled by the Surface EnergyBalance. However, and as you evoked in your comment, air temperature can be considered as a factoror part of this complex process. In our paper, we did not deeply study this complex process becauseit is not the goal of our work but we have been limited to show the impact of low temperature andsoil moisture on decreasing the SUHI effect. Our statement, accompanied by the citation of Lofgren’spaper [1], has been mentioned in this perspective. Also, our expected results do not depend on thetopic of potential evapotranspiration and its physical drivers in detail.

Seasonal Variation of Urban Heat Island and Its Impact on Air-Quality Using SAFAR Observations at Delhi, India

This paper discussed the urban heat island (UHI) intensity and local air quality by using observational data of project of the System of Air Quality Forecasting and Research (SAFAR) over Delhi during the month of May and December 2013. It is found that UHI magnitudes ~2.2°C and ~1.5°C are formed at the evening traffic hours during May and December respectively. Also, intensity of UHI < 0°C over daytime is referred as Urban Cool Island (UCI) during May and December. The diurnal PM2.5 concentration shows a bimodal pattern with peaks at morning and evening traffic hours during May and December. The planetary boundary layer height (PBLH) values show higher in magnitude during the daytime and lower in magnitude during the night-time. Whereas, the Ground Heat Flux values are lower during the daytime and higher during the night-time. The wind speed shows lower values during the UHI and higher magnitudes during the UCI formation hours. Concentration of PM2.5 and wind speed shows a strong negative correlation during May (r = -0.56, p = 0.002) and December (r = -0.57, p = 0.001) at C V Raman (CVR) site, however, high values in the concentration of PM2.5 during the low wind speed favour the condition for the formation of UCI. The regression analysis indicated that PM2.5 plays a significant role in the daytime cooling and nighttime warming over the urban areas during the low wind speed condition.

Diurnal and Seasonal Variation of Surface Urban Cool and Heat Islands in the Semi-Arid City of Erbil, Iraq

The influence of land surface temperature (LST) makes the near-surface layer of the troposphere a key driver of urban climate. This paper assesses the temporal formation of the daytime Surface Urban Cool Island (SUCI) and night-time Surface Urban Heat Island (SUHI) effect in Erbil, Iraq, situated in a semi-arid climate region. LST retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua and Terra and MODIS Normalized Difference Vegetation Index (NDVI) from January 2003 to December 2014 are analysed. The relationships of LST with NDVI and the Normalized Multi-band Drought Index (NMDI) are investigated in order to assess the influence of vegetation and moisture on the observed patterns of LST and the SUCI/SUHI. The results indicate that during the daytime, in summer, autumn and winter, densely built-up areas had lower LST acting as a SUCI compared to the non-urbanised area around the city. In contrast, at night-time, Erbil experienced higher LST and demonstrated a significant SUHI effect. The relationship between LST and NDVI is affected by seasonality and is strongly inverted during spring (r2 = 0.73; p &lt; 0.01). Contrary to previous studies of semi-arid cities, a SUCI was detected, not only in the morning, but also during the afternoon.

Impact of Urban Cool Island measures on outdoor climate and pedestrian comfort: Simulations for a new district of Toulouse, France

One of the first airmail services in the world, the Aeropostale, was located at the Montaudran airport of Toulouse, France. This landmark and its surroundings will be refurbished in the years to come and will be transformed into a mixed-use development, the Aerospace Campus, with residential buildings, and commercial, sports, educational and cultural facilities. This new district has been planned mostly on the basis of heritage and functional rules. The UCI project (from “Urban Cool Islands”) is a French national research project that discusses procedures aiming at incorporating a set of reasoned measures for local climate adaptation into this new urban area that will be set as a landmark development. This research aims to analyse and compare various suitable, resilient urban design strategies to provide support for their application in the Montaudran district, focusing on mitigating urban heat island effects in summer conditions. Three main methodological steps were undertaken: (1) the initial urban plan was assessed relative to a set of well-known urban morphology parameters and a microclimate analysis; (2) a set of urban design variations was established based on important microclimate adaptation measures as well as on the constraints and opportunities of the original plan; (3) the microclimate was modelled and the thermal comfort of pedestrians analysed. Results pointed out the major influence of water and green features on the mitigation of urban heat islands, notably in daytime. Besides creating an important urban cool island for pedestrians, the results showed great opportunities for supporting decision makers on specific integrated actions towards a truly sustainable neighbourhood.

Impact of Three-Dimensional Greening of Buildings in Cold Regions in China on Urban Cooling Effect

With the speeding up urbanization, urban heat island problems are becoming increasingly acute. Three-dimensional greening of buildings not only contributes to expansion of urban green space but also creates good living environment. It also helps improve the climate of building surroundings, which effectively reduces the urban heat island effect. It usually means combine the building roof, wall, balcony, window and other special space with greening design according to the characteristics of different plants. In this article, the cooling effect of building three-dimensional greening is simulated according to the climate characteristics in cold regions by using PHOENICS software. It is to analyze the impact of building greening in different patterns of structure on urban heat island effect and to provide theoretical support for the maximum utilization of building three-dimensional greening which can reduce urban heat island effect.

An Analysis of Urban Cooling Island (UCI) Effects by Water Spaces Applying UCI Indices

This research was supported by a grant (15AUDP-B102406-01) from Architecture & Urban Development Research Program (AUDP) funded by Ministry of Land, Infrastructure and Transport of Korean government.

Urban Cooling Potential: System Losses from Microclimates

Temperatures in the cities are amplified through the urban heat island effect by an additional 2-4 ̊C. The trapping of solar energy in urban canyons and infrastructural plays the most significant role. But use of air conditioning for cooing during summer months is very prevalent in large cities such as New York, and it converts even more energy (electricity) into heat that is rejected into the external environment. The temperature of the environment actually directly controls the efficiency of the common refrigeration cycle found in air conditioning systems by the second law of thermodynamics. A city, with its complex topography of urban canyons and skyscrapers, produces small microclimates with varying temperatures. This project investigates three urban settings that create microclimates that are detrimental for the efficiency of cooling in New York. First, the overall urban heat island effect, second the effect of roof temperature on rooftop package air conditioning units, and third, the impact of local heat emission from agglomerations of window air conditioners. The efficiency loss is investigated by considering the range of temperature changes that can be observed in the surrounding environment of air conditioning system, and determining the subsequent impact on the Coefficient of Performance (COP). Our COP analyses indicate a range of potential energy increases of around 10% to 70% due to increases in environmental temperature around air conditioners. An analysis of the building stock of New York City showed that the electrical energy demand is potentially increased by these effects by 500 TJ from UHI, 75 TJ from rooftop package units, and 370 TJ from window units.

Spatial variation of the daytime Surface Urban Cool Island during the dry season in Erbil, Iraqi Kurdistan, from Landsat 8

Differences between the energy balance of cities and their non-urban surroundings exist due to modification of surface properties. In temperate and sub-tropical climates, these differences are manifest as the Urban Heat Island (UHI) effect. However in more arid environments man-made modifications of the environment can cause urban cooling relative to the surrounding landscape particularly during the dry season. This research examines the spatial formation of the daytime Surface Urban Cool Island (SUCI) effect of Erbil city in Iraq, as a case study of cities in semi-arid climates. Six satellite images acquired by Landsat 8 during the period from 1st July to 19th September 2013 are used to retrieve Land Surface Temperature (LST), identify Land Use/Land Cover (LULC) classes and investigate the spatial variation of LST and the SUCI intensity. In order to find out the key drivers of the observed patterns of LST, the relationship with wetness, brightness, bareness, built-up and vegetation index maps are examined. The results indicate that densely built-up areas, such as central districts of the city, green areas and water bodies, had lower LST acting as cool islands, compared to the non-urbanized area around the city. In contrast, the airport, open spaces and new low-density housing developments on the outskirts of the city, experienced higher LST and showed an SUHI effect. A very strong inverse relationship is evident between surface temperature and wetness index (r=−0.9; p<0.01). A strong positive correlation (r=0.75; p<0.00001) is apparent with the brightness index. In contrast, between surface temperature and the greenness index a moderate negative correlation was found (r=−0.39; p<0.01) for a typical dry season day. The results show that during the daytime residential areas in the city centre recorded an LST of 46.2±1.74°C. Urban Cool Island Intensity (UCII) of the city ranged from 3.5 to 4.6°C compared to a 10km buffer zone around the city. This study shows that during the dry season in some cities, such as Erbil, the surface wetness is the main determinant of the UCI effect, and not vegetation cover.

The urban heat budget derived from satellite data

Abstract. The study of the interactions between urban surfaces and the urban boundary layer plays an important role in urban climatology, especially seen against the background of increasing urbanisation in most parts of the world. Measurements of radiation and heat fluxes suffer from the extreme heterogeneity of the urban landscape. It is therefore difficult to get accurate and representative measurements. To bridge the gap between accurate point measurements and their spatial representation, satellite data from Landsat-TM are used. Methods and results of the investigation of radiation properties, net radiation and heat fluxes of urban areas in the Basel Region, NW-Switzerland are presented. In addition to field measurements, satellite data from Landsat-TM were linked to numerical models to compute net radiation and heat fluxes of the whole region. By integrating the normalized difference Vegetation index (NDVI) from multi-spectral satellite data, storage heat fluxes could be estimated with high accuracy. The next step was to compute latent and sensible heat fluxes by using a Bowen-ratio approach attributed to a land use Classification. Of interest is the Observation that the idea of an «Urban Heat Island» (UHI) has to be defined very carefully. Very often an «Urban Cooling Island» may be found during daytime and under clear sky conditions. This feature could be explained using the results of the satellite based radiation and heat budget analysis.

Corrosion by Urban Cooling Waters

Abstract Untreated recirculating cooling waters are likely to become corrosive when low alkalinity waters are present in an area where there is a relatively high sulfur dioxide air content. Untreated circulating cooling waters are compared with makeup waters for eight Eastern cities for such factors as pH, alkalinity, chloride content, sulfate/chloride ratio, and corrosion products. New York City water is discussed specifically. Corrosion can be mitigated by alkaline pH adjustment with maintenance of satisfactory chromate concentrations. 4.6.4