Causes Of Urban Heat Island Research Articles

Contrasting Effects of Urban Land Cover Change and Anthropogenic Heat on Summer Precipitation Over the Yangtze River Delta of China: Analyses From an Atmospheric Moisture Budget Perspective

AbstractBoth urban land cover (ULC) change and anthropogenic heat (AH) emission are important causes of urban heat island, but their relative contributions to the changes in urban precipitation and the related mechanism remain unclear. Based on numerical simulations utilizing the latest realistic urban fraction and AH data over the Yangtze River Delta urban agglomeration, we found that ULC and AH resulted in nearly opposite effects on precipitation. Various dynamical and thermodynamic processes were involved according to the atmospheric moisture budget analyses. AH increased precipitation particularly during afternoon, and the increases were stronger during heavy precipitation events because of the enhanced moisture convergence effect together with the release of moisture storage previously accumulated in the atmosphere. Differently, ULC reduced mean precipitation mainly due to suppressed evaporation. During weak precipitation events, the suppressed evaporation was largely balanced by the intensified moisture convergence, but during heavy events, ULC caused more pronounced precipitation reduction because the moisture convergence response disappeared and failed to offset the evaporation effect. The relative contributions of different dynamical and thermodynamic processes such as those related to circulation, moisture gradient, and background moisture availability to the temporal variation in the total moisture convergence were further quantified. Overall, our results help better understand the relative roles of different aspects of urbanization on precipitation, and suggest that compared to ULC, reduction in AH emission that is tightly related to the energy consumption structure could be more efficient for mitigating the risk of extreme precipitation.

Modeling Urban Heat Islands and Thermal Comfort During a Heat Wave Event in East China With CLM5 Incorporating Local Climate Zones

AbstractThe urban expansion‐induced heat can exacerbate heat stress for urban dwellers, especially during heat waves. With a focus on the intra‐urban variability of urban heat islands (UHIs) and thermal comfort, the urban parameterization within the Community Land Model version 5 (CLM5) was modified incorporating the local climate zones (LCZs) framework, named CLM5‐LCZs, to simulate the urban climate during a heat wave (HW) event in the summer of 2013. The evaluation of model performance demonstrated that it did a reasonable job of simulating surface energy balance and thermal regimes in cities against observational fluxes from a flux tower measurement site and temperatures from automatic meteorological stations in Nanjing, China. Then we investigated the characteristics and causes of UHIs associated with local background climate, intra‐urban inhomogeneity and HW intensity in East China. The results exhibited that daytime and nighttime canopy urban heat island intensity (CUHII) were highest in the Compact Low Rise (LCZ3) and the Compact High Rise (LCZ1) areas, respectively, while surface urban heat island intensity (SUHII) peaked in the Large Low Rise (LCZ8) and the Compact High Rise (LCZ1) areas during daytime and nighttime, respectively. Urban dwellers were easier exposed to serious heat environment in LCZ3 and LCZ1 areas over the north subtropical climate zone. Contrasts of CUHII and SUHII among different urban classes could exceed 1.7°C and 5.4°C. The intra‐urban heterogeneity may alter the dominant factors controlling SUHII, which were also modulated by local climate and HW intensity. Unlike other controlling factors, the impact of local climate on the contribution from the urban‐rural contrast of convection efficiency was larger than urban features. Overall, CLM5‐LCZs displayed potential of implementing detailed simulations for inter‐ and intra‐city UHIs at a larger scale, and enhancing the capabilities in modeling urban climate and exploring the causes and controls of UHIs.

How can greenery space mitigate urban heat island? An analysis of cooling effect, carbon sequestration, and nurturing cost at the street scale

Rapid urbanization has contributed to urban heat islands, which can potentially lead to increased energy consumption and carbon emissions, further worsening global warming. The U-shaped street canyon is one of the leading causes of urban heat islands, which may block air circulation and lead to urban heat accumulation. The canyon heat issues can be usually mitigated by nature-based solutions, such as street trees. It is important to increase the greenery space benefits (e.g., cooling effect of trees) with limited canyon space. However, there is an absence of refined greenery space design strategy in various street canyons. This work explored the quantitative design of greenery space (e.g., tree spacing) in different street canyons with complex morphological characteristics, in order to effectively improve co-benefits of trees and mitigate urban heat islands. Eighteen morphological types were considered, including symmetrical & asymmetrical shallow, ideal, and deep street canyons. Co-benefit considering cost of different tree spacings were analyzed, to maximize the benefits of cooling effect and carbon sequestration at minimal nurturing cost. Compared with street canyons without trees, ideal street canyon with tree spacing of 0.2W (W is canyon width) achieved the maximum temperature reduction of 6 °C. The positive correlation between tree spacing and co-benefits was found. The maximum co-benefits of street canyon trees occurred at tree spacing of less than 0.7W, which was largely increased by about 14% compared with 0.2W. This work can provide the guideline for efficient greenery space design, which is crucial for mitigating urban heat islands by nature-based solutions.

Simulation of the Energy Performance of a Building with Green Roofs and Green Walls in a Tropical Climate

Global temperatures have continued to rise for decades, partly due to human-caused greenhouse gas emissions and subsequent urban heat island (UHI) effects. This current research examines the benefits of urban greenery by studying the impact of green roofs and walls of a building on thermal behavior and heat transfer in a warm and humid climate. This simulation study discusses the importance of greening systems in improving thermal comfort and minimizing the causes of UHI by assessing an integrated green building design. Using the simulation software DesignBuilder, the significance of greening systems, green roofs, and walls in enhancing thermal comfort and reducing the factors that contribute to UHI is investigated. The simulation results are based on the building’s energy usage in hot and humid regions while featuring green roofs and walls. The simulation results indicate a considerable positive impact of greening systems in improving the urban environment in hot and humid tropical climates. Air temperature, radiant temperature, humidity, and solar gain are decreased by urban greening. The total energy consumption and district cooling demand of buildings with green roofs and walls are reduced by 10.5% and 13%, respectively. The greening systems substantially improve air quality and building’s energy efficiency. Thus, the present study‘s findings can benefit urban designers and dwellers in devising strategies for establishing green spaces in congested urban environments by integrating green technologies and systems into built environments.

Mitigating urban heat island effects in urban environments: strategies and tools

In the twenty-first century, urban heat islands (UHIs) have become a major problem for humanity as a consequence of urbanization and industrialization. The main causes of UHI are the vast amounts of heat generated by urban structures as they consume and re-radiate solar energy and anthropogenic heat sources. The two heat sources cause an urban area’s temperature to rise above its surroundings, a phenomenon known as Urban Heat Island (UHI). Many approaches, methods, models, and investigative tools have been implemented to study and analysis this phenomenon. In general, green areas in cities are thought to be an effective approach to mitigate urban heat island effects and bring comfort to residents. The improvement of microclimatic conditions in urban environments is mostly influenced by evapotranspiration. Most of the studies show a rising trend in the UHI, which is linked to decreased plant cover and land-use changes. The main objectives of this paper were to explain the concept, formation factors, and influential factors of UHI. In addition, the most common strategies and tools that are applied in mitigating rising temperatures in urban areas were reviewed and summarized. The finding of several studies showed that increasing urban vegetation areas in addition to optimizing their spatial distribution and configuration is an effective strategy to reduce the impact of UHI.

Urban heat islands: a review of contributing factors, effects and data

Urban Heat Island (UHI) is one of the most serious difficulties that humans have faced in the twenty-first century resulted due to urbanization and industrialization. The main causes of UHI are the vast amounts of heat produced by urban structures, which solar energy should be used and re-radiated as well as anthropogenic heat sources. The two heat sources cause an urban area’s temperature to rise in comparison to its surrounding areas, which refers to a phenomenon of the urban heat islands. Large cities and diverse economic activities face even more serious difficulties. This paper defined the concept and types of UHIs. This paper explained a comprehensive review of the phenomenon of UHI from its first appearance with regard to causing factors, its influences on people’s lives and the urban environments, and widely used data for measuring and evaluating the intensity of UHIs. The finding of most studies mentioned that UHI values are different from city to city based on land-use changes. It is revealed that an increasing trend occurred mainly because expand in built-up areas and a reduction in green cover. Also, a number of studies showed that the UHI intensity was low during summer afternoons and high during winter nights in many places around the world.

Various Causes of Urban Heat Islands, effects and their Mitigation Measures for Urban Ecology – A Review

Every human activity creates various adverse effects in our environment. Rapid urbanization and increasedpopulation lead to generation of urban heat islands. It adversely affects urban ecology and creates publichealth damage. The major causes of urban heat islands are destruction of trees, low albedo materials, urbancanopy, human gathering. The increase of land surface temperature caused by urban heat island effect willdefinitely alters the urban ecological balance, atmospheric environment and their functions. Urban heatislands need to be addressed urgently. This review is an effort to understand how cities across the globeaffect urban heat and their mitigation measurements. Green cover and cool pavements are the best methodto reduce the temperature in urban areas. In this paper, we have analyzed major cause, effects and mitigationproperties.

Investigation of color pigment incorporated roller compacted high performance concrete as a mitigation tool against urban heat island

Traditional asphalt pavement is one of the causes of urban heat island due to its low albedo. Although the use of various pavement materials is widely preferred method to mitigate urban heat island effect, the use of color pigment added roller compacted high performance concrete as road pavement is a new approach. Within the scope of this research color pigment incorporated roller compacted high performance concrete (CIP-RCHPC) was produced to mitigate urban heat island. Color pigment was used at the rate of 0.25 % 0.5 % 0.75 % and 1 % of cement by weight. Vebe, compressive strength, splitting tensile strength and albedo determination tests were performed on the samples. Highest albedo was obtained as 0.69 on the RCHPC sample that containing color pigment 0.75 % of cement by weight. A selected area was modeled in ENVI-met software with two configurations. In the base configuration road pavement material was set to be asphalt and in roller compacted concrete (RCC) configuration pavement material was set to be RCHPC with 0.69 albedo value. Air temperature, relative humidity, surface temperature and reflected sort wave radiation results were obtained from the both simulations. In the asphalt configuration the highest air temperature was determined to be 33.72 °C while it was determined to be 32.55 °C RCC configuration. Utilization of color incorporated RCHPC as a pavement material results in 1.63 °C decrease on air temperature, 9.69 °C reduce on surface temperature, 8 % increase on relative humidity and 56 % on sort wave reflectance, in the model area which is an ordinary urban area containing residential buildings and streets where speed limit is 50 km/h.

Impact of urbanization on land surface temperature and surface urban heat Island using optical remote sensing data: A case study of Jeju Island, Republic of Korea

Urbanization changes the existing form of land use and cover (LULC) which can influence the land surface temperature (LST). Therefore, it is important to present the causes of urban heat island (UHI) which is usually linked with anthropogenic activities. There are very few studies on thermal behavior of Korean cities in literature. Hence, in this study we have estimated the LULC of Jeju island and analyzed LST using Landsat (TM/ETM+/OLI) images for the last two decades. The supervised image classification method employed with maximum likelihood classifier algorithm was used to classify the images and thermal band was used to calculate the LST. We have used simplified urban extent (SUE) algorithm to calculate the surface urban heat island (SUHI) and eventually we have correlated the SUHI with mean wind speed of Jeju Island. The results of LULC revealed that urban area increased from 8.69% in 2002 to 20.81% in 2021 and in this period barren land decreased by 34.88% due to urban expansion. Interestingly forest region has been slightly increased which is influenced by decreasing barren land. The accuracy was assessed using confusion matrix for the classified images and results revealed an overall accuracy of 0.87, 0.85 and 0.92 with kappa coefficient of 79.02%, 76.45% and 88.05% for the years of 2002, 2011, and 2021 respectively. The LST of all LULC classes were calculated which revealed the highest temperature for urban class which is followed by barren land. However, the forest cover and water body have the lowest temperature in the island. The intensity of surface urban heat island (SUHI) was increased from 2.47 °C (2002) to 3.10 °C (2021). We have correlated the wind speed and SUHI which revealed that SUHI and wind speed has inverse relationship. The outcome of this research can be utilized by the policymakers, urban planners, architects, and climatologist to develop policy related to climate-resilient cities.

Does land-use affect the temperature distribution across the city of Tuxtla Gutiérrez, Chiapas, México?

A combination of natural (tropical latitudes) and human induced (Climate Change, Urban heat island) conditions give rise and exacerbate extreme hot temperatures, but mechanisms are unclear. Land use and land cover change (LULC) is considered one of the main causes of Urban Heat Island (UHI) but its contribution varies depending on local conditions. This study focuses on determining the influence of land use change on the UHI effect in Tuxtla Gutiérrez City by investigating the relationship between LULC and land surface temperature (LST). Through Landsat 5 and Landsat 8 imagery, this study analyzes historical LST. In 2017, the highest LST (>40 °C) occurred in the metal ceiling land class, which is made up of malls with open-air parking zones. This coverage occupied less than 3% of the total city area. Bare agriculture soil (BAS) class, located mainly on the periphery of the city, represented 11% of the city, and reported a mean LST of 35 °C, followed by asphalt roads with 34 °C and concrete ceiling with 32 °C. The lowest LST (< 28 °C), occurred in contiguous areas of trees greater than 3 ha. The LST variation when land use changed from trees to another coverage (1.3 to 3.1 °C) is higher than in the opposite direction (0.1 to 1.2 °C). The elimination or replacement of tress with impervious surfaces are the main causes for LST increase in Tuxtla Gutiérrez.

Analysing the day/night seasonal and annual changes and trends in land surface temperature and surface urban heat island intensity (SUHII) for Indian cities

• Day/night seasonal and annual changes and trends (2001- 2018) in land surface temperature and surface urban heat island intensity (SUHII) for Indian cities (Lucknow, Kolkata and Pune). • Nighttime LST is a better indicator to study the urban heating effect due to its sensitivity towards urbanization. • Positive trends in mean annual daytime (0.003 to 0.059 °C/year) and nighttime (0.030 to 0.078 °C/year) LST are observed in all cities, except in Pune where urban cooling occurs during daytime. • Nighttime SUHII is positive for all cities with mean annual SUHII ranging from 1.34 to 2.07 °C. Understanding the patterns and possible causes of urban heat islands (UHI) effect due to urbanisation-induced anthropogenic activities is an important area in urban climatic research. Few studies on long-term thermal behaviour of Indian cities are available in the literature. Towards this, the present study investigates the day/night seasonal and annual changes and trends in land surface temperature (LST) and surface urban heat island intensity (SUHII) during the last two decades for three Indian cities (Lucknow, Kolkata and Pune) with distinct urban, physiographic and climatic setting. Moderate Resolution Imaging Spectroradiometer (MODIS) LST data products acquired during daytime and nighttime from 2001 to 2018 are processed and analysed for this purpose. MODIS-based aerosol optical depth (AOD) and normalised difference vegetation index (NDVI) products are used to understand the plausible reasons of change in LST and SUHII patterns. The urban (contiguous built-up) and non-urban (surrounding rural) areas are delineated by applying city clustering algorithm on MODIS Land Cover datasets of 500 m spatial resolution. Mann-Kendall and Seasonal-Kendall tests along with Theil-Sen estimator are used for trend analysis. The findings show that the diurnal temperature range (DTR) has decreased from 2001 to 2018 due to higher increase in nighttime LST as compared to daytime. Nighttime LST is a better indicator to study the urban heating effect due to its sensitivity towards urbanisation. Positive trends in mean annual daytime (0.003 to 0.059 °C/year) and nighttime (0.030 to 0.078 °C/year) LST are observed in all cities, except in Pune where urban cooling occurs during daytime. Increase in aerosol loading (with consequent decrease in air temperature and surface insolation) and vegetative cover over time are attributed as the primary reasons of urban cooling effect in Pune during daytime. Seasonal analysis indicate high warming rates during monsoon and summer seasons, particularly during nighttime (0.062 to 0.1 °C/year). Nighttime SUHII is positive for all cities with mean annual SUHII ranging from 1.34 to 2.07 °C. Statistically significant ( p <0.01) increasing trends are observed in mean annual SUHII (0.009 °C/year to 0.022 °C/year) for Kolkata and Pune. Decreasing trend in SUHII is observed over Lucknow due to higher rate of increase in LST in non-urban area as compared to urban area. The study highlights that geographical, physiographic and climatic setting along with anthropogenic processes in urban and its surrounding non-urban areas are critical towards understanding the causative factors in exacerbating the phenomenon of urban heating.

Characteristics of Winter Urban Heat Island in Budapest at Local and Micro Scale

Abstract Most of the urban heat island (UHI) researches focused on the phenomenon in summer. They mainly studied the causes, different functional areas, and possible mitigation measures to reduce the high temperature in urban areas. However, UHI also exists in winter, but there are a limited number of studies on winter UHI. The characteristics and causes of UHI in winter have not been received much attention or consideration yet. This study aims to characterize the UHI feature in winter in Budapest, Hungary, based on the analysis of land surface temperature (LST) in relation to the factors of elevation, slope exposure, residential type, and snow coverage. Five different Landsat images in the winter season were applied to detect the surface temperature; besides, pictures of the thermal camera at a micro-scale were also used. Results showed that UHI intensity was not strong in winter; built-up areas were warmer than other urban areas. Topography was one of the significant factors affecting the surface temperature in winter. The surface temperature of the hills (300 m asl) was lower than that of the lowlands (below 120 m asl). The south-facing slopes and south oriented buildings were warmer than north-facing slopes and buildings oriented to the north. Areas with snow coverage had a lower temperature than no snow coverage areas. These findings could give general guidance for further UHI research, urban planning as well as landscape design.

Novel Asphalt-Mix Design with High Thermal Diffusivity for Alleviating the Urban Heat Island

AbstractExcessive surface temperature of asphalt pavement in summer is one of the main causes of urban heat islands (UHIs), so reducing pavement temperature is a potential research perspective for ...

Quantitative methods for identifying meteorological conditions conducive to the development of urban heat islands

Meteorological conditions significantly influence the development of urban heat islands (UHIs); hence, a reliable assessment and understanding of UHIs depends primarily on the days selected for observation and analysis. Numerous studies were conducted on the characteristics and causes of UHIs; however, few studies focused on the methods and criteria for identifying meteorological conditions conducive to UHI development. The objective of this study is to investigate the performances of four quantitative methods for identifying meteorological conditions conducive to UHI development based on three years of observations in Nanjing, China. The methods include the widely used method based on cloud cover and wind speed and three methods that combine the effects of multiple meteorological variables on UHIs into a single indicator. Correlations analysis between the daily maximum urban heat island intensity (UHIImax) and the meteorological variables indicates that it is inadequate to identify meteorological conditions conducive to UHI development using a single variable, such as solar radiation, cloud cover, or wind speed. The correlations between the daily UHIImax and the indicators are significant (r > 0.60, P < 0.001). The observations are split into calibration and validation sets. Then, the performances of the four methods are evaluated, and the appropriate thresholds for identifying meteorological conditions conducive to UHI development in Nanjing are determined. The results demonstrate that all four methods can be used for identifying meteorological conditions conducive to UHI development in Nanjing. The features and limitations of the four methods and the potential applications are discussed.

3-D Reconstruction of an Urban Landscape to Assess the Influence of Vegetation in the Radiative Budget

Increased urbanization and climate change have resulted in the intensification of the urban heat island (UHI) effect, particularly in tropical cities. One of the main causes of UHI is the man-made urban surfaces influencing the radiation budget by absorbing, reflecting, and emitting radiation at various wavelengths. The radiative budget of a city is directly influenced by the urban geometry, surface materials, direct solar radiation and incident angle, and atmospheric diffuse radiation. Vegetation cover, in contrast, can decrease UHI by intercepting radiation and through the process of photosynthesis. Better understanding the effect of urban vegetation on the radiative budget can thus contribute towards the mitigation of the UHI effect and ultimately the development of climate resilient urban spaces. To analyze the contribution of vegetation to the radiative budget of a city, a detailed simulation of the complex interaction between the built environment and the vegetation is required. This study proposes an approach for analyzing the 3-D structure of both vegetation and built environment to quantify the contribution of vegetation to the radiative budget of an urban landscape. In a first step, a detailed 3-D model of Singapore including buildings and vegetation was reconstructed using a combination of free and commercial Earth Observation data. Then, the 3-D Discrete Anisotropic Radiative Transfer (DART) model was repurposed to estimate the radiation absorbed by the urban surfaces accounting for the presence of vegetation cover with changing Leaf Area Density (LAD) conditions. The presence of trees in the scene accounted for a significant reduction of the absorbed radiation by buildings and ground. For example, in the case of a residential low-building neighborhood, although having low tree cover, the reduction of the absorbed radiation by buildings and ground was up to 15.5% for a LAD =1. The field validation shows good agreement (R2 = 0.9633, RMSE = 10.8830 and Bias = −1.3826) between the DART-simulated shortwave exitance and upwelling shortwave measurements obtained from a net radiometer mounted on a local flux tower in the urban area of Singapore, over the studied period. Our approach can be used for neighborhood-scale analysis, at any desired location of a city, to allow test scenarios with varying surface materials and vegetation properties.

Spatial structure of surface urban heat island and its relationship with vegetation and built-up areas in Melbourne, Australia

Due to the intensity of urban development around the world, there is an increasing body of studies attempting to investigate urban heat island (UHI) in various spatial and temporal scales. In surface heat urban island (SUHI) studies, extended periods of time, broader regions and local government area (LGA) level have become more crucial and will shed light on causes of UHI. Moreover, the spatial pattern and structure of SUHI will be useful for policy-makers to develop mitigation strategies. This study focused on three objectives. Firstly, analyzing land surface temperature (LST), normalized difference built-up (NDBI) and vegetation (NDVI) indices. Secondly, investigating interrelationships among LST, NDVI, and NDBI. Thirdly, identifying LST patterns in the Melbourne metropolitan area. These objectives were achieved through three different methods. The modified automatic mapping method for the first objective, the correlation analysis for the second, and spatial statistical methods for the third. The methodological innovations of this study were considering LGA in interrelationship analysis among LST, NDBI and NDVI, and calculation of NDVI for each acquisition date. The results indicated that the clustering pattern of LST expanded toward the north-west and south-east during the period of the study. Furthermore, the north-west part of the city has the highest positive (0.6) correlation between NDBI and LST, and the south-east part of the city has the lowest negative (−0.8) correlation between NDVI and LST. The most significant increase and decrease in mean LST happened respectively from January 6th to 22nd 2017, and January 14th to 30th January 2014. The temperature degree altered from 19.61 °C to 27.86 °C in inner western suburbs, and from 35.49 °C to 26.88 °C in most LGA's. These findings are critical for planners to localize UHI mitigation action plans, target hot spots in LGA's and allocate resources to respond to the adverse effect of UHI.

Energy and Hydraulic Performance of a Vegetated Roof in Sub-Mediterranean Climate

Widespread overbuilding, the prevalence of asphalt surfaces on green areas, and the use of building materials with low heat dissipation abilities are among the main causes of Urban Heat Islands. Within urban areas, evapotranspiration and shade from plants can significantly reduce the UHI phenomenon, help in stormwater management, and reduce building energy consumption. The goal of this work is to analyze the hydraulics and energy performances of an experimental extensive green roof at the University of Calabria (Italy) in Mediterranean area. This study confirmed that green roofs significantly mitigate storm water runoff generation in terms of runoff volume reduction and peak attenuation, and improve the thermal performance of buildings and the internal comfort of indoor spaces.

A Multi-Temporal Analyses of Land Surface Temperature Using Landsat-8 Data and Open Source Software: The Case Study of Modena, Italy

The Urban Heat Island (UHI) phenomenon, namely urban areas where the atmospheric temperature is significantly higher than in the surrounding rural areas, is currently a very well-known topic both in the scientific community and in public debates. Growing urbanization is one of the anthropic causes of UHI. The UHI phenomenon has a negative impact on the life quality of the local population (thermal discomfort, summer thermal shock, etc.), thus investigations and analyses on this topic are really useful and important for correct and sustainable urban planning; this study is included in this context. A multi-temporal analysis was performed in the municipality of Modena (Italy) to identify and estimate the Surface Urban Heat Island (SUHI, strictly correlated to the UHI phenomenon) from 2014 to 2017. For this purpose, Landsat-8 satellite images were processed with Quantum Geographic Information System (QGIS) to obtain the Land Surface Temperature (LST) and the Normalized Difference Vegetation Index (NDVI). For every pixel, LST and NDVI values of three regions of interest (ROI, i.e., Countryside, Suburbs, and City Center) were extracted and their correlations were investigated. A maximum variation of 6.4 °C in the LST values between City Center and Countryside was highlighted, confirming the presence of the SUHI phenomenon even in a medium-sized municipality like Modena. The implemented procedure demonstrates that satellite data are suitable for SUHI identification and estimation, therefore it could be a useful tool for public administration for urban planning policies.

Optical properties and field test results of spectrally-selective solar control window film that enables not increasing downward reflection

Recently high reflective materials such as high reflectance paint for wall and solar control film or Low-E glass for window are utilized as a building façade aiming for reducing the cooling load of the buildings. From a view point of the direction of reflected rays, almost half of the energy goes upward due to diffuse reflection in case of paint, meanwhile most of energy goes down to the ground due to specular reflection by the glass. This leads to deterioration of thermal environment of surrounding buildings, and is one of causes of urban heat island. Retro-reflective façade materials are promising candidates for mitigation of urban heat island. The authors successfully developed a new type of transparent solar control window film Albeedo that selectively reflects near-infrared rays towards the sky. The film has a saw-tooth microstructure with a spectrally-selective reflection layer. This microstructure is able to reflect infra-red rays toward the sky. The upward reflectance of the film is measured by our custom made spectroscope. Firstly, the sample is placed in a large scale integrating sphere. Then, solar energy absorptive materials are arranged under the sample to absorb the transmitted component and positioned where the downward reflection goes to. Finally, a halogen lamp irradiates the sample at an incident angle of 60° as a light source. The result of the upward reflectance of near-infrared is about 33%. We also conducted field test measurements to evaluate the energy savings of the interior of the building and the degree of reflection of the exterior.

Impact of remotely sensed albedo and vegetation fraction on simulation of urban climate in WRF‐urban canopy model: A case study of the urban heat island in Los Angeles

AbstractModeling the climate of urban areas is of interest for studying urban heat islands (UHIs). Reliable assessment of the primary causes of UHIs and the efficacy of various heat mitigation strategies requires accurate prediction of urban temperatures and realistic representation of land surface physical characteristics in models. In this study, we expand the capabilities of the Weather Research and Forecasting (WRF) model by implementing high‐resolution, real‐time satellite observations of green vegetation fraction (GVF) and albedo. Satellite‐based GVF and albedo replace constant values that are assumed for urban pixels in the default version of WRF. Simulations of urban meteorology in Los Angeles using the improved model show marked improvements relative to the default model. The largest improvements are for nocturnal air temperatures, with a reduction in root‐mean‐square deviation between simulations and observations from 3.8 to 1.9°C. Utilizing the improved model, we quantify relationships between surface and 2 m air temperatures versus urban fraction, GVF, albedo, distance from the ocean, and elevation. Distance from the ocean is found to be the main contributor to variations in temperatures around Los Angeles. After conditionally sampling pixels to minimize the influence of distance from the ocean and elevation, we find that variations in GVF and urban fraction are responsible for up to 58 and 27% of the variance in temperatures. The satellite‐supported meteorological modeling framework reported here can be used for studying UHIs in other cities and can serve as a foundation for testing the efficacy of various heat mitigation strategies.