Urban Surface Albedo Research Articles

Impacts of high-albedo urban surfaces on outdoor thermal environment across morphological contexts: A case of Tianjin, China

The urban heat island (UHI) effect can exacerbate various environmental challenges related to high temperatures in urban areas. Increasing urban surface albedo is an effective strategy to mitigate UHI. However, the efficacy of high-albedo urban surfaces across varying urban contexts remains poorly understood. In this study, we leverage Urban Weather Generator to systematically simulate the effects of high-albedo roads, walls, and roofs on urban microclimate across Tianjin. Our simulation covers a typical meteorological year, representing typical weather conditions from January to December. Our results reveal that increasing road albedo is more effective in mitigating UHI in fringe areas, whereas increasing wall and roof albedo is more effective in mitigating UHI in central areas. Local climate zones with an urban aspect ratio of about 0.5 can obtain a maximum reduction of road surface temperature (-6 °C) and wall surface temperature (-3 °C). The temperature changes induced by albedo changes show evident seasonal characteristics: the road temperature decreases significantly in summer, while the wall temperature decreases significantly in spring and autumn. Our results could help guide UHI mitigation policies and urban planning in cities hoping to enhance urban surface albedo to balance urban growth and climate resilience.

Neural network models for predicting urban albedo of urban surfaces with different reflection directional properties

The urban albedo is regarded as an important indicator for mitigating the urban heat island (UHI) phenomenon. The goal of this paper is to create neural network models that forecast the urban albedo of various reflection directional characteristic urban surfaces by simply inputting the sun’s position (altitude and azimuth) and solar radiation. In this study, two urban models with diffuse highly reflective (DHR) and retro-reflective (RR) urban coatings are created, and their urban albedo for two sunny days is calculated using standard (ASTM E1918A). Using the measured urban albedo for one sunny day, two predictive neural network models, Gaussian process (NNGP) and hyperbolic tangent function (NNTanH) are developed (August 1, 2021). The two developed neural network models are used to forecast the urban albedo for another sunny day (July 19, 2021). In the case of DHR urban coatings, the NNTanH model is more accurate with higher R-squared (R2) and lower root mean squared error (RMSE) than the NNGP model, and there is no significant difference between the two neural network models. However, it is demonstrated that the NNGP model is more accurate than the NNTanH model in predicting urban albedo in the case of RR urban coatings, with higher R2 and lower RMSE.

Effect of urban heat island mitigation strategies on precipitation and temperature in Montreal, Canada: Case studies

High-resolution numerical weather prediction experiments using the Global Environmental Multiscale (GEM) model at a 250-m horizontal resolution are used to investigate the effect of the urban land-use on 2-m surface air temperature, thermal comfort, and rainfall over the Montreal (Canada) area. We focus on two different events of high temperatures lasting 2–3 days followed by intense rainfall: one is a large-scale synoptic system that crosses Montreal at night and the other is an afternoon squall line. Our model shows an overall good performance in adequately capturing the surface air temperature, dew-point temperature and rainfall during the events, although the precipitation pattern seems to be slightly blocked upwind of the city. Sensitivity experiments with different land use scenarios were conducted. Replacing all urban surfaces by low vegetation showed an increase of human comfort, lowering the heat index during the night between 2° and 6°C. Increasing the albedo of urban surfaces led to an improvement of comfort of up to 1°C during daytime, whereas adding street-level low vegetation had an improvement of comfort throughout the day of up to 0.5°C in the downtown area. With respect to precipitation, significant differences are only seen for the squall line event, for which removing the city modifies the precipitation pattern. For the large-scale synoptic system, the presence of the city does not seem to impact precipitation. These findings offer insight on the effects of urban morphology on the near-surface atmospheric conditions.

Spatial Heterogeneity and Temporal Variation in Urban Surface Albedo Detected by High-Resolution Satellite Data

Albedo is one of the key parameters in the surface energy balance and it has been altered due to urban expansion, which has significant impacts on local and regional climate. Many previous studies have demonstrated that changes in the urban surface albedo are strongly related to the city’s heterogeneity and have significant spatial-temporal characteristics but fail to address the albedo of the urban surface as a unique variable in urban thermal environment research. This study selects Beijing as the experimental area for exploring the spatial-temporal characteristics of the urban surface albedo and the albedo’s uniqueness in environmental research on urban spaces. Our results show that the urban surface albedo at high spatial resolution can better represent the urban spatial heterogeneity, seasonal variation, building canyon, and pixel adjacency effects. Urban surface albedo is associated with building density and height, land surface temperature (LST), and fractional vegetation cover (FVC). Furthermore, albedo can reflect livability and environmental rating due to the variances of building materials and architectural formats in the urban development. Hence, we argue that the albedo of the urban surface can be considered as a unique variable for improving the acknowledgment of the urban environment and human livability with wider application in urban environmental research.

The Future of Climate-Resilient and Climate-Neutral City in the Temperate Climate Zone.

The urban heat island (UHI) effect is the main problem regarding a city’s climate. It is the main adverse effect of urbanization and negatively affects human thermal comfort levels as defined by physiological equivalent temperature (PET) in the urban environment. Blue and green infrastructure (BGI) solutions may mitigate the UHI effect. First, however, it is necessary to understand the problem from the degrading side. The subject of this review is to identify the most essential geometrical, morphological, and topographical parameters of the urbanized environment (UE) and to understand the synergistic relationships between city and nature. A four-stage normative procedure was used, appropriate for systematic reviews of the UHI. First, one climate zone (temperate climate zone C) was limited to unify the design guidelines. As a result of delimitation, 313 scientific articles were obtained (546 rejected). Second, the canonical correlation analysis (CCA) was performed for the obtained data. Finally, our research showed the parameters of the UE facilities, which are necessary to mitigate the UHI effect. Those are building density and urban surface albedo for neighborhood cluster (NH), and distance from the city center, aspect ratio, ground surface albedo, and street orientation for street canyon (SC), as well as building height, material albedo, and building orientation for the building structure (BU). The developed guidelines can form the basis for microclimate design in a temperate climate. The data obtained from the statistical analysis will be used to create the blue-green infrastructure (BGI) dynamic modeling algorithm, which is the main focus of the future series of articles.

Narrow-to-Broadband Conversion for Albedo Estimation on Urban Surfaces by UAV-Based Multispectral Camera

Urban surface albedo is important for investigating urban surface–atmosphere radiative heat exchanges. For modeling surface energy balance (SEB) at local and neighborhood scales, ground or unmanned aerial vehicle (UAV)-based multispectral remote sensing (RS) can be used to obtain high-spatial-resolution multispectral information for both horizontal and vertical urban surfaces. The existing narrow-to-broadband (NTB) conversion models, developed for satellite/high-altitude observation and large homogeneous rural/vegetated/snow zones, may not be suitable for downscaling to the local and neighborhood scales or the urban complex texture. We developed three NTB models following published methodologies for three common UAV-based multispectral cameras according to Sample_D, a sample group of extensive spectral albedos of artificial urban surfaces, and evaluated their performance and sensitivities to solar conditions and surface material class. The proposed models were validated with independent samples (Sample_V). A model considering albedo physics was improved by multiplying different variables with respect to the camera (termed as “Model_phy_reg”), which initially proved to be the most accurate with a root mean square error of up to 0.02 for Sample_D and approximately 0.029 for Sample_V, meeting the required accuracy of total shortwave albedo for SEB modeling. The accuracy of Model_phy_reg was not much prone to the solar conditions.

Analysis of the cooling effects of higher albedo surfaces during heat waves coupling the Weather Research and Forecasting model with building energy models

In this paper, the numerical Weather Research and Forecasting (WRF) model is coupled with a Building Effect Parameterization (BEP) and Building Energy Model (BEM) to investigate the effects on the urban microclimate and building energy demand of increasing the albedo of urban surfaces. For the scope of this study, while the WRP performs real-data applications numerical weather prediction, the BEP serves to predict the heat and moisture fluxes from the urban canopies to the atmosphere and the BEM simulates the effects of anthropogenic heat emissions. As such the overall simulations are capable of taking into account the several factors that influence the urban microclimate and couple their mutual interactions. Outdoor microclimate results for the air temperature and wind speed are compared against measurements in Toronto, ON, during the 2018 heatwave period. The validation confirms the reliability of the WRF model and the selected approach. Then, in order to assess possible urban heat island mitigation strategies, the albedos of roofs, walls, and ground surfaces are increased from 0.2 to 0.65, 0.60, and 0.45, respectively. Results show that the maximum decrease in air temperature through these higher albedo values is nearly 2 °C at noon with a slight increase in wind speed. Moreover, the daily averaged decrease in the urban surface temperature and in the absolute humidity is 3.3 °C and 0.6 g/kg, respectively. Increasing the surface albedo would reduce the solar radiation balance at noon by almost 30 W/m2. Finally, the combined effect of decreased solar heat gains by urban surfaces and decreased air temperature would reduce the cooling energy demand by almost 10%.

The impact of increasing urban surface albedo on outdoor summer thermal comfort within a university campus

The impact of increasing urban surface albedo on outdoor thermal comfort was studied in two phases:Firstly, the thermal conditions of three locations with different ground surface materials were compared. The study used CFD modelling followed by a measurement campaign to validate the control simulation. It was observed that the physiological equivalent temperature (PET as the outdoor thermal comfort index) in the campus park (covered with grass) was 11.0 °C lower than the parking lot (paved with concrete) at 16:00 CET.As the next step, the albedo of the roofs and walls were increased from 0.2 (control) to 0.3, 0.4, 0.5 and 0.6. It was found that increasing the albedo made the open space of the courtyard uncomfortable due to the higher reflectivity of high-albedo materials. An increase of every 0.1 albedo of the surfaces led to 1.2 °C higher mean radiant temperature, and consequently, 0.8 °C higher PET. The study also showed that the increase of albedo radiated more sun to the ground surface. This increased average ground surface sensible heat flux (6.7 W/m2) and surface temperature (0.4 °C) during the day. This finding shows that the position and orientation of high albedo materials can significantly affect pedestrians' thermal comfort in urban open spaces.

Effect of urban surface albedo enhancement in India on regional climate cooling

In this work, the impact of surface albedo change (by using white paint on roofs in India) on radiative forcing and land surface temperature change has been quantified based on the principles of Physics using energy balance equation and one-layer atmospheric model. The reduction in temperature from enhanced surface albedo was related to the offsets in temperature increase due to carbon dioxide concentration change from pre-industrial to present times. For Indian region, land surface temperature was found to decrease by ~0.63±0.04K considering water vapour feedback effect and total outgoing radiation increased by ~1.314W/m2, for an increase in area weighted surface albedo by 0.0037. However, in order to take into account all the feedback effects and the effect of ocean, the mesoscale model, Advanced Research Weather Research and Forecasting (WRF) model version 3.7 was used and the effect of urban albedo enhancement on the land surface temperature change was simulated and it was found to be in the range of −1 to +1K with the mean value of −0.043K. Radiative forcing due to Carbon dioxide concentration change from pre-industrial to present times was calculated as 1.797W/m2 and corresponding land surface temperature change estimated was ≈0.86K. The results obtained by the analytical model were found to be the subset of the results obtained using WRF model. The study concluded that use of cool roofs in India can compensate the heating due to increase in CO2 concentration (from pre-industrial times to present times) by ~5% by taking into account the average value of temperature change obtained by using WRF model.

Evaluation of albedo enhancement to mitigate impacts of urban heat island in Rome (Italy) using WRF meteorological model

The extreme effects of urban heat island (UHI) on energy consumption, air quality, and human health are significantly detrimental. Increasing the albedo of urban surfaces has been proposed as a potentially efficient mitigation strategy. In this study the Weather Research and Forecasting (WRF) mesoscale model has been used to simulate the urban climate of Rome (Italy).Four different scenarios have been analyzed: the Base Scenario as control case; the Base-ALB Scenario, in which the albedo of roof, walls and road have been increased; the Morph Scenario in which the morphology of urban area has been parameterized more accurately; the Morph-ALB Scenario in which the urban albedo of the improved model has been increased.This study demonstrates that a more accurate parametrization of the urban morphology leads to a more accurate representation of UHI phenomenon. The simulation results show that albedo increase leads to the decrease of the 2-m air temperature at day-time and at night-time. Albedo increase offers very promising results in terms of UHI mitigation, reducing the temperature in the urban area by up to 4°C at daytime and a little increased (up to 1°C) in some locations at night time, compared to the control cases.

On the Effects of Albedo Increase through Reflective Roofing on Philippine Urban Atmospheric Temperature: Real-Time Parameter Inputs

With global warming as one of the major concerns today, people try to find ways to control factors that affect atmospheric temperature. One of which is surface albedo. Akbari, et al. (2009), asserts that the use of modern reflective roofing and pavement materials can increase urban surface albedo by 0.1. This study utilized an agent-based simulation model in an experiment to verify whether this increase would bring about significant cooling of Philippine urban atmosphere. Real data obtained from reliable sources were used to set parameter values in the model. Simulation results showed that if the current levels of CO2 concentration and cloud cover would persist, such increase in albedo is not enough to significantly lower urban atmospheric temperature.

A New Performance Indicator to Assess Building and District Cooling Strategies

The goal of this study is to bring out the efficiency of district landscaping on local urban heat island mitigation. Two cooling strategies at the district scale are evaluated through the coupled effect computation of microclimate and building energy demand: vegetation (trees, green walls and roofs) and high albedo values (cool roofs and façades). The study focus on an existing district, Part-Dieu, in Lyon (France). This district has a high urban density and is composed of buildings for which rehabilitation is expected to be difficult. The city is particularly sensitive to summer heatwaves that are supposed to be more regular with global warming. Two specific urban blocks, of about 10 building blocks each, are modeled: a 60,000 m2 site where the urban fabric is a mix of historical buildings and 60s’ buildings, and a 70,000 m2 new residential site under construction, more homogeneous compared to the first one. The simulation results are processed for a summer period (hourly time step) andcooling strategies are analyzed individually and combined. Two indexes, Energy Performance Index (EPI) and Ambient Temperature Mitigation Index (ATMI), are defined to evaluate strategy efficiencies. In our case studies, cooling energy demand is mitigated by around 3% with tree growth strategy, 35% with green roofs, and 76% with increased albedo of urban surfaces. Whereas detailed results of coupled simulation are useful here for engineers, global performance indicators defined in this study give useful hints for stakeholders and urban cooling strategy design.

Comparative assessment of various heat island mitigation measures

With the rapid intensification of urban heat islands worldwide, measures for its mitigation are gaining attention. Some of the most popular measures are based on employing evaporative cooling, altering the surfaces' albedo or making use of shading. In this paper, we numerically investigate the influence of each of these three methods on the comfort of a pedestrian in a typical urban street canyon. The environmental conditions in the urban street canyon are obtained from a detailed microclimatic model, and serve as input for a comfort model based on the Universal Thermal Climate Index (UTCI). Simulations are conducted for average summer conditions, taken from a typical meteorological year (TMY), and for heat wave (HW) conditions. The results show that evaporative cooling can considerably reduce the air temperature and the mean radiant temperature, but that the corresponding increase in vapor pressure limits the net gain in comfort. Shading results in significantly reduced surface temperatures, in addition to decreasing the intensity of direct solar irradiation, both leading to an increased comfort sensation. Increasing the local albedo of urban surfaces also leads to lower surface temperatures, but does not affect the amount of direct solar irradiation, explaining the lower comfort gain compared to shading. The cooling effect of all methods proved to increase during a HW in comparison to a TMY. These observations show that the effectiveness of mitigation measures highly depends on the climatic conditions and demonstrate the potential of microclimatic models to determine the optimal combination of measures for a given context.

Effects of white roofs on urban temperature in a global climate model

Increasing the albedo of urban surfaces has received attention as a strategy to mitigate urban heat islands. Here, the effects of globally installing white roofs are assessed using an urban canyon model coupled to a global climate model. Averaged over all urban areas, the annual mean heat island decreased by 33%. Urban daily maximum temperature decreased by 0.6°C and daily minimum temperature by 0.3°C. Spatial variability in the heat island response is caused by changes in absorbed solar radiation and specification of roof thermal admittance. At high latitudes in winter, the increase in roof albedo is less effective at reducing the heat island due to low incoming solar radiation, the high albedo of snow intercepted by roofs, and an increase in space heating that compensates for reduced solar heating. Global space heating increased more than air conditioning decreased, suggesting that end‐use energy costs must be considered in evaluating the benefits of white roofs.

알베도 증가에 따른 도시 기온 하강 효과에 관한 수치연구

In order to clarify the influence of the change of urban surface albedo on mesoscale meteorological factors during the summertime, numerical experiments with various albedo of urban surface were carried out. Numerical model used in this study is PSU/NCAR MM5 V3.6. As a result of the numerical simulation intended of Busan assumed the increase of albedo of roofs, buildings, or roads, the increase of albedo can make decrease radiation effect of surface, so that it caused drops in ambient air temperature from to on the average, and the downtown of Busan formed along the trough presented a substantial drop in ambient air temperature about . Modeling studies suggest the increased surface albedo in urban area can reduce surface and air temperatures near the ground and affect related meteorological parameters such as winds and the depth of the mixed boundary layer.

An Evaluation of Urban Land Use Regulations from View of Urban Surface Albedo

abstractUrban surface albedo, which is one of the important factors of the urban heat island phenomenon, was used to evaluate urban land use regulations. Broadband albedo at the surface level of Sakai City, Japan, was calculated, and effects of land use regulations and actual land use were estimated using satellite remote sensing data, a land use regulation map and detailed digital information expressing actual land use. Land use regulations define land use, maximum building coverage ratios and maximum floor area ratios, which relate the heights of buildings to the areas of their sites. These factors have effects upon broadband albedo and each narrowband albedo corresponding to wavelengths observed by Landsat ETM+. Albedo values differed according to land use regulation zones. The combination of land use regulation and actual land use is also meaningful in grasping urban surface albedo. But this relationship may provide only slight insight into the urban heat island phenomenon.

466各種土地被覆情報との分析による都市地表面アルベドの特性に関する研究

The urban surface albedo was estimated using four methods; 1) Panchromatic, 2) NVI (Normalized Vegetation Index), 3) Land cover ratio calculated by the constrained least square method, 4) Detail mesh data on land uses. The methods 1) -3) were derived from Landsat TM data. The histgram s of the estimated albedo were compared with each others. The result estimated by the panchromatic method was best. The result estimated by Detail mesh data on land uses had a problem based on the discrete values.