Heat Island Mitigation Measures Research Articles

Mapping urban heat islands and heat-related risk during heat waves from a climate justice perspective: A case study in the municipality of Padua (Italy) for inclusive adaptation policies

Climate change has led to a dramatic increase in extreme events worldwide. Predictions for a + 1.5 °C world indicate that 13.8% of the global population will be exposed to heat waves (HWs), a proportion rising to 36.9% in a + 2 °C scenario. At present, about 9.6 M people in the EU and UK are exposed to extreme heat every year. Overheating has various impacts on cities, including urban infrastructure failures and changes in ecological processes. However, scarce attention is currently paid to the distribution of HWs impacts and the differential vulnerabilities of different social groups, raising the issue of climate justice in cities. HWs directly impact the health of the most vulnerable social groups resulting in an increase in mortality and morbidity rates. This research focuses on the city of Padua (Italy) as a pilot study to assess the effects of urban HWs and heat islands (UHI) combined. By framing the unequal spatial distribution of socially vulnerable groups, this study aims to i) provide a replicable spatially explicit open-access methodology to assess the heat-related risk of UHI; ii) propose the first climate justice heat-related risk index to be adopted in inclusive and just adaptation plans. Specifically, it aims to i) identify HWs and map critical hotspots during summer 2022 at suburban scale; ii) assess the spatial correlations among impervious areas and UHI; iii) map the climate risk to vulnerable social groups; and iv) propose a global climate justice risk index for all the vulnerable groups considered. Images from Landsat 8–9 were processed, and territorial data were acquired from public databases. It was found that three extreme HWs hit Padua in summer 2022, on 2–7 June, 21–23 July, and 4–8 August, when maximum temperatures were 35.1 °C, 36.1 °C, and 35.8 °C, respectively. The intensity and magnitude of UHIs were considerable, with land surface temperatures of 33.8 °C on average (σ = 1.7, min = 27.9, max = 41.4). UHI intensity reached 5–8 °C of difference with rural contexts, mainly in strongly urbanized sectors. Ordinary least squares regression indicated a positive correlation with impervious surfaces, with a β coefficient showing an average increase of 0.3 °C per 10% of soil sealing. Six different hotspots were identified both in industrial areas and within the city centre. However, the integrated climate risk analyses highlight that most critical areas are in sectors where there is a large number of the elderly, migrants, children, and low-income households. Our findings reveal the need for urgent heat island mitigation measures and that the distributive dimension of climate justice should be respected in adaptation planning.

Mitigation measures for urban heat island and their impact on pedestrian thermal comfort

A multiscale coupled model is presented that allows for the detailed analysis of the local impact of urban heat island mitigation measures. The model uses coupled computational fluid dynamics (CFD) simulations with unsteady heat and moisture transport (HAM) in porous urban materials in order to take into account the dynamic heat and moisture storage in the built environment. A realistic case study is performed for a public urban square in the City of Zurich during heat wave conditions. The impacts of two different mitigation strategies, i.e. adding artificial wetting of pavements and adding vegetation, on pedestrian thermal comfort are evaluated and compared with the existing situation. The results show an improvement in thermal comfort in both conditions. The improvement resulting from the addition of trees is larger and lasts longer due to shadowing effects, even though a reduced ventilation and an increased relative humidity by trees have an adverse effect on the thermal comfort.

Sensitivity analysis of the PALM model system 6.0 in the urban environment

Abstract. Sensitivity of the PALM model 6.0 with respect to land-surface and building properties is tested in a real urban environment in the vicinity of a typical crossroads in a densely built-up residential area in Prague, Czech Republic. The turbulence-resolving PALM is able to simulate the urban boundary layer flow for realistic setups. Besides an accurate representation of the relevant physical processes, the model performance also depends on the input data describing the urban setup, namely the building and land-surface properties. Two types of scenario are employed. The first one is the synthetic scenarios altering mainly surface and material parameters such as albedo, emissivity or wall conductivity, testing sensitivity of the model simulations to potentially erroneous input data. Second, urbanistic-type scenarios are analysed, in which commonly considered urban heat island mitigation measures such as greening of the streets or changing surface materials are applied in order to assess the limits of the effects of a particular type of scenario. For the synthetic scenarios, surface parameters used in radiation balance equations are found to be the most sensitive overall followed by the volumetric heat capacity and thermal conductivity of walls. Other parameters show a limited average effect; however, some can still be significant during some parts of the day, such as surface roughness in the morning hours. The second type, the urbanistic scenarios, shows urban vegetation to be the most effective measure, especially when considering both physical and biophysical temperature indicators. The influence of both types of scenario was also tested for air quality, specifically PM2.5 dispersion, which generally shows opposite behaviour to that of thermal indicators; i.e. improved thermal comfort brings deterioration of PM2.5 concentrations.

Persistent Increases in Nighttime Heat Stress From Urban Expansion Despite Heat Island Mitigation

AbstractUrban areas generally have higher near‐surface air temperature and lower air humidity than rural areas. Little is known about how heat stress, the combined effect of high air temperature and high humidity on human physiology, will be affected by future urban land expansion. Here we use a mesoscale numerical weather prediction model to examine the effects of urban land expansion from 2000 to 2050 on heat stress (measured as wet‐bulb globe temperature, WBGT) in the urban areas of China, India, and Nigeria, which are projected to account for one‐third of global urban population growth through 2050. Our results show that urban expansion slightly reduces heat stress during the day (∼0.2°C) but substantially intensifies it at night, by ∼1°C on average and by up to 2–3°C in five mega‐urban regions (MURs). These effects exist with or without climate change induced by rising concentrations of greenhouse gases (GHGs). Installing cool roofs—an urban heat island mitigation measures—can reduce the daytime WBGT by 0.5–1°C, partially offsetting the heat stress conditions caused by GHG‐induced climate change. However, even with cool roofs, the nighttime WBGTs are higher by 0.3–0.9°C over the whole countries studied, and by 1–2°C in the MURs under the urban expansion scenario, compared to the situation in which urban areas remain unchanged. These results show that future urban expansion and heat island mitigation can result in potential daytime benefits but also persistent nighttime risks.

Impact of urban heat island mitigation measures on microclimate and pedestrian comfort in a dense urban district of Lebanon

The interaction between microclimate and buildings is predominant in urban areas. Thus, the urbanization constitutes the main cause of UHI formation. During the last years, several studies exposed the critical effects of UHI in the Mediterranean climate. As a result, there is a need for sustainable systems and urban policies to reduce heat islands in urban areas. This paper aims to present the impact of some UHI mitigation strategies in a Mediterranean city, Beirut Lebanon. Numerical microclimate simulations of an existing Lebanese district of Dora in Beirut, are performed using ENVI-met software. The study focuses on the potential of UHI mitigation scenarios to create freshness in summer conditions. The strategies are based on the increase of green surfaces, on the installation of water spray and fountains, and on the increase of the albedo of the facades and roofs of the buildings. A focus is carried out on white models, dealing with the albedo modification of roads and buildings. The effect of the proposed strategies on the pedestrian comfort is also evaluated by using the physiologically equivalent temperature PET. Results pointed out that implementation of water features contributes to a significant reduction of ambient temperature reaching a maximum of 5 °C. In term of outdoor thermal comfort, the results showed the important role of green areas on the improvement of pedestrian comfort during daytime. Finally, this research supports a set of actions that could be helpful for urban designer to create cool islands in the Lebanese neighborhoods.

Heat Vulnerability and Heat Island Mitigation in the United States

Heat waves are the deadliest type of natural hazard among all weather extremes in the United States. Given the observed and anticipated increase in heat risks associated with ongoing climate change, this study examines community vulnerability to extreme heat and the degree to which heat island mitigation (HIM) actions by state/local governments reduce heat-induced fatalities. The analysis uses all heat events that occurred over the 1996–2011 period for all United States counties to model heat vulnerability. Results show that: (1) Higher income reduces extreme heat vulnerability, while poverty intensifies it; (2) living in mobile homes or rental homes heightens susceptibility to extreme heat; (3) increased heat vulnerability due to the growth of the elderly population is predicted to result in a two-fold increase in heat-related fatalities by 2030; and (4) community heat island mitigation measures reduce heat intensities and thus heat-related fatalities. Findings also show that an additional locally implemented measure reduces the annual death rate by 15%. A falsification test rules out the possibility of spurious inference on the life-saving role of heat island mitigation measures. Overall, these findings inform efforts to protect the most vulnerable population subgroups and guide future policies to counteract the growing risk of deadly heat waves.

Secondary effects of urban heat island mitigation measures on air quality

Abstract This study presents numerical simulations analysing the effect of urban heat island (UHI) mitigation measures on the chemical composition of the urban atmosphere. The mesoscale chemical transport model WRF-Chem is used to investigate the impact of urban greening and highly reflective surfaces on the concentrations of primary (CO, NO) as well as secondary pollutants (O3) inside the urban canopy. In order to account for the sub-grid scale heterogeneity of urban areas, a multi-layer urban canopy model is coupled to WRF-Chem. Using this canopy model at its full extend requires the introduction of several urban land use classes in WRF-Chem. The urban area of Stuttgart serves as a test bed for the modelling of a case scenario of the 2003 European Heat Wave. The selected mitigation measures are able to reduce the urban temperature by about 1 K and the mean ozone concentration by 5–8%. Model results however document also negative secondary effects on urban air quality, which are closely related to a decrease of vertical mixing in the urban boundary layer. An increase of primary pollutants NO and CO by 5–25% can be observed. In addition, highly reflective surfaces can increase peak ozone concentration by up to 12% due to a high intensity of reflected shortwave radiation accelerating photochemical reactions.

都市流域の地表面地物要素へのヒートアイランド緩和策に対する実流域シミュレーション評価

The promotion of heat island mitigation measures is an urgent need in Tokyo. This study simulated evapotranspiration, land surface temperature, and watershed averaged land surface temperature for each individual land-surface feature in the Upper Kanda River Watershed, implementing a water-permeable pavement for roads and green roofs as heat island mitigation measures. The simulation took an advantage of TET model that can express the amount of evapotranspiration, accounting for changes in soil moisture and permeable characteristics of individual land-surface features. As a result, it was found that the state of the soil moisture had a significant effect in the impact assessment of urban heat island mitigation measures.

Impact of urbanization and land-use/land-cover change on diurnal temperature range: A case study of tropical urban airshed of India using remote sensing data

Diurnal temperature range (DTR) is an important climate change index. Its knowledge is important to a range of issues and themes in earth sciences central to urban climatology and human–environment interactions. The present study investigates the effect of urbanization on the land surface temperature (LST) based DTR. This study presents spatial and temporal variations of satellite based estimates of annually averaged DTR over megacity Delhi, the capital of India, which are shown for a period of 11years during 2001–2011 and analyzes this with regard to its land-use/land-cover (LU/LC) changes and population growth. Delhi which witnessed massive urbanization in terms of population growth (decadal growth rate of Delhi during 2001–2011 was 20.96%) and major transformations in the LU/LC (built-up area crossed more than 53%) are experiencing severity in its micro and macroclimate. There was a consistent increase in the areas experiencing DTR below 11°C which typically resembled the ‘urban class’ viz. from 26.4% in the year 2001 to 65.3% in the year 2011 and subsequently the DTR of entire Delhi which was 12.48°C in the year 2001 gradually reduced to 10.34°C in the year 2011, exhibiting a significant decreasing trend. Rapidly urbanizing areas like Rohini, Dwarka, Vasant Kunj, Kaushambi, Khanjhawala Village, IIT, Safdarjung Airport, etc. registered a significant decreasing trend in the DTR. In the background of the converging DTR, which was primarily due to the increase in the minimum temperatures, a grim situation in terms of potentially net increase in the heat-related mortality rate especially for the young children below 15years of age is envisaged for Delhi. Considering the earlier findings that the level of risk of death remained the highest and longest for Delhi, in comparison to megacities like Sao Paulo and London, the study calls for strong and urgent heat island mitigation measures.

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.

Modeling methods to assess urban fluxes and heat island mitigation measures from street to city scale

The urban microclimate is due to complex physical interactions with the contribution of water balance, thermo-radiative exchanges and airflows. In this paper, we present and discuss modeling of heat island effects and mitigation techniques in order to give consistent results considering different time and space scales, and different fluxes (heat, water and winds) from ground to urban canopy, including buildings. The models and numerical descriptions are presented in detail and illustrated on typical examples of heat island mitigation techniques. At the neighborhood scale, alternative rainwater management techniques are studied by considering their impact on both seasonal water table depth and surface-atmosphere heat fluxes. Assessing the building thermal performance interactions with the microclimate requires adapted models that have to be refined for a better description of building envelope and systems effects. Two examples at the street and the neighborhood scale, modifying the building radiative properties or using green envelopes, show how simulation brings out the potential benefits of these techniques for the heat island mitigation and building energy performance.

Relationships between the properties of an urban street canyon and its radiant environment: Introduction of appropriate urban heat island mitigation technologies

The relationships between the properties of urban canopy components and the radiant environment in an urban street canyon are examined considering the introduction of appropriate urban heat island mitigation technologies. Radiant heat transfers between walls and roads are calculated according to Gebhart’s radiant absorption coefficients and using the Monte Carlo method. Roads are classified as either north–south or east–west; intersections are also considered. The key property of an urban street canyon is expressed by its aspect ratio W/H. A simple street canyon model and two actual urban street canyon areas are used as the objects of examination. Distributions of surface temperatures and solar radiation gains on street canyon roofs, roads, and walls are analyzed. The top priority for the implementation of urban heat island mitigation measures concerns the buildings with large roof areas. The other high-priority areas for implementing mitigation measures focus on smaller roofs and roads for which the street canyon aspect ratio W/H is greater than 1.5; the lowest-priority area is the walls.

Development of a Zero-Dimensional Mesoscale Thermal Model for Urban Climate

Abstract A simple energy balance model is created for use in developing mitigation strategies for the urban heat island effect. The model is initially applied to the city of Phoenix, Arizona. There are six primary contributions to the overall energy balance: incident solar radiation, anthropogenic heat input, conduction heat loss, outgoing evapotranspiration, outgoing convection, and outgoing emitted radiation. Meteorological data are input to the model, which then computes an urban characteristic temperature at a calculated time step for a specified time range. The model temperature is shown to have the same periodic behavior as the experimentally measured air temperatures. Predicted temperature changes, caused by increasing the average urban albedo, agree within 0.1°C with comparable maximum surface temperature predictions from the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The present model, while maintaining valid energy-balance physics, allows users to quickly and easily predict the relative effects of urban heat island mitigation measures. Representative mitigation strategies, namely changes in average albedo and long-wavelength emissivity are presented here. Increasing the albedo leads to the greater reduction in daytime maximum temperatures; increasing the emissivity leads to a greater reduction in nighttime minimum temperatures.

Environmental Impact of Urban Heat Island Phenomena —Cause-effect chain and evaluation in Osaka City—

The impacts of urban heat island phenomenon on human activity and ecosystems have already been obvious in Japanese large cities. To discuss importance of urban heat island mitigation measures, it is necessary to identify and quantify urban heat island impacts such as change in energy use (cooling, heating and hot water supply), change in water use, heat stress, damage to urban ecosystem, air pollution, change in precipitation and floods, etc. In this paper, cause-effect chain of urban heat island impacts is proposed and some examples of heat island impacts in Osaka city are identified and evaluated. Change in energy consumption in residential and commercial sector is evaluated quantitatively by numerical simulation. The result shows the difference in total energy consumption by different air temperature is small since decrease in heating and hot water energy use compensate for increase in cooling energy use.

Cool communities: strategies for heat island mitigation and smog reduction

Adopting our ‘cool communities’ strategies of reroofing and repaying in lighter colors and planting shade trees can effect substantial energy savings, directly and indirectly. In our target city of Los Angeles, annual residential air-conditioning (A/C) bills can be reduced directly by about US$100 M and, because these strategies serve to cool the air in the Los Angeles basin and reduce smog exceedance levels by about 10%, an additional savings of US$70 M in indirect cooling and US$360 M in smog-reduction benefits—a total savings of about US$1/2 B per year—is possible. Trees are most effective if they shade buildings, but the savings are significant even if they merely cool the air by evapotranspiration. In Los Angeles, avoided peak power for air conditioning can reach about 1.5 GW (more than 15% of the city's air conditioning). Generalized to the entire US, we estimate that 25 GW can be avoided with potential annual benefits of about US$5 B by the year 2015. Recent steps taken by cities in the warm half of US towards adoption of cool communities include (1) incorporation of cool roofs in the revised ASHRAE building standards S90.1 and (2) inclusion of cool surfaces and shade trees as tradeable smog-offset credits in Los Angeles. Other step underway include (1) plans by the US Environmental Protection Agency (EPA) to approve heat island mitigation measures in the state implementation plan to comply with ozone standards and (2) plans for ratings and labeling of cool surfaces.

Mitigation of urban heat islands: materials, utility programs, updates

Elevated temperatures in urban `heat islands' increase cooling energy use and accelerate the formation of urban smog. Urban shade trees and light-colored surfaces can offset or reverse the heat island and conserve energy. Implementation of heat island mitigation measures is now a prominent part of President Clinton's Climate Change Action Plan to control the emissions of greenhouse gases, necessitating a better understanding of the quantitative benefits of these control measures. We present recent measurements of the air-conditioning savings for houses in Sacramento and Florida, and air temperature measurements at White Sands National Monument, New Mexico. We also discuss the results of meteorological and smog simulations for the Los Angeles Basin. The albedo of a city may be increased gradually if high-albedo surfaces are chosen to replace darker materials during routine maintenance of roofs and roads. Such high-albedo surfaces may last longer than their conventional dark counterparts. Utility-sponsored incentive programs, product labeling, and standards could promote the use of high-albedo materials for buildings and roads, and several paint manufacturers have expressed interest in participating in a ‘cool surfaces’ labeling program. We examine the spectral reflectance of various white coatings and building materials that might be labeled in such a program.