Interaction of urban heat islands and heat waves under current and future climate conditions and their mitigation using green and cool roofs in New York City and Phoenix, Arizona (original) (raw)
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Journal of Applied Meteorology and Climatology, 2013
Cities are well known to be hotter than the rural areas that surround them; this phenomenon is called the urban heat island. Heat waves are excessively hot periods during which the air temperatures of both urban and rural areas increase significantly. However, whether urban and rural temperatures respond in the same way to heat waves remains a critical unanswered question. In this study, a combination of observational and modeling analyses indicates synergies between urban heat islands and heat waves. That is, not only do heat waves increase the ambient temperatures, but they also intensify the difference between urban and rural temperatures. As a result, the added heat stress in cities will be even higher than the sum of the background urban heat island effect and the heat wave effect. Results presented here also attribute this added impact of heat waves on urban areas to the lack of surface moisture in urban areas and the low wind speed associated with heat waves. Given that heat ...
2000
In this project, the urban heat island effect (UHI) was examined under current and projected climate conditions in the greater Newark, New Jersey region using a combination of spatial analyses of remotely-sensed thermal data and of meteorological station data. Mitigation strategies for the UHI, including increased urban vegetation and lighter-colored surfacing, were explored using CITYgreen, a GIS application developed by the American Forestry Association. The urban heat island effect is a reflection of the microclimatic changes brought about by human alterations of the Earth's surface in densely populated areas. It is a good indicator of the sensitivity of the climate system to human forcing on a local level. Heat islands of varying extent and magnitude have been observed in most urbanized areas in the world. (Landsberg, 1981). Urban Heat Island Effect The urban heat island effect is a complex, site and episode-specific phenomenon and can therefore vary in time and space. Many factors can alter the strength of the urban heat island on any given day or in any given location. Some of these factors include: season, climate, weather conditions, urban characteristics, and human moisture and heat sources present in the area (Taha 1997). Heat islands develop in areas that contain a high percentage of non-reflective, water-resistant surfaces and a low percentage of vegetated and moisture-trapping surfaces. There are three main causes of the urban heat island. First, heat islands form as a result of urban surfaces, such as asphalt, pavement, roofs and walls. These surfaces have low albedos and absorb much of the incoming solar radiation. They re-radiate solar energy in the form of infrared heat. This is seen most often at night, when a city will remain warm in comparison to surrounding areas even with the absence of the sun (Taha 1997). Second, the lack of urban vegetation such as trees and shrubs contributes to the heat island effect. Trees serve to cool surrounding air through evapotranspiration (Sailor 1998). When plants undergo photosynthesis they release water vapor, which evaporates upon release and cools the surrounding air. In general, it is thought that vegetation plays a larger role in the regulation of surface temperatures than do non-reflective surfaces (Goward 1985). Third, the urban heat island is also affected by a lack of moisture availability in cities due to the large fraction of impervious surfaces. Stormwater runs off quickly, leading to a reduction in the cooling effects of evaporation (Sailor 1998). Non-impervious surfaces will absorb precipitation, and it can be evaporated slowly from the soil. In the absence of favorable meteorological conditions, elevated surface temperatures may not translate into elevated air temperatures. In general, clear skies and low winds provide the optimal conditions for the development of a heat island (Oke, 1982, Morris, 2001, Kidder, 1995). Cloud cover plays a leading role in the radiative exchange in an urban area, while windspeed drives the turbulent exchange of heat in and around the urban region (Morris, 2001). Moisture availability also plays a large role in the development of heat islands, but this effect has been studied less thoroughly.
An examination of urban heat island characteristics in a global climate model
International Journal of Climatology, 2011
A parameterization for urban surfaces has been incorporated into the Community Land Model as part of the Community Climate System Model. The parameterization allows global simulation of the urban environment, in particular the temperature of cities and thus the urban heat island. Here, the results from climate simulations for the AR4 A2 emissions scenario are presented. Present-day annual mean urban air temperatures are up to 4°C warmer than surrounding rural areas. Averaged over all urban areas resolved in the model, the heat island is 1.1°C, which is 46% of the simulated mid-century warming over global land due to greenhouse gases. Heat islands are generally largest at night as evidenced by a larger urban warming in minimum than maximum temperature, resulting in a smaller diurnal temperature range compared to rural areas. Spatial and seasonal variability in the heat island is caused by urban to rural contrasts in energy balance and the different responses of these surfaces to the seasonal cycle of climate. Under simulation constraints of no urban growth and identical urban/rural atmospheric forcing, the urban to rural contrast decreases slightly by the end of the century. This is primarily a different response of rural and urban areas to increased long-wave radiation from a warmer atmosphere. The larger storage capacity of urban areas buffers the increase in long-wave radiation such that urban night-time temperatures warm less than rural. Space heating and air conditioning processes add about 0.01 W m −2 of heat distributed globally, which results in a small increase in the heat island. The significant differences between urban and rural surfaces demonstrated here imply that climate models need to account for urban surfaces to more realistically evaluate the impact of climate change on people in the environment where they live.
Urban Heat Island: Mechanisms, Implications, and Possible Remedies
Annual Review of Environment and Resources, 2015
Urban heat island (UHI) manifests as the temperature rise in built-up urban areas relative to the surrounding rural countryside, largely because of the relatively greater proportion of incident solar energy that is absorbed and stored by man-made materials. The direct impact of UHI can be significant on both daytime and night-time temperatures, and the indirect impacts include increased air conditioning loads, deteriorated air and water quality, reduced pavement lifetimes, and exacerbated heat waves. Modifying the thermal properties and emissivity of roofs and paved surfaces and increasing the vegetated area within the city are potential mitigation strategies. A quantitative comparison of their efficacies and costs suggests that so-called cool roofs are likely the most cost-effective UHI mitigation strategy. However, additional research is needed on how to modify surface emissivities and dynamically control surface and material properties, as well as on the health and socioeconomic ...
Journal of Applied Meteorology and Climatology, 2012
Urban heat island (UHI) effects can strengthen heat waves and air pollution episodes. In this study, the dampening impact of urban trees on the UHI during an extreme heat wave in the Washington, D.C., and Baltimore, Maryland, metropolitan area is examined by incorporating trees, soil, and grass into the coupled Weather Research and Forecasting model and an urban canopy model (WRF-UCM). By parameterizing the effects of these natural surfaces alongside roadways and buildings, the modified WRF-UCM is used to investigate how urban trees, soil, and grass dampen the UHI. The modified model was run with 50% tree cover over urban roads and a 10% decrease in the width of urban streets to make space for soil and grass alongside the roads and buildings. Results show that, averaged over all urban areas, the added vegetation decreases surface air temperature in urban street canyons by 4.1 K and road-surface and building-wall temperatures by 15.4 and 8.9 K, respectively, as a result of tree shadi...
The effectiveness of cool and green roofs as urban heat island mitigation strategies
Environmental Research Letters, 2014
Mitigation of the urban heat island (UHI) effect at the city-scale is investigated using the Weather Research and Forecasting (WRF) model in conjunction with the Princeton Urban Canopy Model (PUCM). Specifically, the cooling impacts of green roof and cool (white/highalbedo) roof strategies over the Baltimore-Washington metropolitan area during a heat wave period (7 June-10 June 2008) are assessed using the optimal setup of WRF-PUCM described in the companion paper by Li and Bou-Zeid (2014). Results indicate that the surface UHI effect (defined based on the urban-rural surface temperature difference) is reduced significantly more than the near-surface UHI effect (defined based on urban-rural 2 m air temperature difference) when these mitigation strategies are adopted. In addition, as the green and cool roof fractions increase, the surface and near-surface UHIs are reduced almost linearly. Green roofs with relatively abundant soil moisture have comparable effect in reducing the surface and near-surface UHIs to cool roofs with an albedo value of 0.7. Significant indirect effects are also observed for both green and cool roof strategies; mainly, the low-level advection of atmospheric moisture from rural areas into urban terrain is enhanced when the fraction of these roofs increases, thus increasing the humidity in urban areas. The additional benefits or penalties associated with modifications of the main physical determinants of green or cool roof performance are also investigated. For green roofs, when the soil moisture is increased by irrigation, additional cooling effect is obtained, especially when the 'unmanaged' soil moisture is low. The effects of changing the albedo of cool roofs are also substantial. These results also underline the capabilities of the WRF-PUCM framework to support detailed analysis and diagnosis of the UHI phenomenon, and of its different mitigation strategies.
Mitigating the Scale of Urban Heat Island Effect in Cities with Implementation of Green Roofs
Variations of the urban heat island effect is different from city to city. To understand the variations in the heat island effect, in the center of the Colombo City the capital of Sri Lanka, with the locations of buildings and roads, the temperature and humidity was measured with the height. A detailed analysis was done to find the specific height of which the heat island effect prevails and the temperature variations with height. With that a theoretical model was developed to identify the possibilities of controlling the heat island effect. The model was developed with replacing existing flat roof areas with green roofs, in randomly selected area (0.5 Km²) in the Colombo City. These findings were coupled with the energy balance in the selected area and with a computer simulation. The reduction in the temperature in the canopy air layer in the roof level was calculated. The results show that there is a variation in the heat island effect with height. When, the vertical distance incr...
2017
The climate change projections for the Austrian cities indicate that the observed warming trend, including frequent occurrences of extreme heat events, is expected to continue in the coming decades. Due to the Urban Heat Island (UHI) effect, caused by modification of energy balance in the built-up environment, the cities are warmer than their rural surroundings and therefore more exposed to negative impacts of climate change. During prolonged heat wave events, the excess in heat combined with reduced night-time cooling, decreased ventilation and possible air pollution can cause severe health impacts on the urban population. Developing measures for reduction of the UHI effect is important in the context of sustainable urban development and climate sensitive urban planning. Number of counteracting measures such as increase in vegetation, green open spaces, green roofs, unsealing of paved surfaces, decreasing absorption of solar radiation by increasing the reflectiveness of buildings a...
A B S T R A C T Urban heat island (UHI) has been evidenced as a phenomenon having a series of negative consequences in energy use, human thermal comfort, citizens' health, wellbeing and air quality. Thus, all professions, faculties and disciplines of society are actively seeking for effective UHI mitigation techniques and strategies. Previous studies have indicated that synoptic variables such as wind, precipitation, cloud coverage, fog and air quality have significant impacts on UHI phenomenon. In accordance with this basis, we devise to develop UHI mitigation techniques and strategies based on meteorological characteristics and synoptic conditions. This paper therefore has reviewed the influences of meteorological characteristics and synoptic conditions, such as precipitation, wind, cloud coverage, fog, air pollution and haze on UHI effects. Through this work, people can obtain better understandings of using them to mitigate UHI effects. Meanwhile, some suggestions on urban planning and development have been briefly presented for the alle-viation of UHI effects.