POSTER 19: CCMA - UHI mitigation strategies based on vegetation
Optimizing urban irrigation schemes for a trade-off between energy and water consumption
Arizona State University, United States of America
In the United States, building energy accounts for around 40% of the total energy consumption in cities, with an increase in cooling load due to urban heat island (UHI) effect. Irrigation of green spaces in cities helps to alleviate thermal stresses in hot seasons, but calls for an intricate – and rarely intuitive – balance between energy and water resource usage. While the objective for agricultural irrigation is mainly focused on the yield of produces, irrigation of urban vegetation necessarily requires a new paradigm by considering the water-energy nexus. In this study, we investigate a variety of uncontrolled and controllable (“smart”) urban irrigation schemes in Phoenix, Arizona, US, using a cutting-edge urban canopy model with realistic representation of urban hydrological processes. We devise a measure of energy-water trade-off based on the combined monetary saving/cost analysis to evaluate the environmental sustainability of individual irrigation schemes and their impact on human thermal comfort as well as building energy efficiency. It is found that that the soil-temperature-controlled irrigation scheme is the most efficient in saving (with comparison to “no irrigation” case) building energy in an annual scale, as compared to the constant-rate and moisture-controlled cases. The total annual saving, combining savings of energy and water based on local rates, depends on the threshold of soil temperature for irrigation activation, and can be up to $2.5 per square meter wall area per month. In addition, this scheme can substantially enhance outdoor thermal comfort of pedestrians in summers. This study helps to deepen our insights into a potential new paradigm for urban irrigation using a measure of trade-off between energy and water consumption.
Does urban vegetation enhance carbon sequestration?
1Singapore-MIT Alliance for Research and Technology, Singapore; 2National University of Singapore, Singapore; 3Molina Center for Energy and the Environment, USA
Many cities are developing policies to promote greenery as a measure to reduce their greenhouse gas emissions. However, the potential to directly remove carbon dioxide (CO2) from the atmosphere by urban vegetation is still poorly supported by scientific evidence. Current assessments consider only the carbon accumulated by trees and usually neglect the contribution from soil respiration and the emissions associated with greenery management. Studies in mid-latitude cities suggest that the carbon uptake by urban vegetation is small compared to the magnitude of the anthropogenic emissions. The usually evergreen vegetation in (sub)tropical cities may have the potential for a larger carbon sequestration. To investigate this, the CO2 flux data from two sites in Singapore and Mexico City were analyzed. Results suggest that (sub)tropical vegetation may act as either an emission source or sink depending on the species and characteristics of the trees and the amount of impervious surfaces for soil respiration.
Characterization of the behavior of watered urban materials
1Paris City Hall, Water and Sanitation Department, France; 2Univ Paris Diderot, Paris Sorbonne Cité, MSC, UMR 7057, CNRS; 3Université Paris Est, EIVP, Lab'Urba, EA 3482
Materials used in urban environments have strong impacts on urban climates and consequently on pedestrian thermal comfort. Depending on their thermal properties, they contribute more or less to the urban heat island (UHI) effect. Several UHI countermeasures involving cool materials, urban greening or urban watering have been studied.
Previous work by the authors has focused on the field study of pavement-watering as a climate change adaptation measure for Paris against increasing and intensifying heat waves. Testing over the summers of 2013 and 2014 has shown that the method reduces surface temperatures an average 13°C during pavement insolation in a N-S street paved with asphalt concrete. Air temperatures reductions of up to 1°C as well as 2°C mean radiant and 1°C UTCI equivalent temperatures reductions were also found, while relative humidity was increased by 4%RH at most. Finally, the optimal watering rate was determined to be 0.16-0.21 mm/h during shading and 0.31-0.41 mm/h during pavement insolation.
Additional work has focused on characterizing the behavior of five types of pavement frequently used in Paris in heat-wave conditions: asphalt concrete road surfaces, stabilized sand, asphalt and modular granite sidewalks and grass. Cylindrical road samples approximately 32 cm tall and 16 cm in diameter were submitted to an identical 24-hour climatic signal similar to heat-wave conditions for Paris. It was found that dark materials were the warmest day and night, grass was the coolest at all times, while stabilized sand demonstrated intermediate behavior. The modular granite sidewalk sample behaved similarly to stabilized sand in the day, but surprisingly was as warm as the asphalted materials at night.
This paper proposes to analyze the behavior of the pavement samples in the same conditions, but with active pavement-watering during the extent of the insolation period. The climatic signal consists of 8 hours of daytime and 16 hours of nighttime. In the daytime, conditions are forced to 35°C air temperature and 35% relative humidity. At night, conditions are set to 25°C and 70% humidity. A 7-bulb dichroic halogen lamp is used to insolate the samples artificially, concentrating 85% of its radiative power in the 0.3-3 µm band. In addition to surface and multi-depth temperature and heat flux measurements, surface evaporation is monitored with an automated evaporation chamber based on a LICOR LI-840A CO2/H2O gas analyzer.
This study aims to provide insight into the effects of pavement-watering on different urban pavements and how to adapt the method to each of them.
Research on Outdoor Thermal Environment of Lingnan Garden in Hot-humid Region, China -Taking Yu Yin Shan Fang as an Example
South China University of Technology, China
The aim of this paper is to explore how water body, which is one of the important landscape elements, affects the outdoor thermal comfort for gardens in hot-humid region, China. The research object is Lingnan Garden which is located in hot-humid region. The summer thermal environment of typical Lingnan Garden was simulated, based on the 3D microclimate model ENVI-met. Analysis of the spatial distribution and temporal variation of climatic factors, which include temperature, humidity, wind speed and mean radiation temperature. On this basis, use standard effective temperature (SET*) to comprehensively evaluate the outdoor thermal environment quality of Lingnan Garden in summer. Try to use modern methods to describe the characteristics of space environment in Lingnan Garden, which is a unique type of residence blended well with the natural scenery, to do a research further to grasp the effects of water body on microclimate in traditional Lingnan Garden. To explore how to use low technology strategy to solve the ecological problems in ancient times, and to let ecological wisdom about Lingnan Garden on water design get inheritance. Results show that water has the function of decreasing temperature, increasing humidity in Lingnan Garden in the summer daytime. The stronger sun radiation is, the more effect of whether there is water in courtyard is. In other words, influence of water on air temperature in the Linnan Garden is to be positively correlated with intensity of solar radiation. The space structure of underlying surface can affect the spatial distribution of air temperature around water. Air humidity change amount is relatively small on the water. In addition, the stronger sun radiation is, the more obvious the differences of standard effective temperature (SET*) in the condition of whether there is water. At 12:00 to 14:00, the difference exceeds 1 ℃. The standard effective temperature (SET*) of semi-outdoor space on the water is lower than the standard effective temperature (SET*) of semi-outdoor space on the ground. These show water can reduce the value of SET* in the garden. Water body not only can improve the summer garden outdoor thermal comfort, but also can properly adjust the indoor thermal environment. In the design process, paying attention to the vegetation and water mix and layout is help for creating a comfortable space for outdoor activities in the daytime.
Analysing green roof and albedo effects on thermal urban climate in support of climate adaptation to summer heat conditions
Deutscher Wetterdienst, Germany
The 3-dimensional micro-scale urban climate model MUKLIMO_3 was used to test the sensitivity of different albedo and green roof experiments under typical summer heat conditions using idealized squared cities. The idealized city of 5 km by 5 km is in the centre of the model domain, which extends 10 km in south-north and west-east direction with a horizontal resolution of 100 m. The size and dimension of the city and model domain as well as the meteorological conditions are kept constant in all simulations. The area around the idealized city is modelled as 80% pervious surface with low canopy vegetation and 20% bare soil. Each idealized city comprises one out of nine building classes/characteristics typically for Germany. The characteristics of the buildings within each grid cell are described by the surface fraction of buildings per grid cell, the wall area density, and the mean building height. Furthermore, impervious (without buildings) and pervious surface (bare soil and low canopy vegetation) fractions are considered.
The sensitivity experiments include changing fraction of green roofs with no green roofs as the reference and changes between 10% and 100%. Furthermore, the albedo of the roof, the walls and the impervious surface was adjusted between 0.1 and 0.8, with 0.2 as the reference albedo for the roof and the impervious surface and 0.3 as the reference albedo for the walls. In total 315 model simulations were analysed.
Increasing the roof albedo or the fraction of green roofs has similar impacts on air temperature at street level. Maximum reductions between 0.4 K and 1.2 K occur for increasing roof albedo from 0.2 to 0.6 on 100% of the roofs. Installing green roofs on 40% of the roofs can reduce the surface air temperature by a maximum of 0.7 K. An increase of the wall albedo and the albedo of the impervious surface is found to be less effective for urban adaption to summer heat conditions compared to the roof albedo. The partial shading of the walls and the street surface reduces the influence of the albedo increase of these surfaces on air temperature.
Surface air temperature reductions differ between building classes. An increase of the roof albedo of 0.1 on commercial and industrial units can reduce air temperature by ~0.3 K, while the simulated reduction is ≤0.2 K per 0.1 albedo increase for terrace houses and tenement block residential. In summary green roofs and increasing roof albedo have only a significant effect when they are applied city-wide. Both measures were most effective for building classes with a high fraction of buildings per surface area and low building heights. An increase of the albedo of the impervious surface fraction was most efficient when the surface fraction was above 45%.
The influence of different tree species on outdoor thermal comfort in the tropical urban environment
The Chinese University of Hong Kong, Hong Kong S.A.R. (China)
Urbanization has become a global trend and transformed our life enormously. While urbanization generated many benefits to citizens and also contributed to economical developments, the impact on local environment has been mostly detrimental. For example, the best-documented phenomenon is called urban heat island (UHI) effect, which has been shown to influence both local and global climate change. In residential areas, UHI effect can greatly affect people’s sensation of thermal comfort. One way to mitigate the UHI effect is urban greening as plants can provide evaporative cooling effects as well as shading benefits.
Despite the advances made on the thermal benefits and energy saving of urban greenery, including green roofs and green walls, little is known about the thermal performance of different tree species. Furthermore, little is known on the impact of diverse tree species on UHI effect and also thermal comfort. In this study, we propose to investigate the thermal performance of different urban tree species, in terms of their shading benefits and transpiration. We anticipate that our study could provide the necessary information on tree selection in urban greening, which would better inform the modern urban planning and design.
Key words: urban greening, tree, urban heat island (UHI) effect, outdoor thermal comfort
Towards “green” streets – climate change adaptation and mitigation using the suburban street; a case study from Western Sydney.
University of Sydney, Australia
South Eastern Australia and its cities have suffered record heat waves over the summer of 2013. Alongside increasing heat are projected increases in urban populations and development. Much of this new urban development has centered on the western fringes of Sydney, an area which have 4 times as many hot days as Sydney (Steffen et al, 2012.p.3).
These contemporary suburbs, characterized by single dwellings with large footprints and limited private open space are vulnerable to increased heat. While housing in these new suburbs have been built to higher environmental performance criteria, less attention has been paid to the performance of the public domain including streets within these precincts. With less space for large trees to grow on private land, streets and their trees become key environmental resources.
Research indicates that large scale tree planting in cities can impact on urban microclimate (Akbari et al 2001, 2002; McPherson & Simpson, 2003; McPherson et al 2005, Taha et al 1996). Street trees form a key component in addressing urban heat, requiring less space to reduce urban temperatures reductions than other measures. (Rosenweig et al 2006). This study builds on this research, by explore streetscape and street tree modifications and their impact on local microclimate and household electricity use and emissions.
A case study in the north-western growth corridor in Sydney is used to test street design opportunities. The research project uses a combination of new and existing modeling techniques including terrestrial laser scanning and climate housing simulation software.Outcomes indicate that simply modifying street tree configurations can reduce CO² emissions by up to 7 times that of typical streetscapes. This study lays the basis for design professionals and authorities to test and calculate optimum street configurations and allows governments to make informed decisions, set targets and measure outcomes for these essential public assets.
Trait-based species selection for urban forests under climate change scenario
1International Desert research Center, University of Tehran, Iran, Islamic Republic of; 2School of Biology, University of Tehran, Iran, Islamic Republic of
Concern of climate change has not been integrated in procedures of species selection for urban vegetation. However if climate changes as projected, species composition of urban vegetation should be changed accordingly. Plant species adapt to changing environmental condition by modifying their morpho-functional traits. A thorough review of literature resulted in a classified and annotated checklist of major plant functional traits that increase plant tolerance to environmental stresses, particularly drought and high temperature. A trait-based species selection conceptual framework was designed. Current and future climatic condition of Tehran city along with habitat requirements, tolerance and mitigating effects of plant species on climatic fluctuations were used in this framework. As a sample, urban plant flora of Tehran city was surveyed and most abundant taxa were listed. Current climatic condition of Tehran city was analyzed using data of climatic stations. We used projected data provided by worldclim database for reconstruction of future climatic condition. Using the designed framework, suitability of current vegetation of Tehran for its future conditions was analyzed. According to our results, if climate changes as projected, most of the current cultivated plant taxa should be replaced.
The impact of urban growth and mitigation measures on urban temperatures in Hamburg, Germany
University of Hamburg, Germany
Temperatures in many urban areas are expected to rise over the coming decades not just due to climate change but also through urban growth. A possible measure to reduce urban temperatures during critical periods is to implement green roofs on residential as well as industrial buildings in order to mitigate the increased fraction of sealed surfaces regularly connected with growing cities. Such green roofs can be of extensive (passive) or intensive (irradiated) type.
Hamburg, situated in the North of Germany, is expected to experience population growth over the next decades. The increased demand for residential as well as commercial properties can mostly be fulfilled by new buildings which leads to densification of current developments and to entirely new developments on previously green areas. Both measures therefore increase the sealed fraction which causes increased urban temperatures due to the urban heat island effect. To compensate for some of the temperature increase in Hamburg, the administration encourages to implement green roofs throughout the urban area.
This study estimates the temperature change due to the expected urban growth and the effectiveness of the implementation of green roofs of both extensive (passive) and intensive (irrigated) green roofs throughout Hamburg. In order to quantify both effects the Mesoscale Transport and fluid Stream Model (METRAS) is set up in a nested approach for a domain centred around the area of Hamburg in a horizontal resolution of 250m and the model is employed for a set of meteorological situations representing summer.
Results show that if green roofs are implemented on an ambitious but feasible scale, a noticeable mitigation is possible albeit not an overall compensation of the urban heat island effect. For a successful mitigation a high fraction of intensive green roofs needs to be ensured.
Examination of the interrelation between woody vegetation and urban rainwater management: stormwater runoff decrease as ecosystem service of trees in Szeged (Hungary)
University of Szeged, Hungary
Increasing continentality and need for water requires more efficient water supply management. According to recent stormwater management practices in Hungary the precipitation goes through the grey stormwater system fast and directly into a recipient (in Szeged it is Tisza River), thus the rainwater leaves the system as soon as possible. This practice is unfavorable urban ecologically, because in dry periods deteriorate the water supply of the urban vegetation and the human comfort conditions. One of the best ways to retain the stormwater in the system is the appropriate green infrastructure. Unfortunately, this current practice almost completely ignores this type of ecosystem services of urban forests. The aim of our study is to examine the runoff water reducing capacity of the trees in Szeged by applying the i-Tree Eco model. Simulation is performed on various urban spaces to assess the effect of changing vegetation on the runoff and intercepted water ratio. Our results focus attention to that engineering practice should take into account the effect of trees on managing stormwater runoff. The existing opportunities can be turning to advantages consciously.
Evaluation of greening and highly reflective materials from three perspectives
1Tohoku University, Japan; 2Shimizu Corporation, Japan
In recent years, an increasing number of people worldwide have suffered from severe heat during the summer, and various countermeasures against urban warming have been adopted. There are two aspects to these countermeasures: mitigation and adaptation. Mitigation refers to the removal of the causes of the phenomenon, whereas adaptation refers to a reduction in the effects of the phenomenon, even though the magnitude of the phenomenon does not change. Additionally, ongoing urban warming is being caused by both global warming and urban heat islands (UHIs). Global warming is caused by the rising concentration of greenhouse gases. On the other hand, UHIs are caused by the modification in the land-use from a natural environment into a built environment and anthropogenic heat release which is the result of the intensive energy consumption in urban area. Thus, completely different countermeasures are needed to mitigate these two phenomena, global warming and UHIs, and to adapt to urban warming. However, the distinctions between the countermeasures (1) to mitigate global warming, (2) to mitigate UHIs, and (3) to adapt to urban warming have remained vague. In this study, to assess the effects of countermeasures used to mitigate global warming and UHIs, and to simultaneously adapt to urban warming, a new assessment system was proposed.
This system was used to investigate the impact of greening and highly reflective materials on the vertical walls of buildings, based on coupled simulations of radiation, conduction, and convection. Under the conditions assumed in this study, greening and highly reflective materials had a positive impact on the mitigation of global warming and UHIs. However, in terms of adapting to urban warming, greening was not very effective and the highly reflective material had a clearly negative impact.
Evaluation of cool/green roof in mitigating urban heat island in a tropical city, Singapore
1Singapore-MIT Alliance for Research and Technology, Singapore; 2Department of Architecture, Massachusetts Institute of Technology
With a population reaching 5 million in 2011, Singapore has undergone rapid and dramatic urbanization during the past 45 years. These significant changes of landscapes have profoundly affected the urban thermal environment.
Previous numerical study has shown that the UHI intensity has a strong diurnal cycle, peaking in early morning and keeping almost constant during late night and early morning. The modeled ensemble peak UHI intensity is 2.2 degree Celsius and it can reach 2.4 degree Celsius during early morning in the intensive industrial region in west Singapore. The effect of different urban land use types was most pronounced during the night and least visible during the noon.
To mitigate the UHI effect and for other benefits, Singapore has applied the green roof technique recently in both commercial and residential buildings. Numerical simulations are carried out in this study to evaluate the mitigation effect of the cool/green roof. The sensitivity tests of cool/green roof implementations have been conducted and their impacts of the mean and peak surface temperature are compared. The deployment of cool roof can greatly reduce the surface air temperature during the daytime, but has little effect during nighttime. On the contrary, the deployment of green roof can reduce the surface air temperature by a rather constant value during the whole day, except during noon when the temperature reduction is only slightly lower than during other time. Based on the results of this numerical sensitivity study, it is suggested that the green roof’s performance in mitigation UHI in Singapore is better, although some uncertainties are identified in this study.