Session Overview
CCMA9: UHI mitigation strategies V : vegetation based strategies
Thursday, 23/Jul/2015:
4:30pm - 6:00pm

Session Chair: Matthias Demuzere, KU Leuven
Location: Cassiopée Room


Urban green belt outdoor thermal evaluation via leaf area index

Kuo-Tsang Huang1, Ruey-Lung Hwang2, Meng-Chieh Tung1

1Dept. of Bioenvironmental Systems Engineering, National Taiwan University, Taiwan; 2Dept. of Architecture, National United University, Taiwan

With the widespread of urban living area and due to the urban heat island effect, people in urban outdoors is experiencing more severe heat stress than in rural area during summer period. Street trees can enhance pedestrians’ thermal comfort mainly by providing sun shade via its leaf canopy to reduce human body’s solar radiant exposure. Urban green belt is an important greening strategies in fighting with urban heat island effect in Taiwanese major metropolitan area and has been promoting for years. It is a consecutive urban open space allocated alongside the street with tree greening to serve as recreation purpose. However, tree planting location and tree selection would significantly influence the space users’ thermal perception. As the density of a tree’s leaf would crucially affect the amount of radiation received underneath, this study aimed to identify the minimum required leaf density in terms of leaf area index (LAI) for tree planting selection to provide pedestrian sufficient thermal comfort in summer under hot and humid climate context of Taiwan. Correlation model of solar radiation reduction rate with LAI had been established and was used for calculating radiation received underneath tree’s canopy. In assessing hourly thermal condition, local typical meteorological year (TMY3) data containing hourly weather elements including total horizontal radiation, dry bulb temperature, relative humidity, wind velocity, and cloud cover, was used as outdoor climate conditions. These weather data together with the model estimated reduced solar radiation by trees were afterwards used for estimating the physiological equivalent temperature (PET). The outdoor thermal comfort PET range of Taiwan suggested by T.P. Lin was adopted as criteria in assessing overheating occurrence frequencies and overheating severities under various LAI values, thus to reversely identify the appropriate monthly LAI needed for ensuring thermal comfort. The results show that a minimum of LAI 2.0 is required to maintain outdoor thermal comfort in hottest July to ensure the overheating occurrence probability less than 5%. Other suggested values of minimum required LAI in June, August, and September are 1.44, 1.85, and 1.30, respectively. These results could be serve as a tree selection reference in planning or designing urban green belts in the viewpoint of outdoor thermal comfort.


How Do Green Roofs Mitigate the Urban Heat Island Effects under Heat Waves?

Ting Sun, Guang-Heng Ni

Tsinghua University, China, People's Republic of

Green roofs (GRs) are widely recognized effective in the mitigation of rooftop surface and near air temperatures at the building-scale due to their passive cooling function. This study investigates the mitigation of the Urban Heat Island (UHI) effect of GRs at the city-scale via numerical simulations using the Weather Research and Forecast (WRF) model coupled with the Princeton Urban Canopy Model (PUCM). The WRF-PUCM is set up in the Greater Beijing Region (GBR) during a heat wave period (July 1– July 5 2010) to assess the impacts on air temperature (T2) and humidity(Q2) at 2 m AGL (above ground level) and wind speed (U10) at 10 m AGL of GRs by varying their coverage in the urban area. Results indicate that (1) T2 and U10 significantly decrease whereas Q2 remarkably increase with GR implemented and (2) all the variations in T2, Q2 and U10 demonstrate strong linear correlation with the increase of GR coverage. The investigation also reveals that the mitigation of UHI results from the combined effect by GRs in the surface energy balance and regional advection: GRs dissipate more available energy vertically by latent heat flux and consequently prohibit the advective heating of urban area from its surrounding region under heat waves. This study underlines the effectiveness of GRs in the mitigation of UHI effect at the city-scale and shed light in employing the WRF-PUCM framework to support detailed analysis and diagnosis of the UHI phenomenon.

Evaluation of greening scenarios to reduce Paris city vulnerability to future heat waves

Cécile, Sylvie DE MUNCK1, Aude LEMONSU1, Valéry MASSON1, Marion BONHOMME2, Julien LE BRAS1

1CNRM-GAME,CNRS UMR3589, Centre National de Recherches Météorologiques, Toulouse, France; 2LRA-GRECAU, Toulouse, France

Climate projections predict an amplification of global warming, potentially exacerbated in urban areas by the urban heat island effect. More frequent extreme events such as heat waves may have severe public health, ecological, and economic consequences as cities concentrate population. Among the measures aiming at improving thermal comfort or energy demand in the context of heat waves, a sustainable adaptation strategy, urban greening, is evaluated, based on urban climate simulations across the Paris area.

The modelling of urban climate under various greening scenarios relies on the Town Energy Balance model (TEB, Masson 2000) and its most recent developments. These include an improved parameterization of building energetics (BEM) and the Vegetation module which now encompasses parameterizations for better accounting for low vegetation in urban canyons (Lemonsu et al. 2012), for simulating green roofs (de Munck et al. 2013) and for representing watering practices. This study, which is a component of the MUSCADE project, is realized by running simulations with climate forcings and a dynamic urban heat island generator. This simulation configuration allows greening scenarios to be evaluated for a short heat wave event (2003) as well as for a 10-year period in order to determine the mean impacts of a summer-orientated strategy at the scale of an entire year. Simulation results are analyzed in terms of energy consumption, outdoor thermal comfort and level of water resource induced by watering.

The scenarios tested consist in an increase in ground-base vegetation (with various greening rates) or an implementation of green roofs on compatible buildings, or the two combined, with the option of watering green roofs or not in summer. Results show that increasing the ground cover has a stronger cooling effect than implementing green roofs, and even more so when the greening rate and the proportion of trees are important. The green roofs are however the most effective way to reduce energy consumption, not only in summer but also on an annual basis, mainly due to their insulating properties.

Green infrastructure and ecosystem services to tackle climate change in Chilean cities.

Alexis Vásquez1, Pamela Smith1, Cristian Henriquez2, Dayán Martin1

1Universidad de Chile, Chile; 2Pontificia Universidad Católica de Chile, Chile

Considerable literature has described ecosystem services of urban green spaces. However, only some of them are especially important for climate change mitigation and adaptation in urban environments. First, the paper presents and discusses the arguments in favor of urban green infrastructure and ecosystem services provision as key components of resilient urban ecological systems to climate change. Second, the developed analytical framework is applied at two spatial scales (1) green infrastructure was mapped and analyzed in six Chilean cities, and (2) riparian corridors in Santiago were analyzed in terms of their current and potential contribution for tackling climate change by evaluating three key ecosystem services (a) cooling effect, (b) routes for non-motorized transport, and (c) flood mitigation.


Evaluation of CO2 Reduction Effects of Buildings with Green Roofs by Using a Coupled Urban-Canopy and Building-Energy Model

Yujiro Hirano1, Yukitaka Ohashi2, Toshiaki Ichinose1

1National Institute for Environmental Studies, Japan; 2Okayama University of Science, Japan

In recent years, global warming has become more serious, and it is now an urgent priority to reduce energy consumption and CO2 emissions. In particular, energy consumption for cooling in the summer continues to grow every year, and thus it is highly important, from the perspective of measures to address global warming, to reduce cooling demand in residential and living spaces. On the other hand, increasing temperatures due to urbanization (i.e., the urban heat island phenomenon) is becoming more serious in urban areas, and the loss of comfort and increase in cooling demand in urban spaces have become serious problems. Therefore, improving the thermal environment in urban areas is an important issue linked to both the global and urban environment.

Rooftop greening is one measure for improving the urban thermal environment. Although vegetation is known to be effective for reducing temperature via transpiration, there is only limited leeway for on-ground greening in urban areas, and thus in recent years rooftop greening has garnered particular attention. Therefore, this research focused on rooftop greening in office building districts, and its purpose was to evaluate the effectiveness of such greening in mitigating heat island conditions and reducing CO2 emissions, while taking into account the amount of water needed for evapotranspiration.

In this research, a coupled urban-canopy and building-energy model (CM-BEM) was used to carry out simulation-based evaluation of the effectiveness of rooftop greening in mitigating heat island conditions, and reducing CO2 emissions due to a decreased need for cooling energy. The rooftop greening assumptions were set to 3 levels of 0%, 50%, and 100%. Next, the amount of water needed for evapotranspiration was calculated from the latent heat flux, which in turn was obtained from the results of calculating the roof surface heat balance. In addition, the effect of rooftop greening in reducing CO2 emissions was clarified based on its effectiveness in reducing demand for cooling energy, and this was done by calculating CO2 emissions produced when the needed amount of water is supplied by watering using tap water.

The main results of this research were as follows:

(1) The heat island mitigation effect of rooftop greening was evaluated. The results showed that, under the calculation conditions of this research, the temperature reduction in the case of large-scale adoption of rooftop greening was a maximum of about 0.13°C. The relationship between this temperature reduction effect and evapotranspiration was also clarified.

(2) The CO2 reduction effect of rooftop greening was evaluated. In particular, when evaluation was carried out in this research taking into account both CO2 reduction due to cooling energy and CO2 emissions due to watering, it was shown that the former is clearly greater in terms of direct effects due to the reduction in surface temperature, and a CO2 reduction effect can be achieved in buildings where rooftop greening is adopted.

(3) In addition to the standard conditions assumed in this research, calculation conditions were also assumed in which the roof insulation performance and evaporation efficiency of the rooftop greening area were varied, and the same examination was carried out. The results showed the relationships, under each calculation condition, between water amount, heat island mitigation effect and CO2 reduction effect.