POSTER 13: UCP - Influence of urban vegetation
Tree transpiration as a potential mechanism to mitigate the urban heat island in Mexico City
1Instituto de Ecología, UNAM, Mexico; 2Centro de Ciencias de la Atmósfera, UNAM
Transpiration rates and stomatal and canopy conductances were monitored in Eucaliptus camaldulensis, Fraxinus uhdei, Liquidambar styraciflua and Ligustrum lucidum in México City, to explore the tree potentiality to reduce the urban heat loads. The experiment was carried out over a 2-week period between 11 and 27 April 2013. Four trees of each species were used. Total conductance was obtained from daily measurements of transpiration and vapor pressure deficit between 22 and 27 April, and canopy conductance from stomatal conductance and leaf area index measurements. L. styraciflua registered the highest averaged (4.35 L d-1) transpiration whereas F. uhdei registered the minima (3.64 L d-1). Averaged canopy conductance registered values between 39.8 mm s-1 (E. camaldulensis) and 49.74 mm s-1 (L. lucidum). Decoupling coefficient were low, averaging from 0.086 (L. lucidum) and 0.133 (F. uhdei). These results show that transpiration was strongly dominated by vapor pressure deficit (VPD) and controlled by stomatal conductance. According to the envelope function model, stomata were more sensitive to VPD than irradiance or air temperature. Finally, the presented transpiration rates are capable to reduce up to 20 % of net radiation in Mexico City. With these results, it is possible to build tree arrangements to dissipate the greatest possible amount of heat produced in the city.
Leaf-turning tree species and their local climatic impacts on the city
University of Kaiserslautern, Germany
Based on a research project, initiated by the Dept. of Physical Geography, University of Kaiserslautern, Germany, in cooperation with the Bavarian regional authorities for viniculture and horticulture, silver lindens of the type Tilia tomentosa are under examination. A leaf with a silver-white colored bottom-side is the main feature of this kind of tree. According to observations especially the leaves of the crown of younger trees are able to protect themselves upon hard solar radiation, because the dark green and sun absorbing leaf side turns away from the intense sunlight and uses the silver-white bottom-side of the leaf instead. According to this, the incidental solar radiation is well reflected to gain a cooling effect in the crown area of the tree. Researches on several silver lindens of different ages were started to verify this effect more precisely to be able to analyze the cooling effect of these impacts and furthermore, to transfer their possible positive consequences on local climate.
Air temperature, relative humidity, wind speed as well as long- and short-wave radiation are measured and a thermal imaging camera is additionally applied for the in-situ measurements. Furthermore, photo cameras are utilized to locate clearly heat stress points in the crown of the tree and to comprehend the change of a tree. Additional to the silver linden tree more species of urban trees are supposed to be researched, because they are, additional to heat stress, susceptible to more intense environmental pollution. The aim is to achieve a better comparability of these tree species. But not only typical urban trees will be included in the researches. Tree species comparable to silver lindens will be included, too. Hereto counts the silver poplar (Populus alba) that contains very flexible leaves. This tree also shows a comparable sun manner to the silver linden tree, so this tree species will also be included in the project.
In case that the researches will be able to show up, that these species cause a verifiable cooling effect to their surroundings, this recognition needs to be transferred to urban planning. In the context of landscape planning arrangements and urban development concepts, these trees can be specifically used for e.g. roadside or park area plantings, and probably have a cooling effect on the mostly overheated urban areas. Therefore it is required to sensitize the positions responsible for green planning in cities and to make the measurement results reasonable for urban planning.
Relevance of geometry and other basic parameters of urban trees on conditions of water and heat stress in long lasting heat waves – a simulation approach
Environmental Modelling Group, Germany
Many studies indicate that climate change will lead to an increased amount and a higher intensity of severe heat waves and dry spells. These extreme conditions will be even more concerning in urban areas where increased heat loads due to the density of urban structures and the amount of sealed surfaces are issues already. Many climate change adaptation strategies developed these days advice to introduce more so called “green technologies” into the urban structure to mitigate the effects of the urban heat island and to increase thermal comfort. Vegetation in general and trees in particular, however, will only fulfil their intended beneficial effects on the microclimate under unstressed biological conditions. Conditions typical of heat and dry waves such as limited soil water supply and increased air temperatures, on the other hand, impede the biological active complex and lead to water and heat stress of the vegetation. In conditions like these, the desired effects of the green technologies are significantly reduced, and even damages to the trees may occur.
This article will analyze how plant vitality and consequently the effectiveness of vegetation is influenced by basic parameters like plant geometry, leaf area density, and root distribution. In order to do so, the microclimate model ENVI-met is used to model various trees and to simulate plant-water-atmosphere processes in long lasting heats waves and for varying microclimate sites in a typical urban environment. The simulation results show that the complex combination of microclimate parameters, plant geometry, and leaf area density does indeed have a significant impact on plant water stress and vitality and thus the beneficial effects of plants with regard to the attenuation of the urban heat island.
Impact of Greening Area Ratio on Urban Climate in Hot-summer and Cold-winter City
1Tohoku University, Japan; 2Wuhan University, China; 3Wuhan University of Technology, China
Hot-summer and cold-winter region is the most populous and economical-developed area of China, and its hot, humid summers and cold, humid winters trouble local people very much. The weather research and forecasting model (WRF), with a single-layer urban canopy parameterization scheme, was used to simulate the impacts of different greening area ratio on urban climate in Wuhan, a typical city in hot-summer and cold-winter zone. The center city of Wuhan was divided into three zones: business zone, residential zone, and industrial zone. According to the comprehensive planning of Wuhan city from 2010 to 2020, 3 cases were designed to investigate the impacts of different greening area ratio. It was clarified that the influence of different greening area ratio in business zones on air temperature and wind velocity was greater than that in other zones. In the daytime, the air temperature decreased when greening area ratio increased; but at nighttime, the air temperature increased with greening area ratio. In the residential and industrial zones, different greening area ratio had a less effect on wind velocity during the daytime and a greater effect in the nighttime.
Cooling effects of large green park on urban atmosphere observed at the Osaka Castle Park in Osaka, Japan
1Rissho University, Japan; 2Okayama University of Science, Japan; 3Meisei University, Japan; 4The University of Tokyo, Japan; 5Osaka City University, Japan; 6National Institute for Environmental Studies, Japan
It is important for urban planning and heat island countermeasure to clarify cooling effects of urban green park on the urban atmosphere. We investigated those at the Osaka Castle Park in Japan during the summer in 2007 and 2013. The Osaka Castle Park is located at the center of Osaka City (about 2.6 million population) and has an area of about 1 million square meters. Because this park is the largest of urban green parks in the Osaka city, urban cooling is expected here.
Our observations were conducted by static and moving measuring methods for the air temperature, wind speed and wind direction at some observational points inside and outside the park.
During the 2007 August observations, the cool island intensity of the Osaka Castle Park was recorded as the magnitude of 2-3°C at 04-05 Local Standard Time (LST). This cooler park condition was maintained from sunset (around 18 LST) to the next sunrise (around 06 LST). In addition, very weak winds (about 0.5m/s) were observed outside the park, which flowed outward from the park. In particular, this phenomenon was remarkable at the east side of the park and the cold flow extended leeward of about 250m.
Next, we analyzed the temperature difference between the lawn area and forest area inside the park in order to specify a source of the cold air of the park. As a result, a downward motion process of the air cooled at the upper layer of park forests was suggested rather than an outward transport process of the air cooled on park lawn areas. Hence, generations of the cold air within dense forests at the east of the park are probably related to the cold outflow from the Osaka Castel Park as mentioned above.
On the other hand, the daytime air temperature of the park was higher than or comparable to that of surrounding urban areas. This phenomenon was probably induced by shading effects and large heat capacity of tall and dense buildings. The result means that the daytime park air is insignificant to decrease the surrounding urban atmosphere.
Because the nighttime Osaka urban temperature hard to decrease due to the urban heat island, cold outflows from the park have a potential cooling the surrounding urban atmosphere. In the present study, the recent 2013 observational results will be also included and summarized.
The TERRACES project - Qualifying and quantifying the changes in the urban energy balance using vegetative green roofs (VGR).
1CSTB, France; 2CEREMA Est, France; 3CEREMA Ile de France, France; 4GEMCEA, France
Currently, different urban planning strategies are proposed to cool of refresh micro-climate (especially in summer) acting on the temperature, wind and humidity. The frequently cited cooling devices are physical proceedings such as the vegetation cover expansion. The most prominent examples are the living walls and roofs. The proposed project intends to deeply develop the scientific analysis of the performance and impacts of vegetative green roofs (VGR) on urban climate, environment and health. The scientific approach is conducted with three tasks: 1 - to assess the refreshing potential of a VGR; 2 - to develop relevant indicators dedicated to VGR environmental impacts; 3 - to establish a link between VGR performance and spreading potential in urban zones.
This communication addresses methodology selected, the experimental protocols developed and the first results of the task 1. The aim is to establish a comprehensive coupled hydrological and energy balance of a VGR at the building scale, able to compute microclimate impacts under experimental measurements conditions and climate change scenarios at the right scale.
Estimation of evapotranspiration by VGR is important to assess the cooling potential of this system, and the purpose is to combine experimental and modeling approaches. In this project, long-term micro-meteorological, thermal, hydrological measurements are achieved each day on three fully instrumented roof platforms, in the three different partner’s locations: Nancy, Nantes and Trappes. Moreover, monthly measurements of evapotranspiration fluxes have been performed with a specific experimental device. We present a comparison of these detailed measurements in each experimental site to show the influence of the geographic context and of the roofing intrinsic composition. We also introduce a first comparison of the effective evapotranspiration measurements to the potential evapotranspiration theoretical computation. Albedo and Leaf Area Index experimental evaluation are also ongoing and will be discussed. They directly impact the surface energy balance and are difficult to assess with classical methods derived from agronomy.
TERRACES project (ImpacT of vEgetative Roofs on uRban Ambiances: Cooling effects, Environment and Spreading) consists of five partners (CEREMA Directions territoriales Est et IdF, CSTB, LEESU and GEMCEA) and is financially supported by the ADEME.
Influence of the urban vegetation fraction on the urban heat island effect in different climate zones across Europe
Wageningen University, Netherlands, The
The difference in temperature between an urban area and its rural surroundings is known as the Urban Heat Island (UHI). During heat waves, this temperature anomaly is shown to have negative effects on the health of vulnerable groups such as the elderly and cardiovascular patients, as well as decreasing labour productivity. In the light of climate change increasing global temperatures, and the ongoing world-wide urbanisation, it is important to take measures to reduce the UHI. Studies have shown that increased vegetation fraction in cities correlates with a reduction in mean UHI values, due to higher evapotranspiration and sky view factors, among others. Though this relation has been researched for single cities and countries, it is yet unknown whether this relation is universal across different climate zones. A major issue in measuring the UHI is the lack of observational sites in urban areas, and a representative rural background. Here we study the applicability of measurements from hobby meteorologists combined with ECMWF reanalysis data to solve the problem of limited data availability. Using this approach we assess the influence of the vegetation fraction of neighbourhoods on the measured UHI, to establish a relation between vegetation and UHI reduction. This analysis is performed for several European cities in varying climates (Rotterdam, the Netherlands; Oslo, Norway; Madrid, Spain; Basel, Switzerland) to determine the role of climate zones on this relationship.
INFLUENCE OF MANAGEMENT OF THE ARBORICULTURE ON URBAN THERMAL COMFORT
1IPT, Brazil; 2UNICAMP, Brazil; 3USP, Brazil
The arboriculture on streets and avenues in cities, most often is carried out after the implementation of all other urban services such as streets, streetlights, signs, sidewalks and electrical wiring, but without being included in the initial planning of the neighborhood. This fact results in a sequence of problems, dissatisfaction of the population and a lack of possible benefits which can be provided by urban trees, such as thermal comfort in open spaces, which greatly contributes to the quality of life. The influence of vegetation on thermal comfort in open spaces has been widely studied, but there are still gaps in these studies in relation to street arboriculture, especially taking into account the differences in management. So the study of thermal comfort in different situations of street arboriculture in a city, becomes an important tool to support public policies aiming the quality of urban spaces, and to establish guidelines on the management of this vegetation. Trees planted along streets and avenues are fundamental to ensure thermal comfort to urban residents, especially to those who live apart from parks and squares, in apartments without wooded gardens. This benefit is often jeopardized depending on the adopted management procedure for the trees, mainly when they undergo frequent pruning. The aim of this study is to assess the effect of different pruning procedures on thermal comfort provided by the trees in the city of Piracicaba, São Paulo State, Brazil, analyzing the same species, Tabebuia pentaphylla, in three distinct situations: drastic pruning, “V” – shaped pruning and no pruning, all located on the same street, in the same urban setting. A site under plain sunlight was chosen for control. The above mentioned situations were repeated 3 times. We collected data on air temperature, relative humidity, globe temperature, wind speed, global solar radiation and calculated mean radiant temperature. Data were collected in summer and winter, for five days each season, from 06:00 am until 06:00 pm. We also took photographs using a camera with fisheye lens, to obtain the percentage of crowns for the studied trees. We obtained Physiological Equivalent Temperatures (PET) index values using the model Rayman 2.1 and calibrations for data interpretation. Regression analyses were made concerning PET and crown percentage. Therefore 3 types of regression were tested: linear, cubic and square (quadratic), gathering by periods (6:00h-09:00h, 09:00h-12:00h, 12:00h-03:00pm, 03:00h-06:00pm) and seasons (winter, summer). The results show that the different types of pruning influence the thermal comfort. Higher PET index values were found in the control, followed by drastic pruning, V-shaped and no pruning. Regardless of the time of the year, thermal comfort was higher under crown with no pruning. It is worthy noting that the obtained values for PET were always in the warm to hot conditions, in spite of the season. The results of the regression analyses were significant making it possible to establish a relation between the percentage of the crown of the tress and the PET. In all cases, increasing the percentage of the crown, the PET decreases, that is thermal comfort is bigger. Once this relation is established, it allows future simulations of thermal comfort index according to the different arboriculture management adopted. With this study, we demonstrated that pruning does have influence on the PET index for thermal comfort.
Cooling effect of urban green against urban heat island effect - PIV observatian of the airflow from an urban green space
Kanasai University, Japan
Because Japan located in mid-latitudes, urban heat island effect (UHI) is regarded as an environmental problem in summer season. In this climatic zone, urban green, which can supply cold air into the urban, is expected a mitigation role against UHI. This is one of researches to clarify the characteristics of cold air flow from green spaces in urban. I used the Particle image velocimetry (PIV) method to observe the cold air flow from the green spaces. PIV is an optical method of flow visualization. This method enables to observe the movement of air mass and figure of air flow at a glance.
The subject park located in Suita City, upper Osaka metropolitan area, Japan. This observation had done in a summer hot night, started from mid night and ended before sunrise. I carried out this PIV observation at the edge of a urban park covered with tall trees. At the same time, I measured vertical profiles of air temperatures from the ground surface to 3 meters high at the middle of green space and at the edge. The surface temperature observation in green space had done with infrared thermography.
As the result, I found the followings.
- Cooler airflows were observed in comparison with urban air temperature.
- The air flowed out in an adverse wind condition.
- The airflow velocities were around 0.3 meter per second.
- The vertical size of the airflow was more than 3 meters.
- Cooler air mainly contained in the lower layer in the airflow. Cooler part were observed less than 1.5 meter high.
- The air flow at the edge of the park could be divided into 3 patterns: (1) the air flow from the green space to the outside, (2) the air flow from outside into the green space and (3) calm.
- The duration time of the airflow from the green space was 4−7 times in comparison with the duration time of the opposite airflow.
- In the mid night, every surface temperatures in the green space were higher than every air temperatures in the green space. I could not clarify what made the air cool in this space.
- Around one hour before dawn (4:30 LST), surface temperatures in the green space were cooler than air temperature in the green space.
- At 4:30 in the early morning, cool air area was estimated to grow larger include my measurement point at the edge of park. This might be caused by the pond, which faced to this measurement point.
- The estimated cool air volume, which was calculated from the observed wind velocity, the observed hight and the duration time of the out-going airflow from the green space, was as large as the total volume of the green space.
Influence of nearby plants and artificial structures on the surface air temperature statistics: Continuous in-situ measurement at central Tokyo (Otemachi)
Meteorological Research Institute, JMA, Japan
In order to clarify the influence of obstacles on temperature measurement, two years' continuous observation was made in the meteorological enclosure of the Japan Meteorological Agency (JMA) in central Tokyo (Otemachi) by installing an extra thermometer at a spot that was surrounded by trees and was 20m distant from the thermometer for operational observation.
In the early afternoon of the warm season, the temperature at the spot surrounded by trees was found to be higher than the operational temperature. The temperature difference was about 0.5 degrees Celsius on the average, and was correlated with daily solar radiation with a correlation coefficient of over 0.7 at 15JST from March to September. On the other hand, difference of nighttime temperature was generally small, so that the monthly mean difference of daily mean temperatures was less than 0.1 degree Celsius for all the months of the year.
It was also found from observation using supersonic anemometers that the wind speed at the spot surrounded by trees was only one third of that at the center of enclosure. It is speculated that the reduced ventilation under strong insolation is a cause of anomalous temperature in the spot surrounded by trees in the daytime of the warm season.
Influence of Meteorological Variables, Trees and Greens in Open Spaces on Environmental Quality
1Department of Crop, Soil and Pest Managemet, Federal University of Technology, Akure, Nigeria; 2Department of Meteorology, Federal University of Technology, Akure, Nigeria
Vegetation present in open areas contributes to define the microclimate and contributes to thermal comfort. This paper shows the influence of trees and greens in open spaces for environmental performance and on thermal comfort conditions. In order to compare the contribution of trees with greens, greens without trees and bared open space, the study was carried out in the Federal University of technology, Akure selecting nine (9) different locations and it ranges from several species of trees and greens within the campus. Meteorological observation and measurements were carried out on some variables (wind speed, temperature, relative humidity, light intensity, radiation). Data was collected in the first 14days of August when monsoon type (vortex) rainfall was predominant due to the northern position of the Inter-tropical discontinuity (ITD) at about 2 oN. Temperature behavior from the findings showed that a tree with dense canopy tends to have warmer temperature than trees with less dense canopy at early mornings and late in the evenings.
Key word: Tree, Greens, Open space, Thermal comfort.
Impact of surrounding building geometry, vegetation and ground cover on the variation of Microclimatic parameters and thermal comfort within urban open spaces
Visvesvaraya National Institute of Technology, India
Urban open spaces like parks and playgrounds are a big source of refreshment and repose for the urban inhabitants. A comfortable thermal environment is essential for performing these outdoor activities. For a tropical city like Nagpur thermal comfort is of prime concern, especially in summer season. A field of spatial and temporal variation of microclimatic parameters and thermal comfort conducted during 2010-2012 reveals that surrounding building geometry, vegetation and ground cover play a decisive part in modulating the microclimatic parameters and thermal comfort. Shading due to the surrounding buildings and vegetation affects the solar exposure of the soil surface and in turn the radiant component. A thermal environment mapping in the form of overlays can prove a great helper to an urban planner in improving the habitability of these exposed spaces.