UCP11: Influence of urban vegetation II : parks & green roofs
The climatic and bio-climatic impact of a small central city park on the surrounding urban environment during extreme heat events
School of Earth Environment and Atmosphere, Monash University, Australia
Urbanisation and the extensive use of impervious materials has resulted in increased heat storage and runoff within urban areas and reduced evapotranspiration, leading to an air temperature increase within cities. Excess urban heat combined with the effects of global climate change will likely have a deleterious impact upon human health. However green spaces are known to be a most effective solution for reducing air temperatures and mitigating extreme temperatures in urban areas, through shading and increased evapotranspiration. Melbourne, like many Australian cities, has adapted a compact city policy and will become denser in the next few decades to accommodate the urban population growth. Therefore, the role and importance of small green spaces in densified urban environments becomes more critical. This study examines the climatic and bio-climatic effects of a small urban park in Melbourne on the surrounding urban environment, especially in relation to reducing air temperatures and improving human thermal comfort during extreme summer-time heat events. An on-site measurement campaign (i.e. fixed weather stations accompanied by transects) was undertaken for a representative period of the 2014 summer (January- March) to identify the cooling capacity of the park, with a particular focus on air, radiant surface and mean radiant temperature patterns inside the park and in the immediate surroundings, along with the propagation of influence downwind. Air temperatures as high as 44.0°C were recorded during the study period. According to the results, the park was found to be cooler than the surrounding built-up area at all times, whether air or radiant surface temperature was used as a measure. However, the magnitude of park-induced coolness (the park cool island (PCI) effect) was found to vary from 0.5-3.0 °C depending on the time of the day and park’s soil moisture condition (i.e. irrigation). The maximum air temperature difference between the park and its surroundings occurred at midday and immediately after sunrise. Dynamic variations of surface temperature differences within the park and adjacent urban areas could exceed up to 35°C and depended on land surface type and moisture availability. A downwind propagation of cooling effects of the park that exceeded up one park-width of the park could also be observed, depending on the wind direction.
Influence of urban vegetation.
1med ingenierie, France; 2Mairie de Paris, France; 3IRSTV/CERMA, Nantes, France; 4Université de La Rochelle, LASIE, France
Greenery in city has a direct impact on the air temperature. This leads to an air temperature difference between build-up and green areas (usually called "Park Cool Island" effect), which affects the urban heat island effect. The influence of the greenery depends on its type and may vary along the day. The first part of this study analyses the park cool island effect of various types of greenery: large and small parks with different tree density, canal with roadside trees, green roofs. Seven places located in Paris are analyzed. The measurement campaign was carried out during 2014 cooling season, when the heat may have a negative impact on human health. The results show that air temperature of all those greenery devices is lower than air temperature of a reference build-up area during night-time except the canal with roadside trees. It is also shown that large parks better decrease the air temperature than small parks and green roofs. During day-time, no clear trend appears. It is not always possible to highlight a correlation between a temperature signal behavior and a greenery device. In a second part, the cooling effect of fresh or hot air "produced" in the greenery areas is evaluated. For each of the seven previous places, two peripheral stations are implemented at different distances of the greenery area boundary in order to identify a potential cooling effect. The analysis of the results is performed confronting to the low scale observation results that have been obtained by Spronken-Smith and Oke (1999). To conclude on the effect of each greenery device on the cooling effect, temporal evolution of the measurements carried out at the three locations are compared. Only the large park shows a cooling effect both during day-time and night-time. For the most part of the other places and because of the place location, it is not possible to conclude either or not a cooling effect exists. Guidelines concerning new locations choice for a future experimental campaign are selected from our results: at least two places should be located in the greenery area and three in the surrounding. The peripheral places should be located along a same axis in the same direction from the greenery type and in a similar geographical context (similar sky view factor and land-use type). For a better estimation of the cooling effect, several other peripheral axes may be instrumented in an other direction than the original one.
Temporal variations of transpiration and latent heat fluxes from isolated linden crowns and lawns in a park at Strasbourg, France
1UMR 547 PIAF, INRA - U. Blaise Pascal, Clermont-Ferrand, France; 2UMR 7357 ICUBE, CNRS - U. Strasbourg, Strasbourg, France; 3UMR 1137 EEF, INRA - U. Lorraine, Nancy, France
In a context of global climate change, mitigating the “urban heat island” effect is becoming an urging necessity with respect to human health. Among the different mitigating strategies, increasing the vegetated surface areas, such as trees and lawns, is thought to induce a cooling effect by increasing the latent heat flux (E) from the transpiring plants. However, there is nearly no information on direct assessment of plant transpiration to the total latent heat flux of a city center.
The aim of this study is to estimate the latent heat flux emitted by a lawn surface (EL) and isolated linden crowns (ET) in the city centre of Strasbourg (France), and to characterize the seasonal evolution of these fluxes with respect to atmospheric and soil variables.
Six adult linden (Tilia tomentosa) trees grown in an urban park were equipped with Granier’s thermal dissipation probes for continuously monitoring the total crown transpiration starting from April 2013. The estimated total sap flux was assumed to be the total transpiration and converted into ET. The EL values were measured manually on two measurement locations using a closed chamber system during five summer diurnal cycles. Both ET and EL were expressed on ground surface area basis.
During the growing season, the ET varied daily, according to the main meteorological variables (global radiation, air temperature, vapor pressure deficit). The maximum ET values ranged from 183 W.m-2 to 284 W.m-2 during August, according to the crown size (and location). In comparison, the maximum EL values ranged from 290 W.m-2 to 302 W.m-2. When compared to the ETP values, the ET/ETP ratio increased from budburst to mid June according to leaf area development. The ratio remained rather steady within the 0.3-0.4 range (regarding the trees) until October. In late summer and autumn, the ET/ETP dropped below 0.1 several days, indicating a possible stomatal regulation of ET in response to soil water limitation. Further analyses will be conducted to better assess the water balance dynamics in this urban park. The daily variations in diameter of the trees, which reflect their hydric status dynamics, will be compared to stomatal closure. The measured values of linden and lawn transpiration will be used for validating the estimates of latent heat flux performed by the eddy correlation method on the same site.
Thermal effects of Woody Green Areas in Urban Landscapes in Campinas City, Brazil
1UNIVASF, Brazil; 2UNICAMP, Brazil
Urban Landscapes can be described by the density, occupation, land use, height-to-width ratio, soil permeability and green area percentage. These factors affect the local climate, distinguishing the neighborhoods cool and comfortable of the hot and discomfortable. This paper quantifies thermal effects of wooded green areas in different urban landscapes, classified according the urban climatic zone – UCZ. Also, it recommends the minimum percentage of wooded green areas to produce effects in the local climate. Measurement campaigns to obtain air temperature (oC) and specific air humidity (g/m3) were conducted over a 1 year (2010) in three times (9h, 15h, 21h), in seven neighborhood in Campinas City, Sao Paulo – Brazil. Data were collected by sensors located in representative points of the urban zones, above the roughness layer, by mobile transactions, and by tripods into the green areas. Thermal differences between urban landscapes, wooded areas and the official meteorological station were obtained. Air temperature can decrease up to ___ oC and air humidity can increase up to ___ g/m3. Results showed the green area dimension (m2) necessary to decrease air temperature and increment air humidity depends on the urban land characteristics. To the UCZ 1, minimum percentage of wooded green area obtained was %; to the UCZ 2, x%; to the UCZ 5, y%. Minimum of 20%, distanced approximately twice the wooded green area length, produce thermal effects in all urban landscapes studied. The results can be adopted by urban regulations and applied by urban planners to promote cool islands in Brazilian tropical cities.
Leaf area measurements of urban woodlands, parks and trees in Gothenburg, Sweden
University of Gothenburg, Sweden
Leaf area of urban vegetation is an important characteristic since it influences e.g. the urban climate through transpiratory cooling, air quality through air pollutant deposition and water management through rainfall interception. Measurements of leaf area are fundamental to accurately model these processes. Few studies have however presented leaf area measurements in the urban environment.
The aim of this study was to i) describe the urban greenery based on measurements of leaf area index (LAI) of trees in different types of urban environments and ii) compare two different methods to measure LAI of urban trees.
During the summer of 2014, LAI was measured in a central urban deciduous woodland, a suburban mixed forest, a central old park, a grove adjacent to a traffic route and in allotment gardens in Gothenburg, Sweden. In addition, single urban trees of seven common urban tree species in Gothenburg were measured (Acer platanoides, Aesculus hippocastanum, Betula pendula, Fagus sylvatica, Prunus serrulata, Quercus robur and Tilia europaea).
Two different indirect methods were used; the LAI-2200 plant canopy analyzer (Li-cor Inc.) and hemispherical photography. The digital images were analyzed with Hemisfer (Schleppi, WSL). The canopy measurements were performed in a grid or cross with 8-32 points with fixed intervals. For single urban trees, 3-6 specimen of each species was measured.
Average LAI of the measured urban parks and forests ranged between 2.6 and 4.8. A better way to characterize the different sites was to use cumulative density functions, which visualized the degree of heterogeneity. The old urban park had the largest LAI range (0 - 8.3). As a comparison, LAI ranged from 2.4 to 5.9 in the more homogenous urban woodland. LAI based on hemispherical photos was similar to the values received by the LAI-2200 plant canopy analyzer.
Both methods had advantages and disadvantages. The urban environment offers challenges not present in forest canopies, such as interference of buildings. A combination of methods might be necessary for an environment as heterogeneous as the urban, with both single trees and forest canopies.
Employing Terrestrial LiDAR to detail Tree Canopy Structure and Shade for the Cooling Effect Analysis
Nanjing University， China, China, People's Republic of
Urban warming has become a serious problem along with global warming and the rapid urbanization. One important phenomina is the increasing urban heat island (UHI) effect, which has caused serious negative impacts on energy consumption, environmental pollution and human well-being. Trees lower surface and air temperatures by providing shade and through evapotranspiration, and therefore, are a useful strategy to effectively mitigate the UHI effect. Cooling effects of different trees is different due to the different tree crown size, density and optical properties of their leaves. Selecting the best species to plant is important for the mitigation of the urban thermal environment and for saving energy as well as sustainability. In this research, we selected four woodlands with different vegetation structure. Three woodlands were dominated by common species (Cinnamomum camphora, Metasequoia glyptostroboides, Magnolia grandiflora) frequently planted in Nanjing city, China, and one is a mix-woodland with several different species. The HOBO meteorological stations were used to measure the microclimate environment. Terrestrial LiDAR was employed to detail the vegetation canopy structure and capture the three-dimensional point cloud of leaves as well as the shade at each station. The statistical analysis has been used to capture the cooling effect characteristics of different woodland and their related impact factors. The findings of this research revealed that the vegetation could influence the microclimate underneath the tree canopy and their impacts differed among species. The statistical analysis also showed that the woodlands have an obvious temperature reduction in the daytime (5:00 h-19:30 h) but weak during the night (19:30 h- 5:00 h).The temperature reduction of different species was (in decreasing order) Metasequoia glyptostroboides, Cinnamomum camphora, Magnolia grandiflora and then the mixed broad-leaved woodland. Compared with LAI (Leaf area index) and SVF (Sky view factor) with the L_V3DPC and shade respectively, L_V3DPC and shade can more accurately reflect the impact that vegetation canopy have on the cooling effect. The correlation analysis between L_V3DPC and shade, microclimate and cooling effect proved that shading by trees is of prime importance in mitigating the thermal environment. The high significance of L_V3DPC and shade indicate that the tree canopy is a major component that is able to contribute to microclimatic environments- particularly the cooling effect under the tree canopy. The research presents an innovative technique for analyzing tree canopy shade by using ground-based LiDAR data to analyze the cooling effect of trees. The results can be used as a guide for selecting the best species for urban greenspace planning and designing to mitigate the urban thermal environment and enhance energy savings in urban environments.