UDC2: Impact of Urban forms on comfort II : temperate and cold climate cities
The impact of urban geometry on the radiant environment in outdoor spaces
Kent School of Architecture, University Of Kent, United Kingdom
Urban geometry, namely the quantitative relationship of building volumes and open spaces (i.e. built density) and their spatial configuration (i.e. urban layout), is a major modifier of urban microclimate. This paper presents the results of an ongoing research which explores the impact of urban geometry on the radiant environment in outdoor spaces, with direct implications for urban microclimate and outdoor thermal comfort. In particular, the research investigates the relationship between a set of urban geometric indicators (such as Built Density, Site Coverage, Mean building Height and Frontal Area Density) and Mean Radiant Temperature (Tmrt) at the pedestrian level, in different areas of London.
Three representative areas of London were selected to be studied; in central, west and north London which are of high, medium and low built density, respectively. Each area was divided into squares of 500m x 500m size, with a total of 84 urban squares included in the study. The methodology comprises three stages: (i) A set of simple geometric indicators have been computed for all urban squares using special algorithms written and executed in Matlab software. (ii) Radiation simulations have been performed for 10 days of a typical year in London, with the use of SOLWEIG software. SOLWEIG simulates hourly, 3-D radiation fluxes, incoming to / outgoing from the ground, spatial variations of Tmrt, Ground View Factor (GVF) as well as Sky View Factor (SVF). Sunny and cloudy days have been considered, evenly distributed in the year in order for the effect of solar angles to be examined. (iii) Statistical tests have been conducted for investigating the correlation between urban geometry, as expressed by the geometric variables, and hourly, average values of Mean Radiant Temperature in the outdoor spaces of the urban squares.
The simulation results show that at night-time and in fully overcast conditions, the outdoor spaces of central London’s urban squares are warmer than those of west and north London, due to greater longwave radiation emitted and reflected by building volumes. In contrast, on sunny days, average daytime Tmrt values have been found to be higher in North London’s urban squares due to the larger insolation of their outdoor spaces. Additionally, the statistical analysis has shown that in the absence of direct solar radiation, the correlation between the geometrical variables and average values of Tmrt is very high with an almost perfect linear relationship between the geometrical variables and average SVF values (r2= 0.980). In the presence of direct solar radiation, the strength of the correlation varies with the sun altitude angle; the higher the sun altitude angle, the higher the correlation. In particular, a threshold altitude angle of 20 degrees has been identified, above which the correlation of average Tmrt values with urban geometry approximates that of night-time / cloudy hours. Finally, further statistical tests showed that site coverage (built area over site area) and frontal area density (façades’ total area over site area) are the strongest indicators among those considered in the analysis.
Neighbourhood morphology and solar irradiance in relation to urban climate
1Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland; 2Royal Holloway University of London, UK
Assessing the relationship between neighbourhood morphology and urban climate is becoming increasingly important as cities continue to grow and the climate continues to change. Here we focus on the impact of (1) shapes and sizes of buildings, (2) street layouts, and (3) spatial distributions of buildings and streets on the urban climate in the 16 neighbourhoods (zones) within the city of Geneva, Switzerland. The results show that the size distributions of the areas, perimeters, and volumes of the buildings follow approximately power laws, whereas the heights of the buildings follow a bimodal (two-peak) distribution. Using the Gibbs-Shannon entropy formula as a measure of dispersion (spreading), we calculate the area, perimeter, volume, and height entropies for the 16 neighbourhoods in Geneva and show that the entropies have strong positive correlations (R^2 = 0.43-0.84) with the average values of these parameters. By contrast, there are negative correlations (R^2 = 0.39-0.54) between building density or urban compactness (site coverage and volume-area ratio) and the entropies of building areas, perimeters, and volumes. The calculated length-size distributions of the streets show negative correlations (R^2 = 0.70-0.76) with the number of streets per unit area as well as with the total street length per unit area. We compare the neighborhood morphologies with the annual and monthly solar irradiance for each of the 16 neighborhoods using the simulation tool CitySim. The results show a negative correlation between building densities and solar irradiation as well as between street densities and solar irradiation. By contrast there is a positive correlation between entropy of the street lengths and solar irradiation, indicating that the greater the variation in street length the greater the solar irradiation. Further work will include comparison of these results with the average surface temperatures, sky view factors, and daylight factors in the 16 neighborhoods so as to explore the relations between neighborhood morphology and the local climate and, thereby, providing quantitative information to help urban designers in the decision making processes.
Climate-Conscious Development of an Urban Area
1Hungarian Meteorological Service, Hungary; 2Hungarian Urban Knowledge Centre Non-Profit Ltd
Climate-conscious urban planning, especially that of public spaces, has little history in Hungary. Although Hungary’s Environmental Law requires that each settlement prepare a program for the protection of the environment, these documents tend to be either overly theoretical studies or summaries of the initiatives of local non-governmental organisations. In urban planning documents, climate consciousness manifests itself mostly in the parroting of well-known slogans, without any concrete practical suggestions.
Relying on several years’ worth of research identifying and evaluating factors that influence climate, and inviting the contribution of external partners, we tested the effects that climate-related factors of urban development had on a pilot area. Background support for the experiment was provided by a computer program called ENVI-met, which, based on knowledge of the existing situation, is able to use several dozen climatic variables to calculate changes that would occur if the plans were realised.
The methods applied here are not new to public space planning: it is well known both in and outside professional circles that vegetation, for example, cools the environment through evapotranspiration; and these methods represent the primary tools employed in the redevelopment of outdoor public spaces in general.
Within the pilot area, the planners of the Budapest modelling regions used the following tools provided by the climate specialists of UHI:
Single Alleys, Double Alleys, Planters, Green Spaces, Permeable Pavement, Green Walls, Vertical Gardens and Green Roofs.
According to results of ENVI-met simulations it can be stated that within the modelled regions the microclimate – following the localised nature of the intervention – improves in discrete areas due to the proportionate increase of green spaces: cross-ventilation improves, relative humidity increases, mean radiant temperature (MRT) significantly decreases and, in cases of drastic intervention, air temperatures also show significant decreases.
Numerical analysis of heat environment in central Tokyo using tree-crown-resolving large-eddy simulation considering three-dimensional radiation process
1Center for Earth Information Science and Technology, Japan Agency for Marine-Earth Science and Technology, Japan; 2Dept. of Integrated Science and Engineering for Sustainable Society, Chuo University, Japan
Increase of urban green spaces is considered to be one of the countermeasures to mitigate the urban temperature increase since the “cool spot” effect of green spaces was observed by a lot of field measurements. For the effective planning of urban green spaces, it is important to understand the mechanism of the cool spot effect and its effect on the heat attack risk reduction. This study aims to investigate the influence of trees on the urban heat environment by using the "MultiScale Simulator for the Geoenvironment" (MSSG), which is capable of running as a building-resolving large-eddy simulation model considering the three-dimensional radiation process. The tree-crown-resolving heat exchange model implemented in the MSSG solves the heat balance equation on the tree leaves considering the transpiration, sensible heat and the three-dimensional radiation at each time step and each grid cell inside the tree crowns. The performance of the model is confirmed by conducting simulations for an ideal green space case, whose results show that the model can predict the leaf temperature and heat flux distributions inside the tree crowns. The model is then applied to the case of an actual urban area in Tokyo with the New National Stadium Japan, which will be constructed for the 2020 Tokyo Olympics. The computational domain covers 5km x 5km horizontal area, which is discretized by 5m grid mesh. The initial and boundary conditions for the wind, temperature, humidity, etc. are set based on the reanalysis data at noon on a summer-time clear-sky day, on which a typical meteorological condition of the heat island was observed around Tokyo. The results show that the air temperature around the stadium decreases by increasing trees in the stadium premises. The discomfort index and Wet-Bulb Globe Temperature (WBGT) index are estimated directly from the air temperature, water vapor density and radiation flux data. We will show the estimated results of the indexes and discuss the heat environment mitigation effect of greening.
Sensitivity of Perceived Temperature on meteorological variables and urban morphology parameters
1CNRM-GAME, Météo France, Toulouse, France; formerly Meteorological Institute, University of Hamburg, Germany; 2Meteorological Institute, University of Hamburg, Germany; 3Deutscher Wetterdienst, Seewetteramt, Hamburg, Germany; formerly Meteorological Institute, University of Hamburg, Germany; 4UmweltBundesamt, Dessau, Germany; formerly Meteorological Institute, University of Hamburg, Germany; 5Deutscher Wetterdienst, Zentrum für Medizin-Meteorologische Forschung, Freiburg, Gemany
The Perceived Temperature (PT) is a measure for outdoor human thermal comfort developed by the German Meteorological Service. We investigate the sensitivities of PT on meteorological variables and urban morphology parameters. The meteorological input data is taken from simulation results of the mesoscale atmospheric model METRAS for a domain covering the greater city of Hamburg in northern Germany and a selection of typical synoptic situations during the summer season. The influence of the buildings on the shortwave and longwave radiation is calculated by the radiation modification routines of the Building Effect Parameterisation (BEP) which provide the grid cell-averaged radiation fields within typical street canyons. The sensitivities of PT are determined by automatic differentiation. The sensitivities show how accurate the different input variables need to be known in order to obtain a certain desired accuracy in PT and how PT can be influenced most efficiently (e.g. through adaptation measures). The sensitivities of PT on air temperature, water vapour pressure and mean radiant temperature are higher during warm and humid conditions than in situations with thermal comfort. The sensitivity of PT on wind speed is highest for low wind speeds. Around noon, in street canyons with aspect ratios above 0.5, increasing the building heights by 5 m can reduce PT by down to 2.4 K due to shading effects. After sunset, increasing the building heights by 5 m tends to moderately increase PT by 0 to 0.4 K due to the increased longwave radiation. Future work should focus on a separation between the sensitivities at positions exposed to the sun and positions in the shadow. Further, the indoor thermal comfort should be investigated.
An evaluation of the effects of heat ray retro-reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection
1University of Fukui, Japan; 2Tohoku University, JSPS research fellow; 3Tohoku University
It has recently been noted that the adoption of low-e double glazing and heat shading films for windows has a negative impact on the thermal comfort of pedestrians, since these windows usually reflect solar radiation to pedestrian spaces. As a countermeasure to this problem, it is expected that the application of a heat ray retro-reflective film onto window surfaces will have positive impacts by both reducing the indoor cooling load and mitigating effects on the thermal environment in outdoor spaces. This paper describes an evaluation of the effects of a heat ray retro-reflective film applied to a window on the thermal environment of an outdoor space, using a computational method proposed by the present authors. In the former part of this paper, we outline the radiant computational method considering the effects of the directional reflectivity of surfaces. We incorporate these effects into the existing method by extending the computational method proposed by Ichinose et al. (2005) to the evaluation of the outdoor radiant environment. In the latter part of this paper, we investigate the radiant thermal environment around a building during the summer season, using the revised method. Three different windows installed in the building surface are compared: (1) single float glass with a heat ray retro-reflective film, (2) untreated single float glass, and (3) low-e double glass. By comparing the calculation results, it was clarified that the adoption of the heat ray retro-reflective film to the building surface improves the radiant environment for pedestrians during the summer season.
Creating Urban Cool Islands effects for summer season in Toulouse new area: urban microclimate adaptation
1Ecole Nationale Superieure d'Architecture de Toulouse, France; 2Institut National des Sciences Appliquées de Toulouse, France
Urban population has been growing exponentially over the past decades all across the globe. Cities currently concentrate more than half of the world population and of around 80% in developed countries such as in France. Such concentration of people along with their different activities has produced major stress on the natural and built environment. The urbanization models are marked by important changes in the natural surfaces and in the built morphology, which have altered radiation, thermal, moisture and aerodynamic properties of these environments, leading to a new human induced climate. This urban climate has affected environmental quality of spaces, leading to human heat stress, particularly in summer conditions, and a significant increase of energy demand in buildings. This intensive urbanization process brings us to face new challenges of adapting existing and new urban areas to a progressive and local climate change, which requires integrating decisive measures right from the first stages of the design process. One of the first airmail services in the world, the Aeropostale, was located in the Montaudran airport in Toulouse-France. This landmark and its surroundings will be refurbished and transformed into a mixed-used urban site, the Aerospace valley, with residential buildings, commercial, sportive, educational and cultural activities. This new district has been recently planned based mostly on patrimonial and functional rules. The UCI project (from “Urban Cool Islands”) is a national French research project that has discussed procedures aiming at incorporating a set of reasoned measures of local climate adaptation to this new urban area that will be set as landmark reference. This research aims at analysing and comparing different adapted and resilient urban design strategies to provide support for their application in the Montaudran district plan, focusing on mitigating urban heat island effects in summer season conditions. Two main methodological steps were undertaken: (1) the initial urban plan was assessed relating a set of well-known energy-related parameters of the urban morphology and microclimate analysis; (2) a set of variations to adapt the initial plan was undertaken based on the local plan major guidelines and on main climate adapting measures. Results pointed to a major influence of the water bodies and vegetation density on the mitigation of urban heat islands, notably in daytime. The increase of vegetation density all along the ancient airport runway allowed creating an important urban cool island for pedestrian walk, reducing drastically local heat islands effects