Human thermal comfort in summer within an urban street canyon in Central Europe
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Regional climate models predict an intensification of extreme heat waves in Central Europe. Against this background, the significance of human-biometeorologically orientated urban planning strategies is increasing by which the impairment of thermal comfort for people in cities in the future can be minimised. Such strategies require quantitative information on factors determining human thermal comfort within different urban quarters. With respect to these problems, the joint research project KLIMES funded by the German Federal Ministry of Education and Research was initiated. Its methodical approaches and objectives are presented in this article. One part of KLIMES are experimental investigations on human thermal comfort within different urban street canyons, whose variable arrangement generally characterises urban quarters. The investigations are conducted in Freiburg (SW Germany). The experimental design and the concept to analyse the measured data related to the objectives of KLIMES are exemplarily explained based on investigations in the Rieselfeld quarter on a typical summer day in 2007. The internationally well-known physiologically equivalent temperature PET is used as thermal index to quantify the perception of the thermal conditions by a collective of people within cities. During typical summer weather in Central Europe, PET is strongly influenced by the radiation heat, which is parameterised by the mean radiant temperature T mrt. Therefore, the short- and long-wave radiation flux densities from the three-dimensional surroundings of a standardised standing person representing mean properties of a collective of people in cities are analysed in detail. For the specific conditions at the stationary site Rieselfeld (NW-SE oriented urban street canyon, H/W = 0.49, SVF = 0.51, SW oriented sidewalk), the contribution of the total long-wave radiation flux density absorbed by a standing person to T mrt increased during the day from about 70% in the morning to about 90% in the evening before sunset.Keywords:
Street Canyon
Street Canyon
Aspect ratio (aeronautics)
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Street Canyon
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The diffusive characteristics of vehicle emission pollutant and the wind field characteristics in urban street canyons depend on street canyon ration of height to width, A and the symmetry of building height. In this article, a numerical model was used to study the relationship between street canyon characteristics and diffusive and wind characteristics. It shows that when %A% is more than 2.1, a stable vortex formed within the urban street canyon become two vortexes. The diffusive levels of the pollutant from urban street canyons in the order of from strong to weak strength are the street canyons in which the building heights in windward are highter than that of in leeward, the building heights in windward are lower than that of in leeward, and the street canyons with a series of parallel to each others, respetively.
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Submarine canyon
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Street Canyon
Richardson number
Aspect ratio (aeronautics)
Stratification (seeds)
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The thermal environment of an urban street canyon in summer becomes a great concern for human health under rapid urbanization. For accurate prediction of the in‐canyon thermal environment, a realistic representation is required of microscale physical processes within the canyon as well as multi‐scale atmospheric interaction between the canopy air and the overlying urban boundary layer. To accomplish this, the Vegetated Urban Canopy Model ( VUCM ), which interactively parametrizes in‐canyon radiative/dynamic/thermodynamic/hydrological processes based on a combined framework of the two‐dimensional single canyon and the single tree canopy, is implemented into the Weather Research and Forecasting ( WRF ) model. Using the coupled WRF‐VUCM model, a series of simulations is performed for a hot summer day with a finest grid resolution of 0.333 km to investigate the impacts of in‐canyon vegetation (permeable grass/soil surfaces and trees, with the vegatation fraction f v ranging from 0 to 0.4 which corresponds to 0 to 15% in urban patch area) and canyon aspect ratio ( h / w ; ranging from 0.5 to 2) on the thermal environment of urban street canyons over the Seoul metropolitan area. The model simulation compares well with the measured 2 m temperatures (above zero‐plane displacement height) and canopy air temperatures at 13 urban sites in Seoul, with root mean square errors of 1.0 and 0.96 °C, respectively. The increase of the in‐canyon vegetation from 0 to 15% (at h / w = 1) leads to a reduction of the canopy air temperature throughout the diurnal cycle, exhibiting relatively larger cooling effect during daytime (∼1.1 °C on average) than at night (∼0.8 °C on average) under a limited condition for evapotranspiration by the in‐canyon vegetation. Provided that the soil moisture is enough for the hydrological effect, the cooling effects significantly increase by a factor of ∼2.5 in both daytime and night‐time. The increase of h / w from 0.5 to 2 (at f v = 0.2) reduces the daytime canopy air temperature (∼1.3 °C on average) but increases the nocturnal canopy air temperature (∼0.3 °C on average). It is also found that the existence of in‐canyon vegetation at h / w > 1 has a synergic cooling benefit to the thermal environment of street canyons compared to the effects of no vegetation. These results demonstrate the importance of interactive parametrization of the physical processes and the interplay of in‐canyon vegetation and building density (via canyon aspect ratio) effects on accurate prediction of the thermal environment of urban street canyons.
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Urban Heat Island
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Air pollution remains a major environmental and health concern in urban environments, especially in street canyons that show increased pollution levels due to a lack of natural ventilation. Previous studies have investigated the relationship between street canyon morphology and in-canyon pollution levels. However, these studies are typically limited to the scale of a single street canyon and city-wide assessments on this matter are scarce. In 2018, NO2 concentrations were measured in 321 street canyons in the city of Antwerp (Belgium) as part of the large-scale citizen-science project "CurieuzeNeuzen". In our research, this data was used to study the correlation between morphological indices (e.g. aspect ratio (AR), lateral aspect ratio (LAR), presence of trees) and the traffic volumes on a city-wide scale. The maximum hourly traffic volume (TVmax) and AR correlated significantly with the measured NO2 values, making them useful indicators for air quality in street canyons. For street canyons with AR > 0.65, a TVmax of 300 vehicles/hour was found as a threshold value to guarantee acceptable air quality. No significant correlations were found for the other parameters. Finally, a number of typical street canyon types were defined, which can be of fundamental interest for further research and spatial policy making.
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Urban Morphology
Morphology
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With the developed micro-scale mathematical model for studying turbulent flows and the transfer of passive gaseous impurities in a street canyon, the effect of temperature inhomogeneity on the nature of the spread of harmful emissions in a street canyon when the emission source is located at the leeward side of the canyon, was studied.
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