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Increasing heat in urban environments has recently become one of the most dangerous climate hazards due to its adverse impacts on urban populations. Implementing street-level trees could be an effective strategy to mitigate pedestrian heat exposure, particularly due to their ability to block incoming solar radiation. In this study, the PALM model system was applied to simulate the effects of a tree canopy and its location on heat exposure, as quantified by the Universal Thermal Climate Index (UTCI), during a heat wave, using the example of Prague-Dejvice, Czech Republic. Our results show that trees reduce the UTCI under their canopy by 3.5 °C on average, with the greatest UTCI reduction in open spaces during mornings and afternoons. High spatio-temporal variations in the reduction of UTCI by a tree canopy were observed in the study domain, especially in street canyons and courtyards. The effectiveness of trees in mitigating heat exposure was found to be closely related to their individual location with respect to surrounding buildings, specifically: (i) the distance from the nearest building, (ii) the height of the nearest building, and (iii) the azimuthal angle of the vector from the nearest building towards the tree. Model simulations indicate that a particularly small reduction in UTCI (about 2.5 °C less than the mean) can be found under trees located in the shade of taller buildings that are within a few metres and between southwest and southeast of the trees. Our findings illustrate that tree positioning in cities should be undertaken carefully and thoughtfully so that the presence of trees effectively improves thermal comfort and urban quality of life. [Display omitted] • The tree canopy mitigates heat exposure mainly during the morning and afternoon. • The individual location of trees influences their shading effectiveness. • Trees in open spaces significantly reduce the heat exposure of pedestrians. • Tree shading ability is affected by the height and location of nearby buildings. • Trees on the south side of street canyons minimally mitigate heat exposure. [ABSTRACT FROM AUTHOR]

The development of integrated urban services requires the implementation of informative tools that provide a balance between quality, time and costs for air quality assessment. Within this framework, three modeling techniques with different levels of complexity were compared during a winter inversion episode against PM 10 concentrations measured in a built-up area in Prague (Czech Republic) characterized by heavy traffic. Although the Gaussian model ATEM satisfied the common statistical-performance criteria, the predictions poorly represented the spatial variability of concentrations in the study domain. The Lagrangian model GRAL provided a better simulation of the effects of terrain and vortice formation inside street canyons, but tended to overpredict the influence of these phenomena. Finally, the most sophisticated of the three models, the Large-Eddy Simulation model PALM, demonstrated the best performance based on an exhaustive analysis of the model outputs in the temporal and spatial dimensions. After model comparison, a sensitivity test of the selected models to the driving meteorology and emissions inputs was carried out. While advanced models can simulate complex urban environments, their suitability for use in urban planning is subject to further considerations, such as computational cost, user expertise, and the usefulness of the output. Thanks to increasing computation power and intensive work on the entire modeling chain, sophisticated models could become routine tools for use in regulatory applications, contributing to future integrated urban service provision. • Selection of dispersion models should include a spatio-temporal assessment. • Meteorological input and resuspension emissions are important sources of uncertainty. • LES models provide a thorough set of data valuable for integrated urban services. [ABSTRACT FROM AUTHOR]

The regional climate model RegCM4.4, including the surface model CLM4.5, was offline coupled to the chemistry transport model CAMx version 6.30 in order to investigate the impact of the urban canopy induced meteorological changes on the longterm summer photochemistry over central Europe for the 2001–2005 period. First, the urban canopy impact on the meteorological conditions was calculated performing a reference experiment without urban landsurface considered and an experiment with urban surfaces modeled with the urban parameterization within the CLM4.5 model. In accordance with expectations, strong increases of urban surface temperatures (up to 2–3 K), decreases of wind speed (up to −1 ms −1 ) and increases of vertical turbulent diffusion coefficient (up to 60–70 m 2 s -1 ) were found. For the impact on chemistry, these three components were considered. Additionally, we accounted for the effect of temperature enhanced biogenic emission increase. Several experiments were performed by adding these effects one-by-one to the total impact: i.e., first, only the urban temperature impact was considered driving the chemistry model; secondly, the wind impact was added and so on. We found that the impact on biogenic emission account for minor changes in the concentrations of ozone (O 3 ), oxides of nitrogen NOx = NO + NO 2 and nitric acid (HNO 3 ). On the other hand, the dominating component acting is the increased vertical mixing, resulting in up to 5 ppbv increase of urban ozone concentrations while causing −2 to −3 ppbv decreases and around 1 ppbv increases of NOx and HNO 3 surface concentrations, respectively. The temperature impact alone results in reduction of ozone, increase in NO, decrease in NO 2 and increases of HNO 3 . The wind impact leads, over urban areas, to ozone decreases, increases of NOx and a slight increase in HNO 3 . The overall impact is similar to the impact of increased vertical mixing alone. The Process Analysis (PA) technique implemented in CAMx was adopted to investigate the causes of the modeled impacts in more details. It showed that the main process contributing to the temperature impact on ozone is a dry-deposition enhancement, while the dominating process controlling the wind impact on ozone over cities is the advection reduction. In case of the impact of enhanced turbulence, PA suggests that ozone increases are, again as assumed, the result of increased downward vertical mixing supported by reduced chemical loss. Comparing the model concentrations with measurements over urban areas, a slight improvement of the model performance was achieved during afternoon hours if urban canopy forcing on chemistry via meteorology was accounted for. The study demonstrates that disregarding the urban canopy induced meteorological effects in air-quality oriented modeling studies can lead to erroneous results in the calculated species concentrations. However, it also shows that the individual components are not equally important: urban canopy induced turbulence effects dominate while the wind-speed and temperature related ones are of considerably smaller magnitude. [ABSTRACT FROM AUTHOR]