Urban trees influence temperatures in cities. However, their effectiveness at mitigating urban heat in different climatic contexts and in comparison to treeless urban green spaces has not yet been sufficiently explored. Here, we use high-resolution satellite land surface temperatures (LSTs) and land-cover data from 293 European cities to infer the potential of urban trees to reduce LSTs. We show that urban trees exhibit lower temperatures than urban fabric across most European cities in summer and during hot extremes. Compared to continuous urban fabric, LSTs observed for urban trees are on average 0-4 K lower in Southern European regions and 8-12 K lower in Central Europe. Treeless urban green spaces are overall less effective in reducing LSTs, and their cooling effect is approximately 2-4 times lower than the cooling induced by urban trees. By revealing continental-scale patterns in the effect of trees and treeless green spaces on urban LST our results highlight the importance of considering and further investigating the climate-dependent effectiveness of heat mitigation measures in cities. Urban trees influence temperatures in cities. The authors here investigate in spatio-temporal variations in their cooling effect and find 8-12 K decreased temperatures for tree-rich urban areas in Central Europe during hot summers, and up to 4 K for Southern Europe, respectively.

The role of urban trees in reducing land surface temperatures in European cities

Urban environments lie at the confluence of social, cultural, and economic activities and have unique biophysical characteristics due to continued infrastructure development that generally replaces natural landscapes with built-up structures. The vast majority of studies on urban perturbation of local weather and climate have been centered on the urban heat island (UHI) effect, referring to the higher temperature in cities compared to their natural surroundings. Besides the UHI effect and heat waves, urbanization also impacts atmospheric moisture, wind, boundary layer structure, cloud formation, dispersion of air pollutants, precipitation, and storms. In this review article, we first introduce the datasets and methods used in studying urban areas and their impacts through both observation and modeling and then summarize the scientific insights on the impact of urbanization on various aspects of regional climate and extreme weather based on more than 500 studies. We also highlight the major research gaps and challenges in our understanding of the impacts of urbanization and provide our perspective and recommendations for future research priorities and directions.城市环境位于社会、文化和经济活动的结合点,城市下垫面具有独特的生物物理特征。在城市化过程中持续的基础设施建设导致自然景观被建筑物替代。在过去很长时间,绝大部分城市影响天气气候的研究和城市热岛效应(城市及上空温度高于周边地区)有关。除了城市热岛效应,城市化也影响大气湿度、风、边界层结构、云的形成、污染物扩散、降水和暴雨。在这篇综述文章中,我们阅览了超过五百篇文献,从观测和模拟两个方面,首先介绍了用于城市化及影响研究的数据资料和方法,总结了城市化影响区域气候和极端天气的各个领域的科学要点。我们也例举了在理解城市化影响方面目前存在的主要问题和挑战,提出了相应的未来研究重点和方向。. 

/pdf/urbanization-impact-on-regional-climate-and-extreme-weather-1v8dakqv.pdf

Urbanization Impact on Regional Climate and Extreme Weather: Current Understanding, Uncertainties, and Future Research Directions

Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Vegetation alters urban climates via transpirational cooling; however, unlike shorter vegetation, trees additionally provide shade and shelter. Urban canopy models (UCMs) are coupled with mesoscale models for assessment of neighbourhood-scale climate, but their representation of urban trees is limited. We present BEP-Tree, a multi-layer UCM that integrates trees instead of using the ‘tile’ approach that characterizes most urban mesoscale modelling. BEP-Tree resolves the microclimate underneath trees where outdoor human thermal exposure occurs; these conditions are largely inaccessible to current mesoscale modelling and remote sensing approaches. Moreover, BEP-Tree allows trees to protrude above buildings, enabling assessment of low-rise neighbourhoods. The new model combines existing models, including detailed radiative and hydrodynamic models that assess built-tree interactions, and includes new parameterizations for impacts of tree foliage distribution. BEP-Tree is evaluated against unique datasets from three cities enabling assessment of modelled radiation, energy exchanges and road and air temperatures across the diurnal cycle. Urban trees redirect sensible heat into latent heat and reduce pedestrian-level solar radiation, wind, and temperatures during daytime. Thermal climate and energy exchanges are more sensitive to street tree density than height. Coupled with a mesoscale model, BEP-Tree enables assessment of urban forest-induced neighbourhood-to-city scale climate impacts during fair weather periods.

A multi-layer urban canopy meteorological model with trees (BEP-Tree): Street tree impacts on pedestrian-level climate

Turbulent organized structures (TOS) above building arrays were investigated using a large-eddy simulation (LES) model for a city (LES-CITY). Square and staggered building arrays produced contrasting behaviour in terms of turbulence that roughly corresponded to the conventional classification of ‘D-type’ and ‘K-type’ roughness, respectively: (1) The drag coefficients (referred to the building height) for staggered arrays were sensitive to building area density, but those for square arrays were not. (2) The relative contributions of ejections to sweeps (S2/S4) at the building height for square arrays were sensitive to building area density and nearly equalled or exceeded 1.0 (ejection dominant), but those for staggered arrays were insensitive to building area density and were mostly below 1.0 (sweep dominant). (3) Streaky patterns of longitudinal low speed regions (i.e., low speed streaks) existed in all flows regardless of array type. Height variations of the buildings in the square array drastically increased the drag coefficient and modified the turbulent flow structures. The mechanism of D-type and K-type urban-like roughness flows and the difference from vegetation flows are discussed. Although urban-like roughness flows exhibited mixed properties of mixing layers and flat-wall boundary layers as far as S2/S4 was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers.

Large eddy simulations on the effects of surface geometry of building arrays on turbulent organized structures

Effects of trees on mean wind, turbulence and momentum exchange within and above a real urban environment

(1) Department of Civil Engineering, Monash University, Clayton, Australia, (2) Columbia University, Civil Engineering and Engineering Mechanics, United States (marco.giometto@columbia.edu), (3) Environmental Meteorology, Faculty of Environment and Natural Resources, University of Freiburg, Freiburg, Germany, (4) Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland, (5) Department of Civil Engineering, Faculty of Applied Science, University of British Columbia, Vancouver, BC, Canada, (6) Department of Forestry, University of British Columbia, Vancouver, BC, Canada

Effects of trees on momentum exchange within and above a real urban environment

A LiDAR-based approach for anthropogenic heat release modeling by buildings at the urban neighbourhood-scale

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Effects of trees on mean wind, turbulence and momentum exchange within and above a real urban environment

Effects of trees on momentum exchange within and above a real urban environment

A LiDAR-based approach for anthropogenic heat release modeling by buildings at the urban neighbourhood-scale