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Cooling potentials of urban greens to mitigate urban heat island effect
Dr. Mohammad Asrafur Rahman
Technical University of Munich, Germany
Impact of global climate change and local urbanization have led our cities to bear unbearable heat loads. Ever-increasing urbanization along with the global climate change is continuously hitting the bar of so-called “Urban heat island” to new records. For example, we have recorded a difference of air temperature at the centre of Würzburg (a major city in Germany) and five km north of Würzburg, Gerbrunn to an extent of 9 °C (Hartmann et al., in press). This is a real problem for human health and getting increasing attention by the designers and policymakers especially with the advent of climate change where heat waves are expected to become more frequent and intense. Overall, urban greening is conceived as the most feasible option to mitigate those problems to a degree, which can be termed as long-term sustainable for our future eco-cities.
However, urban tree growth and ecosystem service provision are highly influenced by species characteristics, climate and site conditions. With accelerated climate change and time lag needed for the trees to mitigate this ecological disaster, therefore, require immediate attention to choose right strategies regarding urban forest types (composition and configuration), species and site selections for maximizing the benefits.
In practice, it is not always easy to understand the magnitude of the cooling potential of urban trees. Trees with their extended and multi-layered canopies can cool the shaded surfaces by protecting them from the incoming radiation. Moreover, trees can cool down their leaf surfaces through transpiration, consequently the surrounding boundary layer. While blocking the incoming shortwave radiation, trees simultaneously reduce longwave radiation fluxes (otherwise, the absorbed radiation is converted to longwave radiation fluxes). Consequently, all the radiant heat absorbed by a human body, is significantly reduced while under the tree shade.
We investigated surface energy balance, boundary layer air-cooling through evapotranspiration and the human thermal comfort over the last six years across two major cities in Germany – Munich and Würzburg. Firstly, we found that the differences between sun and shade were steeper over the grass surfaces and during the wet spells (evapotranspiration rate > 1.5 L m-2 d-1). In contrast, sensible heat fluxes between grass and paved surfaces were not different during the dry spells (Rahman et al., 2021).
On a separate study in Würzburg, we found that mean air temperature (AT) of inner city sites were higher by 1.3 °C during summer compared to sub-urban sites (Rahman et al., 2022). Regarding species traits, we compared two ecologically contrasting species - Tilia cordata and Robinia pseudoacacia. T. cordata with 35% higher leaf area index and diffuse porous wood anatomy provided four times more transpiration thus, up to 2.8 °C AT reduction (ΔAT) and up to 2.6 g m − 3 (ΔAH) increase in absolute humidity compared to 1.9 °C of ΔAT and 1.9 g m − 3 of ΔAH within the tree canopies of R. pseudoacacia (Rahman et al., 2020). Thus, decrease in physiological equivalent temperature (PET), an index to understand human thermal comfort, was up to 4 °C under the shade of a R. pseudoacacia compared to 11 °C under a T. cordata tree than the open sunny surfaces.
Moreover, we found that public squares with grass lawns and wide street canyon conditions could provide almost 25% more cooling effect compared to a completely sealed and narrow canyon (Rahman et al., 2017). However, there were trade-offs between different indicators for ecosystem services such as carbon gain and transpiration; number of trees and wind flow (Rahman et al., 2019; Zölch et al., 2019). The results feature the importance of urban topography, such as street orientation, surrounding environment such as geometry, urban design to reduce the hindrance of wind flow and proportion of greenspaces in terms of outdoor human thermal comfort.
References
Hartmann, C. et al., in press. The footprint of heat waves and dry spells in the urban climate of Würzburg, Germany, deduced from a continuous measurement campaign during the anomalously warm years 2018 – 2020. Meteorologische Zeitschrift.
Rahman, M.A. et al., 2021. Comparative analysis of shade and underlying surfaces on cooling effect. Urban Forestry & Urban Greening, 63: 127223.
Rahman, M.A. et al., 2022. Spatial and temporal changes of outdoor thermal stress: influence of urban land cover types. Scientific Reports, 12(1): 671.
Rahman, M.A. et al., 2020. Tree cooling effects and human thermal comfort under contrasting species and sites. Agric. For. Meteorol., 287: 107947.
Rahman, M.A. et al., 2019. Comparing the infiltration potentials of soils beneath the canopies of two contrasting urban tree species. Urban Forestry and Urban Greening, 38: 22-32.
Rahman, M.A., Moser, A., Rötzer, T. and Pauleit, S., 2017. Microclimatic differences and their influence on transpirational cooling of Tilia cordata in two contrasting street canyons in Munich, Germany. Agric. For. Meteorol., 232: 443–456.
Zölch, T., Rahman, M.A., Pfleiderer, E., Wagner, G. and Pauleit, S., 2019. Designing public squares with green infrastructure to optimize human thermal comfort. Building and Environment, 149: 640-654.
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