Complex analysis of effects of street tree quantity in pedestrian level using large-eddy simulation model

Abstrakt

ZÁKLADNÍ ÚDAJE: 11th International Conference on Urban Climate. Titles. Sydney: ICUC, 2023, č. článku 452. [ICUC 2023: International Conference on Urban Climate /11./. 28.08.2023-01.09.2023, Sydney]. ABSTRAKT: Trees in urban canyons significantly affect the energy balance of horizontal and vertical surfaces. Moreover, they decrease wind velocity and block night-time radiative cooling of horizontal surfaces. The role of trees in the urban canyon is more complex – they can decrease surface temperature (ST) or mean radiant temperature (MRT) about tens °C, same as related biometeorological indices, e.g. universal thermal climate index (UTCI) or physiological equivalent temperature (PET), maximum decrease is between 10–15 °C. All these decreases are located close to trees, with only a slight effect on their surroundings. Air temperature (T) decrease is typically about several °C, but with a larger effect on surroundings. It is a widely accepted fact by city planners and designers that trees generally optimise the urban canyon cooling at pedestrian level. New studies mostly based on large-eddy simulation principles proved that there are more dependencies as expected, e.g., number of trees planted, distances between trees, tree species, irrigation etc. Moreover, there is still a limited number of studies dealing with the impact of tree planting on wind velocity or air quality. We apply the newly-developed PALM model to quantify the impacts of sidewalk tree coverage on pedestrian ST, MRT, UTCI, PET, T, wind velocity and air quality (PM10, NO, NO2) during summer for a synthetic domain in a low-rise neighbourhood in a midlatitude city. PALM captures the detailed spatio-temporal variation of direct shading and directional longwave radiation loading on pedestrians resulting from tree shade. We found that a relatively equal distribution of trees among sun-exposed pedestrian routes and sidewalks within a block or neighbourhood avoids mutual shading and therefore optimises outdoor radiative heat reduction per tree during warm conditions. But with growing tree-canopy ratios the concentrations of air pollution in urban canyons increase significantly.