Urban climate modelling with explicit representation of street trees

Abstract

Street trees are more and more regarded as a potential measure to mitigate the excessive heat due to climate change and the urban heat island effect. Nevertheless, several aspects of the impact of street trees on the urban climate have still to be investigated, especially at the scale of an entire city. In fact, the vast majority of large-scale urban climate models only represent vegetation outside the street canyon, neglecting important effects such as the shading of trees on the canyon’s surfaces. In this thesis, this gap was addressed by coupling a regional climate model to an urban canopy model with explicit representation of street trees. First, the ability of a coupled urban climate model to represent the intra-urban climate variability was explored. Small-scale features such as urban parks and large railway areas started to be resolved at sub-kilometre grid spacing allowing a better representation of the urban heterogeneity. In order to represent the interactions between street trees, urban elements and the atmosphere in realistic regional weather and climate simulations, we coupled the vegetated urban canopy model BEP-Tree and the mesoscale weather and climate model COSMO. The performance and applicability of the coupled model, named COSMO-BEP-Tree, are demonstrated over the urban area of Basel, Switzerland, during the heatwave event of June-July 2015. Overall, the model compared well with measurements of individual components of the surface energy balance and with air and surface temperatures obtained from a flux tower, surface stations and satellites. The representation of street trees in the coupled model generally improved the agreement with observations. Street trees were found to moderately reduce the 2-m air temperature during the day, but to produce a slight warming at night. The daytime cooling was found to be primarily a local effect and proportional to the local density of street trees. In contrast, the impact was more widespread at night. Apart from the air temperature, street trees reduced the wind speed and altered the canyon surface temperature substantially. Owing to these secondary effects, street trees produced a larger impact on the outdoor thermal comfort than on air temperature. Street trees generally reduced the thermal comfort during the day, where the urban area reached "strong" to "very strong" heat stress conditions. At night, street trees increased the perceived temperature substantially. Nevertheless, the conditions were still within the "no thermal stress" category. Compared to the application of cool (highly reflective) roofs, street trees were found to provide larger benefits in terms of thermal comfort than in terms of air temperature reduction. When applied together, the effects of street trees and cool roofs added up to a remarkable reduction in air temperature.