Utilising Terrestrial Laser Scanning (TLS) for urban tree structure characterization and its impact on modelled human thermal comfort

Urban green infrastructure plays a pivotal role in climate regulation by offering various ecosystem services. One crucial metric in understanding human thermal exposure is the mean radiant temperature (Tmrt), which encompasses the spatial and temporal variations of radiation exposure. In the context of urban microclimate models such as SOLWEIG, the accurate characterization of urban trees is essential, whether incorporating existing trees or assessing the cooling effects of new greenery. Currently, urban tree structures are usually generalised in urban climate models due to the lack of detailed measurements and scientific knowledge about urban tree growth and functioning.

Various vegetation types exhibit distinct effects on the attenuation of direct shortwave radiation through shading. Variations in tree shading are influenced by the configuration, optical and structural properties of planted tree species. Leaf Area Index (LAI), tree height, and trunk height significantly determine shade patterns and solar attenuation. We use state-of-the-art Terrestrial Laser Scanning (TLS) techniques to parameterize these structural properties for the precise implementation of existing trees within urban microclimate models. This enhanced structural understanding of urban trees will facilitate the creation of more realistic tree models, allowing for a comprehensive assessment of their impact on human thermal comfort. 

SOLWEIG operates as a 2.5-dimensional model, where x and y coordinates and associated attributes (e.g. height, emissivity or reflectivity) are utilised for the calculation of Tmrt. Greenery such as trees and bushes are represented in separate Digital Elevation Models (DEMs). TLS allows for the highest degree of parameterisation of urban trees within the given raster environment. By conducting a sensitivity analysis on the modelled Tmrt, we explore the impact of tree and trunk height, canopy area and volume, and radiation transmissivity of vegetation.

The result of our sensitivity analysis provides valuable guidance on the TLS data collection of tree parameters essential for evaluating current cooling effects. Which in turn leads to the identification of tree species with significant cooling potential, and determining the size at which a tree substantially contributes to human thermal comfort.