Development of a 3D microscale urban ecophysiological model: Numerical simulation of radiation dynamics in urban canyons

Urbanization significantly impacts Earth’s systems by increasing CO₂ emissions, creating Urban Heat Islands (UHI), and influencing climate, energy use, and public health. To design effective climate change adaptation and mitigation strategies, it is essential to understand urban microclimates and the energy and CO₂ balances within cities. This study presents the radiation model that will be included in a 3D microscale ecophysiological model for CO₂, water, and energy exchanges between plants and the urban atmosphere, particularly within urban canyons containing tree canopies. The goal is to develop a computationally efficient model that balances simplicity with accuracy, effectively capturing plant-urban interactions and highlighting the differences between vegetated and non-vegetated areas.

Using high-resolution Digital Surface Models (DSMs), land cover data, and Leaf Area Index (LAI), a 3D urban landscape is constructed using isometric voxels categorized as buildings, trees, terrain, or empty. This model employs a ray tracing algorithm to calculate key parameters, such as the Sky View Factor (SVF) and light transmission coefficients through tree canopies, enabling detailed simulations of radiation exchange for each voxel. Meteorological data from a tower in Basel, Switzerland, is used as model input to simulate conditions within urban canyons at street level. To evaluate the model, 4-component net radiometer data within (2 m height) and over (40 m height) a street canyon is compared with the simulation results. Accounting for time-shifts caused by the discretization of the 3D model —where shadows may appear delayed or advanced due to voxel size and representation—the model accurately replicates diurnal and seasonal patterns of shortwave and longwave radiation flux densities.

The preliminary results, focused on radiation exchanges, show the algorithm’s potential to advance urban ecophysiology modeling and enhance urban climate research, offering valuable and cost effective insights into urban energy balance and microclimate dynamics.