Development of SOLWEIG-LEB v1.0: An urban radiation model integrating LAD-based canopy shortwave attenuation and leaf energy balance

Abstract. Urban trees are widely used to mitigate pedestrian heat stress by providing shade and modifying the surrounding radiative environment. Accurately representing these effects is important for evaluating urban greening strategies, yet simplified treatments of canopy transmission and vegetation temperature can limit pedestrian-level radiation and mean radiant temperature (MRT) estimates. SOLWEIG is a widely used urban radiation model for spatially resolving pedestrian radiative environments, but its conventional vegetation scheme applies a fixed canopy transmissivity and assumes vegetation temperature equals air temperature. To address these limitations, we developed SOLWEIG-LEB v1.0, which combines path-dependent shortwave attenuation based on leaf area density (LAD), the leaf projection function, and radiative path length with a voxel-scale leaf energy balance that diagnoses leaf temperature, vegetation longwave emission, and sensible and latent heat fluxes. The model was evaluated using two field datasets. Against leaf-temperature observations in Mainz, Germany, Pearson correlation coefficients (r) were 0.97 and 0.85 and root mean square errors (RMSEs) were 1.39 and 1.05 °C under clear-sky and fully cloudy conditions, respectively. Against six-directional radiation and MRT observations in Göteborg, Sweden, SOLWEIG-LEB reduced RMSEs for total shortwave radiation from 231.74 to 203.66 W m⁻², total longwave radiation from 83.42 to 78.93 W m⁻², and MRT from 4.76 to 4.38 °C relative to Legacy SOLWEIG. Differences were greatest near canopy edges and partially obstructed locations, highlighting the importance of spatially variable canopy transmission. SOLWEIG-LEB provides a more physically explicit framework for assessing urban-tree radiative effects and pedestrian thermal environments.