Improving accuracy in urban climate modeling: addressing uncertainties in PALM’s radiative transfer processes modeling

Cities and urban climates have a highly dynamic relationship. Many factors contribute to the framework of this relationship making its evaluation challenging. Ongoing climate change, lack of urban resilience, and exposure of city dwellers to hazards (e.g., increased thermal stress) are pressing the researchers to offer mitigation solutions. Urban atmospheric processes are not isolated and are affected by urban morphology and the dynamic nature of human activities. Specifically, urban-induced increased heat stress is tightly connected to radiative transfer processes (RTPs) in urban areas. 

Despite the scientists’ efforts to offer solutions through modeling approaches, uncertainties in numerical models are unavoidable. Street-scale level RTP modeling remains challenging for many microscale models. A high-fidelity microscale model with well-developed RTPs is needed to properly capture urban atmospheric processes and provide mitigation strategies. One such model is the Parallelized Large Eddy Simulation Model–PALM with its Radiative Transfer Model (RTM) capable of resolving 3-D multi-reflective radiative interactions in urban areas. However, variability in the orientation and height of buildings, roofs, vegetation, and sunlit/shaded pavements challenge PALM to accurately model the shortwave radiation.

To isolate sources of uncertainties in PALM, its comprehensive validation supported by a measuring campaign was conducted during a heatwave in the summer of 2024. The observations were collected with a spherical camera with angular discretization and a pyranometer, ensuring comprehensive data collection (360-degree photos and incoming shortwave radiation). The modeling domain represents a realistic urban area within Prague coinciding with the measurements. The shortwave radiation from PALM has been evaluated in different locations and the model’s outputs agree with the observations with slight discrepancies during certain hours. This study improved the understanding of uncertainties in PALM’s shortwave radiation assessment in urban areas emphasizing the role of erroneous input data about the position and shape of buildings and trees.