Shedding light on the urban LES grey zone: when to parameterise and when to resolve buildings in PALM simulations

Microscale urban climate simulations provide detailed insights into thermal conditions within cities, supporting research and planning for climate adaptation. However, high spatial resolution simulations are computationally demanding, limiting their applicability for large urban domains. City-wide assessments often require coarser grid resolutions, where explicit representation of three-dimensional urban structures becomes impractical due to computational constraints and impacts the quality of the modelling results especially in the case of large-eddy simulations. Another issue arises from the limited availability of high-quality geodata for building resolving simulations.

To address these challenges, urban parameterisation schemes can be employed in the PALM model system to capture urban effects while significantly reducing computational costs. This allows for the simulation of larger domains without explicitly resolving three-dimensional structures such as buildings and trees. The required input data for the parametrisation schemes can be generated based on Local Climate Zone (LCZ) maps. Furthermore, the LCZ maps can serve as a basis for idealised three-dimensional setups in cases where high-quality geodata are unavailable.

In this study, we systematically compare various urban parameterisation schemes implemented in PALM, as well as an idealised LCZ-based setup, against a baseline scenario with explicitly resolving the three-dimensional urban structure. The simulations are conducted for a summer situation with initial and boundary conditions derived from the ICON-D2 model. Simulations are evaluated using observational data from Dortmund, Germany. Our analysis identifies the suitability and limitations of each approach across different spatial resolutions, providing guidance on when to parameterise and when to resolve urban structures in PALM simulations. Initial results will be presented.