Development of a Multi-Scale Meteorological Large-Eddy Simulation Model for Urban Thermal Environmental Studies: The "City-LES" Model Version 2.0

To bridge the gaps between meteorological large-eddy simulation (LES) models and computational fluid dynamics (CFD) models for microscale urban climate simulations, the present study has developed a meteorological LES model for urban areas. This model simulates urban climates across both mesoscale (city scale) and microscale (city-block scale). The paper offers an overview of this LES model, which distinguishes itself from standard numerical weather prediction models by resolving buildings and trees at the microscale simulations. It also differs from standard CFD models by accounting for atmospheric stratification and physical processes. Noteworthy features of this model include: (a) the calculation of long- and short-wave radiations in three dimensions, incorporating multiple reflections within urban canopy layers using the radiosity method, and accounting for building and tree shadows in the simulations; (b) the provision of various heat stress indices (Universal Thermal Climate Index, Wet Bulb Globe Temperature, MRT, THI); (c) the assessment of the efficacy of heat stress mitigation measures such as dry-mist spraying, roadside trees, cool pavements, and green/cool roofs strategies; (d) the capability to run on supercomputers, with the code parallelized in a three-dimensional manner, and the model can also run on a graphics processing unit cluster. Following the introduction of this model, the study confirms its basic performance through various numerical experiments, including simulations of thermals in the convective boundary layer, coherent structure of turbulence over urban canopy, and thermal environment and heat stress indices in urban districts. The model developed in this study is intended to serve as a community tool for addressing both fundamental and applied studies in urban climatology. This study introduces a new multi-scale urban climate model, City-LES, which is a hybrid model combining meteorological modeling and engineering computational fluid dynamics (CFD) approaches. This model simulates urban climates across both mesoscale (city scale) and microscale (city-block scale). Developed through collaboration among urban climatologists, CFD modeling researchers, and high-performance computing experts, City-LES simulates airflow in urban areas, conducts three-dimensional radiation calculations in the urban canopy, incorporates various atmospheric physics schemes, and operates on supercomputers equipped with graphics processing units, as well as CPUs. Furthermore, the model enables the assessment of the efficacy of heat stress mitigation measures such as dry-mist spraying, roadside trees, cool pavements, and green/cool roof strategies. Validation results indicate that City-LES accurately simulates urban atmospheric dynamics, the thermal environment, and heat stress indices. The model will serve as a valuable community tool for both fundamental and applied studies in urban climatology. This study introduces a multi-scale urban climate model that combines meteorological and computational fluid dynamics modeling approaches The model resolves buildings, simulates 3D airflow and radiation in urban areas, and runs on supercomputers equipped with graphics processing units and/or CPUs The model accurately simulates urban atmospheric dynamics, thermal environments, and heat stress indices