Enhancing Heat Exchange Forcings in Microscale Simulations of Urban Environments with FastEddy®

A realistic representation of urban environments at street scale is crucial for accurately capturing local climate dynamics. Large-eddy simulation (LES) models at meter-scale resolutions are a powerful tool for urban studies. FastEddy^®, NCAR-RAL’s GPU-accelerated LES model, offers a computationally efficient approach to high-resolution, turbulence-resolving urban simulations. It explicitly includes building effects and urban heterogeneity, enabling a detailed representation of flow, transport, and exchange processes.

Building effects in FastEddy^® are modeled using an immersed body force method (IBFM), which represents buildings as momentum sinks typically applied together with isothermal boundary conditions when radiation effects and a building energy model are not considered. Therefore, surface heat fluxes between the ground and urban structures cannot directly modify building temperatures, highlighting the need of alternative energy redistribution mechanisms that can lead to realistic urban-atmosphere thermal exchanges.

This study explores different approaches to redistributing surface heat fluxes within the urban environment to achieve a more physically consistent representation of urban energy balance and to better account for urban morphological complexity. Several strategies are tested, evaluating their impact on heat exchange dynamics and urban microscale simulations. The results yield new insights into heat redistribution processes in urban LES and provide improvements for these types of modeling efforts.

Acknowledgements: The authors acknowledge the ECCE project (PID2020-115693RB-I00) of the Ministerio de Ciencia e Innovación/Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033). ERL thanks her predoctoral contract FPU (FPU21/02464) to the Ministerio de Universidades of Spain.