Tree-crown-resolving large-eddy simulation coupled with three-dimensional radiative transfer model

This paper presents a tree-crown resolving large-eddy simulation (LES) coupled with a three-dimensional radiative transfer (3DRT) model. Real-time coupling of the LES and 3DRT models is achieved by using a simplified 3DRT model based on the volumetric radiosity method. The 3DRT model is validated by comparing with an observation and a Monte Carlo radiative transfer model. The sensitivity test for an ideal green space confirms that the real-time coupling is necessary for the reliable evaluation of the thermal environment in green spaces. The computational time required for the 3DRT calculation is negligibly small for performing the coupled LES. The present model is then applied to an actual urban area, which contains tree crowns along streets and in parks, covering 8000 m x 8000 m with 5-m grid mesh in order to confirm its feasibility in real applications. The error of the estimated WBGT at pedestrian level is smaller than 0.2 K when the optical thickness of a volume element is smaller than 2.5, and thus the local spatial distribution of the WBGT is predicted with an accuracy sufficient for capturing the influence of tree crowns.