Large Eddy Simulation Based Evaluation of an Urban Canopy Model

Urban canopy models (UCMs) are routinely used to diagnose or predict the temporal and spatial variations of urban surface-atmosphere exchanges and associated phenomena. Traditionally, UCM evaluations rely on in-situ measurements, which are inherently local and encompass a wide range of physics that are challenging to quantify comprehensively. This study assesses the strengths and weaknesses of a UCM, Urban Tethys-Chloris (UT&C), by comparing its predictions against highly controlled large eddy simulations (LESs) and observations in Phoenix, Arizona. Simulations are performed over an idealized urban geometry for ten clear sky days. Due to the inability of the UCM to accurately account for buoyancy-driven transport mechanisms, surface temperature errors for the considered days can be significant when compared against LES. A key limitation in the UCM resistance parameterization is the inability to capture the stability-dependent variations in vertical heat flux, resulting in discrepancies with the LES results. Further, while the UCM captures the primary cooling effect of radiative shading by street trees, it fails to fully represent the enhanced tree-induced turbulent heat transfer beneath the foliage, thereby underestimating the net cooling impact of trees.