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Abstract: The photon recollision probability in vegetation canopies, defined as the probability that a photon, after having interacted with a canopy element, will interact again, is a useful tool in remote sensing and ecological applications, enabling to link canopy optical properties at different wavelength and to estimate radiation absorption. In this work, a method is presented to estimate the photon recollision probability for horizontally homogeneous leaf canopies with arbitrary leaf angle distribution as well as for discrete crown canopies. The estimation is based on analytical approximation of the first-order recollision probability. Using the analytical solution of the two-stream equations of radiative transfer and Monte Carlo modeling, the first-order photon recollision probability is shown to slightly underestimate the mean recollision probability. Also, an approximation formula for the mean recollision probability in a horizontally homogeneous canopy is presented as a function of leaf area index. The method to calculate photon recollision probability in discrete crown canopies requires only the knowledge of total and between-crown canopy transmittance and is thus independent of the geometric-optical model used. [Copyright &y& Elsevier]

Urban trees are one of the most effective strategies to mitigate excessive heat stress in cities. To understand the underlying mechanisms of their cooling effect and to assess their use in urban planning, the accurate simulation of how trees interact with the ambient built environment is critical and imperative. However, the representation of urban trees in existing urban canopy models (in particular the single-layer ones) remains oversimplified. Here we develop a new Monte Carlo ray tracing method to explicitly resolve the canopy transmittance and evaluate its impact on radiative view factors between trees and regular building facets. The new method is highly accurate in reproducing analytical solutions. Sensitivity tests of radiative view factors suggest the importance of canopy transmittance in changing the radiation exchange. We then incorporate the ray tracing algorithm into the new version of the Arizona State University (ASU) Single-Layer Urban Canopy Model (ASLUM v3.1). In addition to radiation transmittance, ASLUM v3.1 explicitly resolves the radiative shading, evapotranspiration, and root water uptake of urban trees in street canyons, with significantly improved performance in predictions (especially latent heat flux) when compared to previous versions. We further apply ASLUM v3.1 to evaluate the impacts of trees with varying characteristics on urban radiation exchange and turbulent heat fluxes. Results show that urban trees reduce the net radiation of ground and wall as well as the daytime temperature via shading and transpiration, but may slightly warm the nighttime street canyons through radiative trapping effect. Image 1 • We develop a new stochastic method to include canopy transmittance in view factors. • Sensitivity of view factors to urban geometries and transmittance is evaluated. • The new urban canopy model with tree modeling outperforms its previous versions. • Radiation exchange and cooling effect of trees depend on foliage characteristics. [ABSTRACT FROM AUTHOR]