STMRT: A simple tree canopy radiative transfer model for outdoor mean radiant temperature

Tree canopy shading can significantly decrease the outdoor mean radiant temperature (MRT). However, canopy radiative transfer is quite complex, making it challenging to evaluate the MRT under a tree canopy. In this study, a simple tree canopy radiative transfer model (STMRT) was developed to determine the MRT under a tree canopy. In STMRT, the direct and diffuse solar radiation transfers are calculated based on the leaf inclination angle distribution, and the longwave radiation emitted from a canopy is evaluated based on the simplification that the canopy surface temperature is the same as the surrounding air temperature. The STMRT's accuracy was evaluated by measurement and compared to that of 4SAIL. The results demonstrate that the MRT differences (Delta MRT) in STMRT are typically less than 2.3 degrees C; in Shanghai, China, Delta MRT caused by the differences in direct solar radiation transmissivity between STMRT and 4SAIL are less than 2.0 degrees C, unless the leaf area index is less than 2 and the solar zenith angle is greater than 70 degrees. According to STMRT, it is found that Delta MRT generated by the leaf inclination angle variances across various tree species can reach 9.5 degrees C; Delta MRT caused by a uniform distribution simplification for leaf inclination angle can reach 5.7 degrees C. STMRT will help plan the outdoor thermal environment by evaluating the MRT under tree canopies.