A rapid and scalable radiation transfer model for complex urban domains

An important component of urban microclimate is the radiative heat transfer between the myriad elements that make up the urban fabric. While great progress has been made in developing radiation models for idealized urban spaces, operational simulations of fully resolved city-scale domains remain elusive. As a result, simplifications and assumptions must be made. Such compromises may limit utility, and reveal a need for new, scalable microclimate models. This paper presents a novel, physically-based, building-resolving radiation transfer model (QESRadiant) that utilizes ray tracing techniques accelerated with graphics processing units (GPUs). QESRadiant builds on computer graphics methods, incorporating approaches for global illumination and light transport. Tests show that our methods can rapidly simulate the radiation balance for millions of surfaces with unique shapes and properties using a single consumer-class workstation, requiring times on the order of several minutes. High-resolution (0.5m) street canyon radiation budgets are validated using field data covering nine months. The results show the model is able to predict radiative fluxes in the canyon with an overall average R² of 0.77 and mean error of 14.5Wm^-2. Solar radiation was extremely sensitive to geometric obstructions, demonstrating the need for high-quality field experiments that resolve urban details affecting the radiation balance.