TUF-Pedestrian: A Three-Dimensional Microscale Model for Pedestrian Thermal Exposure in Urban Environments

The majority of the human population today lives in cities, which are expected to expand as population grows. These cities are often warmer than their surroundings due to multiple factors such as reduced wind speeds from the presence of buildings increasing drag, the abundance of hot, impermeable surfaces emitting longwave radiation, and multiple reflections and trapping of radiation by buildings. Temperatures in cities and the number and severity of heat waves are also expected to increase with climate change, so it becomes increasingly important to design cities to optimize the thermal comfort of humans by minimizing their thermal exposure. During heat waves, spatial variability of human thermal exposure in urban environments is often controlled more by radiation, or mean radiant temperature (T_MRT), than by air temperature. To this end, we further developed an existing microscale three-dimensional urban energy balance model (TUF3D) to include calculations for T_MRT relevant to pedestrian thermal exposure and the important shading effect of trees (TUF-Pedestrian). TUF-Pedestrian mimics the six-directional method for T_MRT measurement, and explicitly calculates in three dimensions all radiant exchanges between this “pedestrian” and its surrounding urban environment at high temporal resolution over the entire diurnal cycle. We test this model against six-directional shortwave and longwave radiation fluxes measured by a mobile biometeorological station (MaRTy) in an urban canyon on the Arizona State University campus in Tempe, AZ, USA. Our comparisons of TUF-Pedestrian with MaRTy observations show a reasonable performance by the model during the daytime, when solar radiation is the main contribution to the variation in T_MRT. As such, this model can be applied to assess current and future urban designs during the daytime, when temperatures are hottest, to optimize human thermal exposure and comfort in cities.