Evaluating the PALM-4U representation of outdoor urban thermal exposure in a humid continental climate using MaRTy

Keywords: Microscale modeling, large-eddy simulations, biometeorology, urban climate, UTCI

Human heat stress and the risk of outdoor thermal discomfort under extreme heat conditions are increasing even in traditionally cold climates due to climate change. Modeling micro-scale urban climate to explore outdoor thermal exposure is a highly topical challenge in modern climate research. PALM-4U is a large-eddy simulation (LES) based high-resolution micrometeorological model that includes a complex radiation scheme and an integrated biometeorology module to assess thermal conditions in complex urban environments. To test the performance of PALM-4U, we compared the model with in-situ measured mean radiant temperature (MRT) and universal thermal climate index (UTCI). Model evaluation was performed using the mobile instrument platform MaRTy. Air temperature, relative humidity, wind speed, and longwave and shortwave radiant flux densities in a 6-directional setup were recorded by the MaRTy cart and compared to PALM-4U. The in-situ measured data were collected across 23 locations in Guelph, Ontario, Canada, during multiple times of day and seasons in 2020 and 2021. Data were collected and aggregated for evaluation of PALM-4U in 10-minute and 1-hour intervals. We found that PALM-4U performed better in simulating UTCI than MRT in summer, especially during periods when the midday surface temperature is high. Furthermore, the MRT simulation result deviates from the measured values under certain shade types. The performance under vegetation is slightly better compared to under engineered shade. This work aids in the improvement of PALM-4U under various urban morphologies and advances the microclimatic modeling research of the field.