Effects of cool coatings on urban microclimate and outdoor thermal comfort: A CFD-CitySim pro coupled simulation study

Abstract

This study examines how cool coatings, which modify the radiative properties of building and ground surfaces, affect urban microclimate and pedestrian thermal comfort in a district with heterogeneous building heights. A computational fluid dynamics (CFD) model coupled with CitySim Pro explicitly resolved shortwave and longwave radiative exchanges. The framework was validated against field measurements, demonstrating high accuracy for surface/air temperatures and wind speed/direction in the control experiment (CNTL) using in-situ material properties. Sensitivity experiments revealed that high-rise facade coatings (BiGc, BiGj) reduced the coated facade surface temperature by 4.70-5.56 degrees C, but increased temperatures on nearby low-rise facades and roofs by up to 6.03 degrees C and 4.06 degrees C, respectively. In the combined case (BiGj), adding high-reflectance pavement mitigated the facade-induced ground warming observed in BiGc by similar to 2 degrees C. Ground-only coatings (BcGj) lowered pavement temperature by 1.71-2.24 degrees C while producing only minor thermal changes on surrounding buildings. Regarding air temperature, ground-only coatings produced marginal nighttime cooling and similar to 0.5 degrees C daytime cooling, primarily over open spaces. Facade-involved scenarios achieved up to 2 degrees C daytime cooling near deep canyons but caused localized nocturnal warming of 0.3-2.0 degrees C in medium and shallow canyons. Universal Thermal Climate Index (UTCI) analysis at the Gwanghwamun Square showed ground-only coatings increased daytime UTCI by < 1 degrees C on average, whereas facade-involved treatments raised UTCI by up to 4.13 degrees C due to intensified mean radiant temperature. Overall, ground-centric coatings minimize comfort penalties, while facade treatments require strict radiative control. Prioritizing shaded ground applications in high-activity areas offers a practical balance between urban cooling and pedestrian well-being.