Analysis of the Impact of Emissivity and Albedo on Urban Mean Radiative Temperature

Abstract

This study investigates the impact of urban surface albedo and emissivity on Mean Radiant Temperature (MRT), a key parameter for assessing thermal comfort and urban heat island (UHI) dynamics. Results show that MRT generally follows the diurnal cycle of air temperature, with notable deviations in highly exposed areas, particularly on horizontal surfaces, which are more sensitive due to their orientation, thermal capacity, and roughness. High-albedo scenarios effectively reduce daytime MRT and summer heat stress but may also enhance nocturnal radiative cooling. The influence of emissivity is more subtle, as most urban materials already exhibit high values, yet variations can still affect longwave exchanges, especially at night. Intermediate albedo-emissivity configurations provide a more balanced performance across seasons by maintaining moderate MRT levels both day and night. Findings further indicate that horizontal surfaces exert a stronger control on MRT than vertical ones under open-sky conditions, though facades can become dominant in dense urban canyons due to radiative trapping. Additionally, integrating urban vegetation—through evapotranspiration and shading—proves highly effective in lowering MRT and mitigating UHI. Optimizing the radiative properties of urban surfaces, combined with green infrastructure, can improve outdoor thermal comfort, reduce cooling energy demand, and indirectly contribute to lowering greenhouse gas (GHG) emissions. These results provide operational insights for incorporating MRT into energy-climate planning and guiding urban transition strategies towards more resilient and sustainable cities.