Urban Shortwave Radiation Trapping: An Index for Pedestrian-Level Radiative Exposure in Dense Urban Areas

Urban heat intensification and its implications for environmental health due to solar radiation absorption is a key driver of the Urban Heat Island (UHI) effect, particularly in Mediterranean climates. In dense urban fabrics, shortwave radiation is repeatedly reflected within street canyons, affecting pedestrian-level radiative exposure. This study introduces the Urban Canyon Shortwave Trapping Index (STIc), a dimensionless parameter quantifying geometry-induced shortwave accumulation near the ground. Shortwave fluxes were simulated using the SOLWEIG model over the centre of Nicosia, Cyprus, under typical summer conditions. Wall albedo was systematically varied (0.2–0.8) to represent a range of common building materials and cooling strategies, including Nature-Based Solution (NBS). Results show that STIc increases with both wall reflectivity and canyon depth, reaching values above 2 in high-albedo, high aspect ratio configurations. Maximum thermal trapping occurs between 11:00 and 14:00 local time, coinciding with peak in solar altitude. Spatial variability of STIc also increases with wall albedo, indicating greater sensitivity to morphological differences under more reflective conditions. These findings support the integration of both morphological and material-based strategies in UHI mitigation and thermal comfort improvement and sustainable urban development.