Urban cold stress assessment using computational fluid dynamics and universal thermal climate index: A case study of Gwacheon City, South Korea

This study presents a detailed computational framework to assess outdoor cold stress in urban environments. Considering the increasing climate uncertainty, the frequency and severity of cold-related hazards are projected to increase, particularly in dense urban areas, where the morphology and surface properties exacerbate thermal discomfort. Yet, research on cold stress remains limited compared with heat-related studies. To address this gap, we conducted transient Computational Fluid Dynamics (CFD) simulations across 54 parametric scenarios by varying the radiative properties (emissivity, reflectivity, and transmissivity) of typical urban surface materials, and assessed the pedestrian-level thermal comfort under extremely cold conditions using the Universal Thermal Climate Index (UTCI). The simulation outputs were validated against real-time meteorological observations collected from three urban monitoring stations in Gwacheon, South Korea. Scenario 14, which featured a concrete $\epsilon$ of 0.4 and reflectivity of 0.6, building exterior transmissivity of 0.7, and asphalt $\epsilon$ of 0.7, most accurately replicated the observed temperature patterns (R² > 0.85 across all stations). Spatial UTCI mapping revealed that approximately 886,519 m² experienced strong cold stress (UTCI < –10 °C) at 9:00 on February 8, 2025, particularly in high-rise residential clusters and exposed green zones. These findings highlight the role of surface material configuration and urban form in cold stress distribution. The proposed method offers a robust and physiologically grounded tool for guiding winter climate adaptation strategies in urban planning and design.