Cold stress mitigation by urban trees: Computational fluid dynamics analysis of deciduous and evergreen effects in winter environments

• CFD–radiation coupled model evaluates winter tree effects on UTCI in Daegu. • Wind speed dominates UTCI (R≈−0.97); air temperature and MRT are secondary. • Tall evergreens ease cold stress in wind-exposed corridors, esp. morning and night. • In sheltered zones, evergreen shading lowers MRT and worsens afternoon cold stress. • Deciduous or low trees fit sheltered areas; evergreens suit windy sites. Extreme cold surges, increasingly observed in mid-latitude and subtropical cities, pose serious health and safety risks for pedestrians. While nature-based solutions such as urban trees are widely recognized for mitigating summer heat, their wintertime effects remain underexplored and inconsistent. Trees can simultaneously reduce convective heat loss by lowering wind speeds and diminish solar radiation, decreasing the mean radiant temperature (MRT) and potentially aggravating cold stress. This study employs a computational fluid dynamics (CFD) model, with one-way, offline coupling to a radiation model, to investigate how deciduous and evergreen trees influence pedestrian cold stress, evaluated using the Universal Thermal Climate Index (UTCI), in a high-density urban district of Daegu, South Korea, during a winter cold-surge event. Scenarios varied tree type and height to assess aerodynamic drag, shading, and cooling effects. Results show that wind speed was the dominant driver of UTCI (Pearson correlation coefficient, R ≈ –0.97), while air temperature (R ≈ 0.3) and MRT (R ≈ 0.15) acted as secondary modifiers. Evergreen trees reduced strong wind probabilities and alleviated severe cold stress (UTCI ≤ –13°C) in exposed zones, with tall evergreens most effective during morning and nighttime. In contrast, in sheltered low-rise areas where winds were already weak, added shading lowered MRT and air temperature, leading to worsened thermal comfort in the afternoon. Deciduous trees had limited effects owing to low winter leaf area. These results emphasize that winter planting strategies must consider tree type, canopy height, and local morphology, with evergreens suited to wind-exposed corridors and deciduous or shorter trees more appropriate for sheltered zones.