Urban Tree Planting Strategies for Winter Cold Surges: A CFD-based Assessment of Deciduous vs. Evergreen Effects

 As extreme cold surges become more frequent in mid-latitude cities due to climate variability, the role of nature-based solutions (NBS), primarily designed for summer heat mitigation, requires re-evaluation for winter conditions. This study investigates the impact of urban street trees on pedestrian thermal comfort during a cold wave event in a high-density district of Daegu, South Korea. Using a Computational Fluid Dynamics (CFD) model coupled with a solar radiation model, we quantified the opposing physical mechanisms of trees: the beneficial reduction of convective heat loss via aerodynamic drag versus the detrimental reduction of solar gain via shading. Our results reveal that wind speed, rather than air temperature or mean radiant temperature, is the dominant driver of wintertime outdoor thermal comfort (UTCI). Tall evergreen trees significantly mitigated cold stress in wind-exposed corridors by acting as effective windbreaks. However, in already sheltered areas where solar access is critical, the shading effect of evergreens blocked valuable winter sunlight, paradoxically exacerbating cold stress by lowering the mean radiant temperature. Deciduous trees showed negligible impacts due to their low leaf area index in winter. These findings highlight that "beneficial summer shade" can become a "winter penalty." Consequently, we propose a context-specific planting framework for climate-resilient urban design: prioritizing wind mitigation in exposed zones while preserving solar access in sheltered environments.