A coupled CFD–Microphysics parameterization framework for urban-scale microclimate simulation

Urban heat stress is governed by apparent temperature, yet many building-resolving CFD studies oversimplify humidity or impose it through external forcing. We develop a building-resolving CFD system with an online warm-phase microphysics coupling to simulate meter-scale wind–temperature–moisture variability in dense urban form. The model is applied to Jungnang-gu, Seoul (17–28 March 2024) and evaluated against hourly AWS 409 observations using MAE, RMSE, R², and 3D diagnostics. Relative to LDAPS, it better captures the temporal evolution of near-surface thermodynamic conditions, with RMSE = 1.17 °C for air temperature and 7.4% for relative humidity, and improves wind performance. Precipitation timing and variability are reproduced, though some hours show intensity bias, consistent with point-to-grid representativeness gaps and sensitivity to terminal-velocity assumptions. During rainfall, surface rain rate follows rainwater mass flux set by rain mixing ratio and net downward motion, and weak rain exhibits strong sub-kilometer intermittency. Urban ventilation structures shape coupled heat–moisture contrasts, producing hot–dry pockets under stagnation and cooler, moister conditions along ventilated corridors. These contrasts yield ~2–5 °C apparent-temperature differences over short distances, underscoring that heat-stress assessment should consider ventilation and humidity variability in addition to temperature.

 

Acknowledgments

This study was carried out with the support of 'R&D Program for Forest Science Technology '(Project No. "RS-2025-25404070")' provided by Korea Forest Service (Korea Forestry Promotion Institute).

 

Key words: Urban microclimate; CFD model; Warm-phase cloud microphysics; Humidity variability; Apparent temperature