Development of a building-resolving CFD model coupled with warm-cloud microphysics for simulating urban moisture and phase-change effects

Urban heat stress depends on apparent temperature, yet many building-resolving computational fluid dynamics (CFD) studies simplify humidity or rely on external forcing. We present BECLOUD, a building-resolving CFD model that incorporates a dynamic cloud-model microphysics parameterization for warm-phase processes, resolving wind, temperature, and moisture at meter scale. BECLOUD is applied to a densely built district in Jungnang-gu, Seoul, from 17–28 March 2024, and validated against hourly observations from AWS 409. Skill is quantified using MAE, RMSE, R 2 , and 3D-diagnostic fields. Relative to the Local Data Assimilation and Prediction System (LDAPS), BECLOUD better reproduces the evolution of air temperature and humidity and improves wind statistics. Comparison with AWS precipitation indicates event timing and variability are captured, while intensity biases remain at certain hours; these likely reflect representativeness differences point measurements and grid-cell means and uncertainties in terminal-velocity parameterization. Analysis of precipitating hours shows surface rainfall intensity follows the rainwater mass flux by rain mixing ratio and net downward velocity, with weak rainfall exhibiting sub-kilometer variability. Within urban canopy, ventilation structures organize the coupled heat–moisture fields: stagnation favors locally hot-and-dry zones, whereas well-ventilated corridors are cooler and more humid. Apparent temperature differs by ∼2–5°C across short distances, indicating urban heat-stress assessment must jointly consider ventilation and humidity variability, not temperature alone. Results demonstrate that coupling warm-rain microphysics within CFD provides a physical basis for resolving urban moisture heterogeneity and supports broader microclimate applications. Future work will integrate radiation and land-surface processes and extend evaluation across seasons and sites.