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Urban ventilation under light-wind conditions is a critical factor of thermal comfort and air quality in high-density cities, particularly during extreme heat events when synoptic forcing is weak. This study presents an integrated framework that combines city-scale wind mapping, large-eddy simulation (LES), neighborhood-scale pollutant dispersion analysis, and model parameterization to advance understanding and representation of urban ventilation processes in weak-wind regimes.First, a city-scale wind mapping tool is developed for Guangzhou based on urban morphological parameters. The results reveal extensive low-wind-speed zones at pedestrian level, especially in high-density districts, indicating suppressed wind-driven ventilation and an increased reliance on buoyancy-induced airflow.Second, high-resolution three-dimensional LES is conducted to investigate buoyancy-driven thermal plume dynamics under weak ambient winds. Validation against laboratory experiments demonstrates that the model accurately captures plume bending, vertical transport, and plume merging. The simulations show that surface-heating-induced thermal plumes generate strong near-ground horizontal inflow and coherent plume-merging structures, producing pedestrian-level convergence velocities of approximately 1–2 m/s, which is comparable to ventilation induced by moderate background winds.Third, the CFD framework is applied to assess traffic-related pollutant dispersion at the neighborhood scale. Results indicate that buoyancy-driven ventilation substantially enhances pollutant removal under calm and light-wind conditions. Interactions between weak background winds and rising thermal plumes induce oscillatory flow structures and enhanced turbulence, effectively reducing near-surface pollutant accumulation.Finally, drawing on a large ensemble of LES results, a parameterization scheme for urban ventilation under light-wind conditions is developed and incorporated into the UT&C model. By explicitly accounting for buoyancy intensity and urban morphology, the new scheme improves the representation of air exchange velocity.This improvement directly enhances the assessment of heat and air quality risks in dense urban areas under light-wind and extreme-heat conditions, thereby providing a more robust scientific basis for urban design, planning, and climate-resilience strategies.