<p>Clear-sky, low-wind conditions are typical for low-altitude aircraft operations. However, the Weather Research and Forecasting model coupled with the Single-Layer Urban Canopy Model (WRF/SLUCM) often overestimates wind speeds during forecasts for these conditions. This study introduces detailed local climate zone (LCZ) data into WRF/SLUCM to establish an analytical framework linking urban morphology to momentum exchange and wind field response, aiming to improve simulation accuracy and elucidate how building characteristics modulate low-altitude winds. We compared the simulations using LCZ vs. Moderate Resolution Imaging Spectroradiometer (MODIS) underlying surfaces for an extreme heat event in Jiangsu Province, China. The results showed that the LCZ underlying surface with more complex building morphology improves 10-m wind speed simulations. The median mean absolute error (MAE) across all stations decreased from 1.1 to 0.6 m s⁻¹ and the root-mean-square error (RMSE) from 1.3 to 0.8 m s⁻¹. Mechanistic analysis reveals a dual-action mechanism in the momentum “source” and “sink.” The thermal effect, via a lower impervious surface fraction, weakens sensible heat flux and turbulent kinetic energy, largely reducing downward momentum transport, which is a loss of the “source.” Meanwhile, the dynamic effect alters the “sink” complexly: although increased roughness enhances potential drag, this is counteracted by lower impervious surface fraction and wind feedback, causing the final “sink” to decrease slightly. The substantial reduction of the “source” is far greater than the minor change in the “sink,” which leads to the overall wind speed reduction. This study underscores that accurate urban surface representation is critical for precise low-altitude wind simulation, especially under clear-sky, low-wind conditions, supporting the safe development of the low-altitude economy.</p>