Decomposing form and skin drag over complex urban terrain using Doppler LiDAR, sonic anemometers, and immersed-boundary large-eddy simulation

Quantifying how downtown skylines redistribute momentum remains challenging because form drag and skin drag act at different heights over such complex heterogeneous terrains, yet are often lumped into a single roughness length value. We aim to address this problem using three complementary data collected around Houston’s high-rise core: (i) a year-long sequence of Doppler lidar scans that measure the wind profiles from 50 m to ∼2000 m; (ii) a 5 Hz sonic anemometer at 3 m supplying near-surface friction velocity u_* and roughness length z₀; and (iii) an immersed-boundary large-eddy simulation (LES) resolving turbulence around buildings. Lidar profiles are grouped by flow-direction sectors and inverted to retrieve sector-specific u_* and z₀. Sectors aligned with the skyscraper corridor show an order-of-magnitude increase in z₀ relative to suburban inflow. Moreover, the 3 m sonic reveals only modest variations, indicating the additional drag aloft is primarily form drag. To verify, the LES was used to reproduce both u_* for the sector belonging to the downtown and the contrast between surface-based and lidar-based roughness. The results of the stress decomposition from LES confirm that form drag dominates whenever flow intersects the modeled skyline. By integrating surface fluxes, scanning lidar observations, and high-fidelity LESs, this study delivers a direction-based map of urban form versus skin drag and guides the development of a physics-based benchmark for urban canopy parameterizations.