The urban effects on the planetary boundary layer wind structures of Typhoon Lekima (2019)

Abstract

The urban effects on the planetary boundary layer (PBL) wind structures of landfalling tropical cyclones (TCs) have rarely been explored. In this study, numerical simulations for Typhoon Lekima (2019), with and without multilayer building effect parameterization (BEP) and urban land cover, were executed to investigate the urban effects on TC PBL wind structures. Validations against observations demonstrate that the simulation incorporating BEP and urban surface replicates the track, intensity and 10-m wind field of Lekima better. Based on the comparison between the simulations with and without urban land cover, urban effects were analyzed, and the possible mechanisms were examined from the perspective of turbulent transport. Results show that urban surfaces have a deceleration effect on TC wind fields overall. This deceleration effect is most pronounced near the surface and decreases with height under 1 km above ground level. Urban surfaces reduce tangential winds and radial inflow in the near-surface layer, leading to a slight decrease in TC intensity. This is primarily attributed to the enhanced downward transfer of tangential momentum and upward transfer of radial momentum within the PBL, which results in larger magnitudes of negative tangential wind tendencies and positive radial wind tendencies induced by the divergence of subgrid-scale (SGS) momentum fluxes. Additionally, stronger tangential winds and radial outflow above the elevated PBL height correspond well to the increased tangential and radial wind tendencies in nearby areas. The analysis illustrates that turbulent transport provides insights into how urban surfaces affect the PBL wind structures of TCs.