The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Last, a brief analysis of in situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally homogeneous surface types—which have been well documented by previous studies—are inappropriate for use near strong frictional heterogeneity.
A ~14‐hr long‐lived spiral rainband in Typhoon Longwang (2005) produced catastrophic rainfall in Fujian Province of China on 2 October 2005. In this study, the effects of terrain and landmass near Fujian on the structure and propagation of this rainband are investigated through high‐resolution numerical simulations. Results show that although the terrain and landmass near Fujian played a marginal role in the formation of the rainband, both greatly affected the structure and propagation of the rainband. Namely, convection in the upwind sector of the rainband tended to be maintained and locked up near the coastline in the control experiment with both the terrain and landmass near Fujian retained, but shrank more inland with the terrain near Fujian flattened, and further inland with the landmass near Fujian replaced by the virtual ocean. It is found that due to the land‐sea surface roughness contrast, the upstream tangential winds from ocean would be substantially decelerated over land and thus induced a local subgradient force onshore near the coastline. The radially inward agradient force and the subsequent surface friction helped maintain the moisture convergence, and thus convection and the cold pool in the upwind sector of the rainband near the coastline. Although the orographic lifting and blocking effects were found to be marginal to the moisture convergence in the rainband, the terrains near Fujian enhanced the deceleration of surface winds, enhancing the effect of land‐sea surface roughness contrast on low‐level moisture convergence and thus the lockup of the upwind sector of the rainband.
more » « less- Award ID(s):
- 1834300
- NSF-PAR ID:
- 10453307
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 125
- Issue:
- 22
- ISSN:
- 2169-897X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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