The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

James Hlywiak, David S. Nolan

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Pages (from-to)529-549
Number of pages21
JournalMonthly Weather Review
Volume150
Issue number3
DOIs
StatePublished - Mar 2022
Externally publishedYes

Keywords

  • Boundary layer
  • Buoy observations
  • Coastal flows
  • Hurricanes/typhoons
  • Numerical analysis/modeling

ASJC Scopus subject areas

  • Atmospheric Science

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