On the Myth of the Logarithmic Surface Layer during Hurricane Landfalls: Insights from Observations and Large-Eddy Simulations

Understanding the evolution of boundary-layer wind profiles during hurricane landfalls is crucial for advancing model physics development and facilitating real-time assessments of hurricane intensity and structure. Previous observational studies have reported logarithmic characteristics of horizontal wind speeds with height in the lower hurricane boundary layer or surface layer over both ocean and land. This “log-layer” assumption forms the basis for estimating near-surface drag coefficient and frictional velocity using in-situ sonde data where direct flux measurements are unavailable. It is also applied to estimate the 10-m wind speed during hurricane landfalls when only mid-upper boundary layer wind observations or retrievals are accessible. However, despite its significant applications, the existence of the log layer in hurricanes has been questioned, particularly in the inhomogeneous inner-core region. Additional complexity arises during hurricane landfalls due to the rapid development of an internal boundary layer within the coastal region in response to an abrupt increase in surface roughness. Whether the log layer assumption is valid or how the depth of the log layer evolves within coastal regions remains obscure.

Motivated by this issue, the study investigates boundary-layer wind profiles within the coastal region based on a Velocity Azimuth Display (VAD) analysis of Doppler radar observations in Hurricane Isaac (2012) and a composite of rawinsonde wind data collected from hurricane landfalls spanning 2000 to 2020. Notably, the uncertainties linked to inferring the log-layer relationship from rawinsonde wind profiles are discussed, supported by the results of "virtual sondes" derived from a specially designed large-eddy simulation (LES) of onshore flow. The LES results additionally demonstrate that post-landfall, a near-surface log layer emerges and extends upward, deepening from 50-60 m to approximately 400 m within the 12-km transition zone over land. The increase in log-layer depth aligns with the development of an internal boundary layer following onshore flow landfall. These findings offer insights into the discrepancies noted in prior studies regarding log-layer depth over land and bear significance for onshore tornadogenesis during hurricane landfalls.