Fine-Scale Hurricane Boundary Layer Structures

The hurricane boundary layer (HBL) is comprised of coherent structures that are potentially responsible for significant transport of turbulent fluxes throughout the HBL as well as regions of enhanced damage at the surface. These coherent structures are not well understood and consequently their effects are poorly represented in numerical models. With the exception of HBL rolls/streaks, not all HBL structures are observed in every hurricane. Much is still unknown about the landfalling HBL wind structure, since in situ observations are comparatively limited.

In order to increase the sample size and characterize the HBL structures over a range of hurricane intensities, evolution, and landfall surface roughness, DOWs deployed in several hurricanes during the 2020 season, obtaining both dual-Doppler and rapid single-Doppler observations in the boundary layer of landfalling hurricanes. Results will be presented from three hurricanes: Hurricane Laura, Hurricane Sally, and Hurricane Delta. These results will be discussed in the context of standard hurricane wind models, previously DOW-characterized/observed HBL winds, and the relationship between radar winds and derived quantities to anemometry.

Two DOWs targeted landfalling Category 4 Hurricane Laura. Laura is the 1st ever fine-scale dual-Doppler deployment sampling the HBL and eyewall in a Category 4 hurricane at landfall. The DOWs deployed in southwestern Louisiana during landfall. As is usual, the DOW radars operated in the most intense wind regions of the hurricane, collecting fine-scale, near-surface data in the eyewall near landfall. The dual-DOW baseline was a very short ~6.3 km, providing ~60 m spatial resolution in the center of the dual-Doppler lobes. Multiple sets of dual-Doppler analyses were conducted in order to optimize the retrievals of 2D and 3D structures. Moreover, surface roughness was considered in some of the wind retrievals. Comparisons between the strength and size of the structures in the eyewall vs. rainbands will be presented. Size will be quantified FFTs. Evolution of the boundary layer depth, as derived from VAD analyses, will be presented as a function of hurricane evolution. In addition, anemometer data at 1 Hz were collected from DOW masts ~8-10 m above radar level (ARL). Turbulence characteristics, periodicity, and gust factors will be compared to radar-derivations of similar quantities using Doppler velocity and spectrum width.

Single-Doppler radar data were collected in Hurricanes Sally and Delta. This allowed for the two-dimensional quantification rapidly evolving of boundary layer structures. An array of surface based instruments, including a prototype, “POLENET”, which attaches existing infrastructure, allowing for a customizable observation level, were deployed in Hurricane Delta in order to correlate observations at radar level with surface observations. Using corrections based on turbulence statistics and roughness lengths, a reduction factor was derived for the radar winds, allowing for comparison between radar level winds and winds observed at 1, 2, and 10 m.

WASHABLE (Winds And Structures in Hurricane Associated Boundary Layers Experiment) is planned for 2024-2025. This project will deploy mobile radars to expand our understanding of HBLs, including gust damage relationships and turbulent processes.