Fine-scale dual-Doppler analysis of hurricane boundary layer structures in Hurricane Frances (2004) at landfall

Two Doppler on Wheels (DOW) mobile radars collected fine-spatial scale dual-Doppler data in the front right quadrant and eye of Hurricane Frances (2004) as it made landfall near Stuart, Florida. A 5.7 km dual-Doppler baseline established a dual-Doppler domain south and east of Fort Pierce, FL, encompassing a 5.5km x 5.5km horizontal area, with a grid spacing of 20 m, allowing for the resolution of sub-kilometer scale horizontal structures and associated kinematics. Three-dimensional vector wind analyses of the boundary layer revealed the presence of linear coherent structures with a characteristic wavelength of 400–500m near the surface that increased in size and become more cellular in shape with increasing height. Average horizontal perturbation winds were proportional to average total horizontal winds. Within the eye of the hurricane, the features lost linear coherency despite a high mean wind speed, possibly due to changes in stability. A slight decrease in the characteristic wavelength of boundary layer structures was documented as the winds cross the barrier islands east of Fort Pierce. Vertical flux of horizontal momentum caused by individual vortical structures was substantially higher than values employed in turbulence parameterization schemes, but the domain-wide average flux was substantially lower than that in individual structures, likely due to the transient nature of the most intense portions of the structures. Analysis of the turbulent kinetic energy (TKE) yielded slightly larger values than reported in previous observational studies over the open ocean. Additionally, there was substantial variability in TKE within the dual-Doppler domain, emphasizing the challenge in obtaining representative samples using non-3D measurements such as dropsondes.