Fine-scale Dual-Doppler analysis of the boundary layer in Hurricane Frances (2004)

Dual-Doppler data were collected by the DOW radars in the northern eyewall and eye of Hurricane Frances as it made landfall near Stuart, FL. A 5.7 km baseline was established south and east of Fort Pierce, FL. The range resolution varied from 13 to 38 m, with the crossbeam oversampled resolution of approximately 30 m in the dual-Doppler lobes. This is one of the smallest dual-Doppler baselines ever established in a hurricane, providing an extremely fine scale three-dimensional analysis of the hurricane boundary layer. The DOWs deployed at approximately 1400 UTC on 04 September 2004 and undeployed at approximately 1100 UTC on 05 September 2004. Landfall of Hurricane Frances occurred at approximately 0600 UTC on 5 September 2004. The DOW data indicate the presences of boundary layer structures of different length scales, most of which are substantially < 1 km.

The dual-Doppler domain was chosen to optimize coverage and spatial and temporal resolution. As such, the horizontal domain comprised a 5.5 km x 5.5 km region, with a grid spacing of 20 m, thereby allowing for the resolution of horizontal structures with a characteristic size of O ~ 80m. Furthermore, the domain captured the boundary layer flow on both the windward and leeward sides the barrier island, which allowed for the assessment of the impact of the barrier island on the onshore flow. Three-dimensional analysis revealed updraft/downdraft couplets associated with the boundary layer rolls. Characteristic wavelengths of the boundary layer structure were determined through a 1-dimensional Fourier decomposition. Structures were examined both windward and leeward of the barrier islands, as well as over the entire domain. Horizontal cross-sections were taken through the features at various locations and a characteristic wavelength was determined. The results of these analyses and implications on boundary layer momentum fluxes will be presented.