Wavelet analyses of turbulence in the hurricane boundary layer during landfalls

Using wavelet transform (WT), this study analyzes the surface wind data collected during the landfall of hurricanes in the 2002-2004 season. The WT, which decomposes a time series onto the scale-time domain, provides a means to investigate the role of turbulent eddies in the vertical transport in the unsteady, inhomogeneous hurricane boundary layer (HBL). The normalized WT power spectra (NWPS) show that the HBL roll vortices tend to suppress the eddy circulations immediately adjacent to rolls, but they do not appear to have a substantial effect on eddies smaller than 100 m. For low wind speeds less than 10 m/s, the contribution of small eddies (< 236 m) to the surface wind stress and turbulent kinetic energy (TKE) decreases with the increase of wind speed. The opposite is true for eddies greater than 236 m. However, contributions of both small and large eddies tend to level off as wind speeds keep increasing. It is also found that the scale of peak NWPS of the surface wind stress is nearly a constant with the mean value of approximately 86 m, whereas the scale of peak NWPS for TKE generally increases with wind speed, suggesting the different role of eddies in generating fluxes and TKE. This study illustrates the unique characteristics of the HBL turbulent structure, which need be taken into account in the boundary layer turbulent mixing schemes so that the vertical transport processes in the HBL can be appropriately parameterized in models.