Documenting Coherent Turbulent Structures in the Boundary Layer of Intense Hurricanes through Wavelet Analysis on IWRAP and SAR Data

Abstract

New radar remote sensing measurements of the turbulent hurricane boundary layer (HBL) are examined through analysis of airborne (Imaging Wind and Rain Airborne Profiler; IWRAP) and spaceborne (synthetic aperture radar; SAR) data from Hurricanes Dorian (2019) and Rita (2005). These two systems provide a wide range of storm intensities and intensity trends to examine the turbulent HBL. The central objective of the work is to document the characteristics of coherent turbulent structures (CTSs) found in the eyewall region of the HBL. Examination of the IWRAP data in Dorian shows that the peak, localized wind speeds are found inside the CTSs near the eye-eyewall interface. The peak winds are typically located at lower levels (0.15 - 0.50 km), but sometimes are found at higher levels (1.0 - 1.5 km) when the CTSs are stretched vertically. A SAR overpass of Dorian’s eyewall showed ocean surface backscatter perturbations at the eye-eyewall interface that have connections to the CTSs identified in IWRAP data. Wavelet analysis, including detailed significance testing, was performed on the IWRAP and SAR data to study the CTS wavelengths and power characteristics. Both datasets showed a multi-scale structure in the wavelet power spectrum with peaks at ~ 10 km (eyewall), ~ 4 - 5 km (merger of small-scale eddies) and ~ 2 km (native scale of the CTSs). The ~ 2 km native scale of the CTSs is robust across intensity trends (rapid intensification, weakening and steady-state), storm cases and region of the storm. This information is useful for turbulence parameterization schemes used in numerical models that require the specification of a turbulent length scale.