Polygonal eyewall asymmetries during the rapid intensification of Hurricane Michael (2018)

Polygonal eyewall asymmetries of Hurricane Michael (2018) during rapid intensification (RI) are analyzed from ground-based single Doppler radar. Here, we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds at 5-min intervals. Spectral time decomposition of the retrieved tangential wind structure shows quantitative evidence of low (1-4) azimuthal wavenumbers with propagation speeds that are consistent with linear wave theory on a radial vorticity gradient, suggesting the presence of rapidly evolving vortex Rossby waves. Dual-Doppler winds from the NOAA P-3 Hurricane Hunter airborne radar provide further evidence of the three-dimensional vortex structure that supports growth of asymmetries during RI. Both reflectivity and tangential wind fields show polygonal structure and propagate at similar speeds, suggesting a close coupling of the dynamics and the convective organization during the intensification.