Colorado State University

Refereed Publications

Cha, T-Y., M. M. Bell, W.-C. Lee, A. J. DesRosiers, : Polygonal Eyewall Asymmetries during the rapid intensification of Hurricane Michael (2018). Geophysical Research Letters, 47, e2020GL087919 ,

Key Points

  • Single Doppler radar retrievals capture the structural evolution of Hurricane Michael's polygonal eyewall during rapid intensification.
  • The propagation speeds of asymmetric winds and reflectivity are consistent with linear vortex Rossby wave theory.
  • Coastal radar observations and the generalized velocity track display technique can help to improve tropical cyclone intensity forecasts.

  • Plain Language Summary

    Understanding tropical cyclone (TC) structure and evolution are crucial for improving weather forecasts. Hurricane Michael (2018) was observed by radar imagery to have an evolving polygonal eyewall with elliptical, triangular, and square shapes during rapid intensification (RI). While polygonal eyewall shapes have been seen in previous hurricanes, the corresponding evolution of wind asymmetries has never been quantitatively deduced due to limitations from previous observations. 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 winds at 5‐min intervals from single‐Doppler Next Generation Weather Radar (NEXRAD) observations. The results highlight the value of coastal radar observations to investigate physical mechanisms of TC intensity and structure evolution and will help to improve intensity forecasts in the future.


    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.

    Key Figure

    Key Figure

    Spectral time decomposition of m = 1–4: (a) tangential wind components and (b) reflectivity components. The Nyquist frequency is derived from the measuring frequency of the radar (∼0.00167 1/s). The derived propagation speeds of each (c) tangential wavenumber and (d) reflectivity wavenumber and the theoretical VRWs propagation speed from the linear wave theory (black dot). Figure is derived from Fig. 4 in Cha et al. 2020.


    Supported by National Science Foundation awards AGS-1701225 and OAC-1661663, and Office of Naval Research awards N000141613033 and N000142012069.