Airborne Radar Observations of Eyewall Replacement Cycles in Hurricane Gonzalo

As part of the NASA Hurricane and Severe Storm Sentinel (HS3) field experiment, airborne Doppler radar successfully captured detailed observations of inner core convection in Hurricane Gonzalo (2014) over three consecutive days. During this period, satellite observations indicate that Gonzalo underwent two complete eyewall replacement cycles (ERCs). Each ERC exhibited the expected changes in its inner core convection: an initial spiral rainband complex axisymmetrized to form a circular outer secondary eyewall, then the original inner eyewall decays, followed by the contraction of the recently formed and now primary eyewall. The formation of the secondary eyewall involves a complex interaction of the inner core mesoscale convective features and the overall vortex circulation, with dynamical processes that are not yet fully understood. Here we explore observations of Gonzalo's inner core features, compare with past observational and modeling studies, and interpret their dynamical roles in the context of theories for secondary eyewall formation. The data were collected by the Ku/Ka band HIWRAP Doppler radar, which utilizes two downward pointing, conically scanning beams to provide the three-dimensional reflectivity and wind structure of precipitation features. On the first day of observations, the spiral rainband complex contained a large stratiform precipitation region on the downshear-left side of the storm, which contained mesoscale descending inflow that was likely driven by the latent cooling of melting/evaporation of slowly falling hydrometeors. As this complex transitioned into a secondary eyewall on the second day, it developed a deep overturning circulation and tangential wind maximum with embedded convective-scale features on all sides of the ring. On the last day of observations, the second secondary eyewall developed a similar circulation as seen on the day before but with weaker winds, which was consistent with the overall weakening storm. These observations corroborate past observations of secondary eyewalls and show general consistency with the dynamics of mesoscale features that are involved in ERCs.