HIWRAP Observations of Eyewall Replacement Cycles in Hurricane Gonzalo

The High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) is a dual-beam, dual-frequency, Doppler radar system designed for operation on board high-altitude aircraft. As part of the NASA Hurricane and Severe Storm Sentinel (HS3) field experiment, HIWRAP flew on board the WB-57 and obtained detailed observations of Hurricane Gonzalo (2014) over three consecutive days. During this time frame, satellite observations indicate that Gonzalo underwent two complete eyewall replacement cycles (ERCs). We present here HIWRAP observations showing the reflectivity and kinematic structures of Gonzalo's inner core convection during these ERCs.

In order to retrieve the three-dimensional wind field, we utilize the coplane dual-Doppler analysis technique from Didlake et al. (2015), which was specially designed for HIWRAP's unique scanning geometry. In this scanning geometry, the two antennas of HIWRAP point downward and scan conically at different tilt angles. These observations are interpolated to a track-following cylindrical coordinate system where two components of the wind are readily calculated within each coplane. The third component is obtained by enforcing certain boundary conditions on the wind field and then integrating the mass continuity equation along each arc in the cylindrical coordinate frame.

On the first day of observations, Gonzalo exhibited a spiral rainband complex that contained a large stratiform precipitation region on the downshear-left side of the storm. Within this region, mesoscale descending inflow persisted, 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.