Concentric Eyewall Asymmetries in Hurricane Gonzalo (2014) Observed by Airborne Radar

Two eyewall replacement cycles were observed in Hurricane Gonzalo by the NOAA P3 Tail (TA) radar and the recently developed NASA High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) radar. These observations captured detailed precipitation and kinematic features of Gonzalo's concentric eyewalls both before and after the outer eyewall's winds became the vortex maximum winds. The data were analyzed relative to the deep-layer environmental wind shear vector. During the beginning eyewall replacement cycle stages, the inner and outer eyewalls exhibited different asymmetries. The inner eyewall asymmetry exhibited significant low-level inflow, updrafts, and positive tangential acceleration in the downshear quadrants, consistent with observational and theoretical studies. The outer eyewall asymmetry exhibited these features in the left-of-shear quadrants, further downwind from those of the inner eyewall. It is suggested that the low-level inflow occurring at the outer but not at the inner eyewall in the downwind regions signals a barrier effect that contributes to the eventual decay of the inner eyewall. Toward the later eyewall replacement stages, the outer eyewall asymmetry shifts upwind, becoming more aligned with the asymmetry of the earlier inner eyewall. This upwind shift is consistent with the structural evolution of eyewall replacement as the outer eyewall transitions into the primary eyewall of the storm.