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

Over the years, Bob Houze has worked extensively on airborne observations of tropical cyclones (TCs) and has advanced understanding of the cloud and precipitation processes of a TC’s inner core. He has been involved in several TC-related field projects, including his most recent TC field project, the NASA Hurricane and Severe Storm Sentinel (HS3). During the final year of HS3, airborne Doppler radars successfully captured mapped the detailed wind and precipitation structure of the inner core convection in Hurricane Gonzalo (2014) over two consecutive days. During this period, satellite and aircraft observations indicate that Gonzalo underwent two complete eyewall replacement cycles (ERCs). The data were collected by NASA’s Ku/Ka band HIWRAP radar and by NOAA’s X-band P3 Tail radar. The HIWRAP radar flew on board NASA’s WB-57 which was tasked for ONR’s Tropical Cyclone Intensity experiment.

The data were analyzed relative to the deep-layer environmental wind shear vector. During the beginning ERC stages, the inner eyewall (IE) and outer eyewall (OE) exhibited different asymmetries. The IE asymmetry exhibited robust low-level inflow, updrafts, and positive tangential acceleration in the downshear quadrants, consistent with observational and theoretical studies. The OE asymmetry exhibited these features further downwind from that of the IE, in the left-of-shear quadrants. It is suggested that robust low-level inflow occurring at the OE but not at the IE in the downwind regions signals a barrier effect that contributes to the eventual decay of the IE. Toward the later ERC stages, the OE asymmetry shifts upwind, becoming more aligned with the asymmetry of the earlier IE. This upwind shift is consistent with the structural evolution of eyewall replacement as the OE transitions into the primary eyewall of the storm.