The Unconventional Eyewall Replacement Cycle (ERC) of Hurricane Ophelia (2005)

One of the mechanisms proposed for the spin-up of the tropical cyclone (TC) mean tangential circulation is the convergence of absolute angular momentum above the boundary layer. This mechanism is important for the outer primary circulation and results in the broadening of the TC wind field. We hypothesize that the mid-level inflow associated with the stratiform precipitation in TC rainbands may be instrumental in spinning up the broader circulation, and may be important in the development of secondary eyewalls. Hurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a Category 1 storm located over cold sea surface temperatures near 23°C. The ERC was observed using airborne radar observations during the Hurricane Rainband and Intensity Change Experiment (RAINEX). Data was collected from the single-parabolic X-band radar aboard the National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft and from the dual-beam X-band Electra Doppler Radar (ELDORA) aboard the Naval Research Laboratory (NRL) P-3 aircraft. The two aircraft flew simultaneously along Ophelia’s primary rainband during a research flight beginning around 1700 UTC on 11 September 2005, allowing for quad-Doppler wind retrievals along the rainband. Analyses were conducted using a spline-based three-dimensional variational wind synthesis technique. Results showed a broadened tangential wind field associated with the ERC was observed in the stratiform-dominant rainbands of Ophelia. The broadening of the tangential wind field was collocated with the strongest radial advection of angular momentum through the stratiform mid-level inflow and is consistent with the proposed mechanism for TC intensity change.