Undergraduate Research and Creativity

URECA

2008-2009

Inner vortex evolution of Hurricane Ophelia (2005)

Cory Clifton and Brian Colle
Institute for Terrestrial & Planetary Atmospheres
School of Marine and Atmospheric Sciences

During the past 20 years, the short-term (1-3 day) forecasts of tropical cyclone intensity have not significantly improved. Unlike track forecasts, which have benefited from improved large-scale atmospheric flow forecasts from computer models, intensity changes within hurricanes remain poorly understood and modeled. High-resolution simulations of the hurricane’s inner core, which are necessary for understanding intensity changes, are hampered by limits of computer power, a lack of detailed observations, and an incomplete understanding of inner core dynamics. One of the most difficult dynamical problems is the Eyewall Replacement Cycle (EWRC), which is difficult to predict because its genesis is not well understood. To gain further understanding of EWRCs, as well as general inner core dynamics, the Hurricane Rainband and Intensity Change Experiment (RAINEX) took place in 2005, which conducted numerous WP-3D weather research aircraft missions into hurricanes Katrina, Rita, and Ophelia. Of the three storms, Ophelia on 11-12 September 2005 was the weakest, never exceeding category 1 strength. However, Ophelia did have an interesting EWRC, which was investigated using a unique set of high resolution observations.

This study examines the inner vortex evolution of hurricane Ophelia. Aircraft tail and lower fuselage radar as well as flight-level temperature, wind, and moisture data were analyzed during an EWRC as the storm approached the southern North Carolina coast. Airborne Doppler radar illustrated the detailed flow and convective rainband evolution around the inner eye wall. This aircraft data, augmented with satellite and surface measurements provided a comprehensive dynamical picture of the inner core of the system.

Ophelia’s eye wall went through rapid evolution for a 24h period on 11-12 September 2005. The inner core interacted with an outer rainband located about 100 km from the center, which was the genesis of the EWRC. During this process, the central area of convection disappeared around the eye wall, while the outer rainband eventually wrapped around the entire storm, becoming a large, new eyewall located about 80 km from the center. Meanwhile, the flight-level winds and pressures showed the development of secondary wind maxima associated with a EWRC. This vortex evolution may provide clues to the dynamics of other future storms in which the inner core is replaced by an outer rainband.