Inner Core Structure and Intensity Change in Hurricane Isabel (2003)

Hurricane Isabel developed into a major hurricane in the central Atlantic from 7-11 September 2003. Isabel was estimated to reach maximum intensity on September 11th and maintained major hurricane status, with a notable large eye and eyewall (inner core), from 12-14 September. During this time, Isabel weakened slightly. An eyewall replacement occurred over the period, although the new inner eye, which consolidated late on September 13th or on September 14th, never contracted to the size of the previous inner eyewall on September 12th. Intensity fluctuations from the eyewall replacement were not particularly dramatic. In addition, tangential wind variability and horizontal size of the inner core increased while the vertical depth of convection decreased. This study aims to understand how inner core convection, vorticity distribution, and eyewall replacement affected the structure and intensity of Isabel (2003).

Aircraft observations provide an excellent description of Isabel's inner core during three missions from 12-14 September. Data from Doppler radar, Global Positioning System (GPS) dropwindsondes, flight level sensors, and the Stepped-Frequency Microwave Radiometer (SFMR) are used to describe the kinematic and thermodynamic structures of Isabel in this study. A high-resolution model (MM5) simulation of Isabel is used to describe the evolution of the storm. Airborne data is used to validate the model results.

From September 12th-14th, the distribution of relative vorticity in Isabel's inner core changed, as the ratio of eye to eyewall vorticity increased while overall maximum vorticity values decreased. Tangential wind asymmetry at the radius of maximum wind (RMW) increased, while overall tangential wind maxima at the RMW decreased slightly as the RMW increased. In addition, the theta-e gradient between the eye and eyewall decreased over the period. The broadening of high wind region beyond the RMW while reduction at the RMW from September 12th-14th is related to secondary eyewall development. Inertial stability (IS) has a maximum right inside the RMW on the 12th. It becomes lower near and inside the RMW on the 13th and 14th. As inner core IS decreased, radial inflow, azimuthally averaged updrafts within the RMW, and the intensity and depth of eyewall convection also weakened from September 12-14th. These changes in the secondary circulation again resulted in gradual weakening. The demise of the high vorticity annulus on September 12th and the eyewall replacement cycle beginning that same day lead to a gradual weakening of Isabel. It is found that the secondary wind maxima correspond to a peak in the IS profiles in both observation and model simulations. IS profiles may be an invaluable tool for diagnosing the formation and development of secondary eyewalls.