Hurricane Bonnie (1998): Maintaining Intensity During High Vertical Wind Shear and an Eyewall Replacement Cycle

Compared to hurricane track forecasts, hurricane intensity forecasts have only displayed modest improvements in the last decade. Much of the current research in this area focuses on the effects of vertical wind shear on hurricane intensity, either related to a weakening or rapidly intensifying tropical cyclone. In the latter case, previous studies have examined Hurricane Bonnie (1998) to this effect, which rapidly intensified from 33 – 51 ms^-1 over 36 hours as vertical wind shear increased from 5 – 10 ms^-1. However, none of the research addressed how Bonnie maintained a steady state of approximately 50 ms^-1 under the influence of 10 – 13 ms^-1vertical wind shear over the period of two days. The goal of the present study is to examine the evolution of Bonnie’s thermodynamic and kinematic structure in order to understand Bonnie’s resilience to vertical wind shear. In doing so, flight-level datasets obtained from NASA DC-8, U.S. Air Force, G-IV, and two NOAAP3 aircraft were utilized, which included measurements of thermodynamic and kinematic variables. Numerous dropsondes and radar data provided information on Bonnie’s vertical structure so that a 3D picture was gained of Bonnie’s evolution. Observations reveal that Bonnie underwent an eyewall replacement cycle immediately following the increase in vertical wind shear. The mechanism by which the eyewall replacement cycle occurred is explored through the distribution of equivalent potential temperature, radial and tangential wind, vorticity, and reflectivity data from a shear-relative framework. It is hypothesized that the eyewall replacement cycle may have contributed to Bonnie’s resilience in highly sheared conditions by allowing more convection to fall inside the expanded radius of maximum winds. Hurricane Bonnie’s ability to maintain intensity in spite of high shear and an eyewall replacement cycle, both of which usually result in weakening, provides a new direction from which to view the intensity change issue in hurricanes.