Eyewall Cycles in Vertically Sheared and Unsheared Tropical Cyclones

Eyewall cycles typically occur in strong, symmetric storms (Kossin and Sitkowski 2009). In contrast, Hurricane Bonnie (1998) was an unusually resilient hurricane that maintained intensity while experiencing both 12–16 ms^-1 vertical wind shear and an eyewall replacement cycle. This remarkable behavior was examined using observations from flight-level data, microwave imagery, radar, and dropsondes over the two-day period. Similar to other observed eyewall replacement cycles, Bonnie exhibited the development, strengthening, and dominance of a secondary eyewall while a primary eyewall decayed. However, Bonnie’s structure was highly asymmetric due to the large vertical wind shear, in contrast to the more symmetric structures observed in other hurricanes undergoing eyewall replacement cycles. It is hypothesized that the unusual nature of Bonnie’s evolution arose due to an increase in vertical wind shear from 2 to 12 ms^-1 even as the storm intensified to a major hurricane in the presence of high ambient SSTs. These circumstances allowed for the development of strong outer rainbands downshear, where the formation of the outer eyewall commenced. In addition, the circulation considerably broadened during this time. The secondary eyewall developed within a well-defined beta skirt, consistent with earlier theory. Despite the large ambient vertical wind shear, the outer eyewall extended steadily upshear, supported by 35% larger surface wind speed upshear than downshear. The larger radius of maximum winds during and after the eyewall replacement cycle might have aided Bonnie’s resiliency directly, but also increased the likelihood that diabatic heating would fall inside the radius of maximum winds.

Hurricane Frances (2004) exhibited double and triple eyewalls in the presence of very small vertical wind shear and small asymmetry. The behavior of the two storms with differing shears will be addressed.