Energetics of Hurricane Bonnie (1998)

Despite considerable work in understanding the intensity and intensity changes of tropical storms, few studies have been conducted to gain insight into the energetic characteristics of hurricanes due to the lack of high-resolution data. In this study, a 5-day cloud-resolving simulation of the structures and evolution of Hurricane Bonnie (1998) performed by Zhu et al. (2004) is examined in the context of energetics. It is found that the conversions of kinetic (KE), latent (LE) and potential energy (PE) differ during the partial eyewall and axisymmetric stages of the storm. Results show that vertical wind shear drives significant short-term changes in the storm intensity. The intensity changes are shown through energy analyses to be physical and semi-periodic, and thus potentially predictable.

For example, during the partial eyewall stage, the maximum KE, while well-representing the observed values in a mean sense, fluctuates on a timescale of about three hours. Energy conversion terms related to deep convection in the eyewall exhibit similar high frequency variations while the storm is under the influence of vertical shear. These high-frequency oscillations are closely related to individual convective elements that propagate cyclonically around the downshear left half of the eyewall. As the vertical shear subsides below a threshold, the storm abruptly exits the shear-driven regime and axisymmetrization commences. The short-term intensity fluctuations are much smaller in magnitude during this axisymmetric stage. Convective elements continue to propagate around the eyewall in a similar fashion to vortex-Rossby waves. However, in contrast to the partial eyewall stage, several convective elements exist concurrently and they propagate around the entire circumference of the storm.

Fourier decomposition is conducted to separate the wavenumber-0, -1, and -2 components. While wavenumber-1 features clearly dominate the partial eyewall stage, it is shown that wavenumber-2 wave propagation is more closely tied to individual convective elements during the axisymmetric stage. Using temporally high resolution model data, the evolution of each convective element can be traced as it moves around the eyewall. The closely coupled waves and convection propagate together around the storm with a period of about two hours.