Numerical Simulation of Hurricane Bonnie (1998). Part I: Eyewall Evolution and Intensity Changes

Abstract

In this study, a 5-day explicit simulation of Hurricane Bonnie (1998) is performed using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5) with the finest grid length of 4 km. The initial mass, wind, and moisture fields of the hurricane vortex are retrieved from the Advanced Microwave Sounding Unit-A (AMSU-A) satellite measurements, and the sea surface temperature (SST) is updated daily. It is shown that the simulated track is within 3° latitude?longitude of the best track at the end of the 5-day integration, but with the landfalling point close to the observed. The model also reproduces reasonably well the hurricane intensity and intensity changes, asymmetries in cloud and precipitation, as well as the vertical structures of dynamic and thermodynamic fields in the eye and eyewall. It is shown that the storm deepens markedly in the first 2 days, during which period its environmental vertical shear increases substantially. It is found that this deepening could occur because of the dominant energy supply by a strong low-level southeasterly flow into the eastern eyewall plus the presence of underlying warm SST and favorable upper-level divergent outflow. However, the approaching of a strong upper-level northwesterly flow tends to generate mass convergence and subsidence warming and drying, thereby suppressing the development of deep convection in the western semicircle. This gives rise to wavenumber-1 asymmetries in clouds and precipitation (i.e., a partial eyewall) and the eastward tilt of the eyewall and storm center. Both the observed and simulated storms also appear to exhibit eyewall replacement scenarios in which the storms weaken as double eyewalls appear, and then reintensify as their inner eyewalls diminish and concentric eyewalls develop. The results indicate that the eyewall replacement process may be predictable because it appears to depend on the large-scale flow.