In this study, the evolution of Hurricane Bonnie (1998) is explicitly simulated using the latest version of the Penn State-NCAR nonhydrostatic, two-way interactive, movable, triply nested grid mesoscale model (MM5V3.4) with the finest grid length of 4 km. The model physics package used includes the Goddard Tao-Simpson (1993) explicit clouod microphysics scheme, the Blackadar PBL scheme and radiation scheme. The initial mass, moisture and wind fields of the hurricane vortex are incorporated into the model initial conditions (i.e., at 0000 UTC 22 August 1998) using the AMSU satellite sounding data. The daily SST changes are used to simulate the oceanic feedback under high wind conditions. A 5-day simulation is performed, which covers the initial rapid deepening, steady variation and landfalling stages of the storm.

Hurricane Bonnie had two distinct characters compared to other tropical storms. First, Bonnie deepened rapidly (at a rate of 0.8 hPa/hr) in the first two days after reaching hurricane Category 1 at 0000 UTC 22 August. During the next three days (i.e., 24-27 August), its intensity remained nearly unchanged over the open ocean. It weakened as its being close to landfall. Second, Bonnie had strong asymmetric structures, with a partial eyewall observed during 22-25 August.

As verified against various observations and the best analysis, the 5-day control simulation captures reasonably well the evolution and basic structures of the storm. In particular, the simulated track is within 2 degree lat/lon of the best analysis during the 5-days integration, with the landfalling point close to the observed. The model also reproduces the hurricane intensity changes during the 5-day period. The simulated radar reflectivity shows pronounced asymmetries in the eyewall and rainbands, in agreement with the observed.

The diagnostic analyses are conducted to study the hurricane’s partial eyewall features and the processes that influence the hurricane intensity change. The environmental vertical shear appears to play an important role in the formation of the hurricane asymmetric structures. The specification of the daily SST appears to be one of the reasons why the storm remains a steady intensity for a long period of time. More results will be presented during the conference.