A numerical study of the impact of vertical shear on the distribution of rainfall in Hurricane Bonnie (1998)

Despite the significant impacts of torrential rainfall from tropical cyclones at landfall, quantitative precipitation forecasting (QPF) remains an unsolved problem. A key task in improving tropical cyclone QPF is understanding the factors that affect the intensity and distribution of rainfall around the storm. These include the storm motion, topography, and orientation of the coast, and interactions with the environmental flow. The combination of these effects can produce rainfall distributions that may be nearly axisymmetric or highly asymmetric and rainfall amounts that range from 1 or 2 cm to >30 cm. This study investigates the interactions between a storm and its environmental flow through a numerical simulation of Hurricane Bonnie ( 1998) that focuses on the role of vertical wind shear in governing azimuthal variations of rainfall. The simulation uses the high-resolution nonhydrostatic fifth-generation Pennsylvania State University - NCAR Mesoscale Model (MM5) to simulate the storm between 0000 UTC 22 August and 0000 UTC 27 August 1998. During this period significant changes in the vertical shear occurred in the simulation. It changed from strong west-southwesterly, and across track, to much weaker south-southwesterly, and along track. Nearly concurrently, the azimuthal distribution of convection changed from a distinct wavenumber-1 pattern to almost azimuthally symmetric by the end of the time period. The strongest convection in the core was generally located on the downshear left side of the shear vector when the shear was strong. The azimuthal distributions and magnitudes of low-level radial inflow, reflectivity, boundary layer divergence, and low-level vertical motion all varied consistently with the evolution of the vertical shear. Additionally, the vortex showed a generally downshear tilt from the vertical. The magnitude of the tilt correlated well with changes in magnitude of the environmental shear. The accumulated rainfall was distributed symmetrically across the track of the storm when the shear was strong and across track, and it was distributed asymmetrically across the track of the storm when the shear was weak and along track.