Despite the significant impacts of heavy rainfall that sometimes accompany tropical cyclone landfall, quantitative precipitation forecasting (QPF) remains one of the most difficult problems in tropical cyclone forecasting. One of the key tasks in improving tropical cyclone QPFs is understanding the processes that affect the intensity and distribution of rainfall around the storm. These include the translational speed of the storm, topography and orientation of the coast, and interactions with the environmental flow and moisture fields. These effects can combine to produce rainfall distributions that range from nearly axisymmetric to highly asymmetric with respect to the storm center and rainfall amounts that range from minimal to catastrophic.

In this investigation, the interactions between a storm and its environmental flow field are studied by performing a numerical simulation of Hurricane Bonnie (1998) to investigate the role that vertical wind shear played in azimuthal variations of rainfall. A high-resolution (minimum grid length length of 1.67 km), auto-movable mesh, five-day simulation using the primitive-equation, nonhydrostatic MM5 mesoscale model is performed, beginning at 00 UTC 22 August 1998 and continuing until 00 UTC 27 August 1998. The highest-resolution mesh is used between 00 UTC 25 and 00 UTC 27 August. Particular emphasis is given to this time period, after the storm has reached its peak intensity, in order to study the impact that the evolving environmental flow field has on producing a tilt in the main vortex and how this tilt is related to the simulated azimuthal distribution of precipitation.