The Pennsylvania State University-National Center for Atmospheric Research mesoscale model MM5 is used to simulate Hurricane Bonnie at high resolution (2-km spacing) in order to examine how vertical wind shear impacts the distribution of vertical motion in the eyewall on the cloud scale. As in many previous studies, we find that the shear produces a wavenumber 1 asymmetry in the time-averaged vertical motion and rainfall. Several mechanisms for this asymmetry are evaluated. The simulation results show that the upward motion portion of the eyewall asymmetry is located in the downtilt direction, consistent with the vertical motion being that required to maintain dynamic balance. Variations in the direction and magnitude of the tilt are consistent with the presence of a vortex Rossby wave quasi-mode, which is characterized by a damped precession of the upper vortex relative to the lower vortex. The vertical motion asymmetry is also qualitatively consistent with an assumed balance between horizontal vorticity advection by the relative flow and stretching of vorticity, with relative asymmetric inflow (convergence) at low levels and outflow (divergence) at upper levels on the downshear side of the eyewall.

While the time-averaged vertical motion is characterized by weak ascent in a shear–induced wavenumber 1 asymmetry, the instantaneous vertical motion is typically associated with deep updraft towers that generally form on the southern side of the eyewall and dissipate on the northern side. The updrafts are frequently found on the trailing sides of eyewall mesovortices and result from an interplay between the wavenumber 1 forcing of vertical motion and the eyewall mesovortices, which rotate cyclonically around the eyewall. Specifically, the mesovortices contribute to upward motion by producing enhanced convergence on their trailing sides where outflow associated with the mesovortex cyclonic circulation converges with low-level inflow associated with the large-scale shear.