We performed an ensemble simulation with 270 members using a full physics model by changing the intensity and size of the initial vortex, the magnitude of vertical wind shear, and the translation speed. The processes leading to RI in a representative case with moderate shear are consistent with previous studies for weakly sheared cases. The most distinct changes in TC structure are the vortex tilt and the vortex size that both begin to decrease rapidly 6 h before the onset of RI. A vorticity budget analysis for the upper layer around the low-level center reveals that the vertical vorticity is increased by vertical advection, stretching, and tilting terms before RI, whereas the horizontal advection is small. Thus, the upright vortex structure is not achieved through a vortex alignment process, but rather is built upward by deep convection.
The ensemble members with RI show processes before the onset of RI are consistent with the control case and many previous studies: the intensity gradually increases, the radius of maximum tangential velocity (RMW) at 2 km decreases, the flow structure becomes more symmetric, the vortex tilt decreases, and the radius of maximum convergence approaches the radius of maximum winds. We develop a dimensionless parameter representing a tendency for the formation of the vertically upright structure. The product of the developed parameter with the local Rossby number is significantly larger for TCs that exhibit RI in the next 24 hours. The product is simplified for the purpose of practical usage and the simplified version still shows a significant difference between RI and non-RI cases.