Structural Changes Preceding Rapid Intensification in Tropical Cyclones in a Large Ensemble of Idealized Simulations

Strong tropical cyclones (TCs) often experience rapid intensification (RI). TCs can obtain strong intensity once they experience RI, and most of all strong hurricanes experience RI at least once during their lifetimes in the Atlantic (Kaplan and DeMaria 2003). Understanding of processes leading to RI is critically important and many studies conducted especially in the last decade (e.g., Rogers et al. 2013, 2015; Chen and Zhang 2013; Nguyen and Molinari 2015; Chen and Gopalakrishnan 2015). Miyamoto and Takemi (2013, 2015) have proposed transition mechanisms for RI. However, they only focused on cases with very weak vertical wind shear, which does not usually happen in nature. Therefore, the various mechanisms should be tested under a wider range of environments.

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.