Environmental dynamical control of tropical cyclone intensity—An observational study

A systematic statistical analysis was conducted to investigate the effects of two external dynamical factors, namely, the transitional speed and vertical wind shear, on TC intensification, intensity, and the lifetime peak intensity in the western North Pacific during 1981-2003. In general, both the fast translation and strong vertical shear are negative to TC intensification and the lifetime peak intensity. Both the very intense TCs and the TCs with rapid intensification rate are found only to occur in a narrow range of translational speeds between 3-8 m s-1, and in relatively weak vertical shear. The overwhelming majority of western North Pacific TCs reach their lifetime peak intensity just prior to recurvature when their environmental steering flow and vertical shear are both weak. The results also show that few TCs intensified when they moved faster than 15 m s-1, especially for strong storms, or when their large-scale environmental vertical shear is larger than 20 m s-1. The intensification rate of TCs are found to increase with decreasing vertical shear while the majority of the weakening storms experience relatively strong vertical shear. Overall, strong vertical shear prohibits rapid intensification and most likely results in the weakening of TCs, much the same as the fast storm translation. Based on the observations, a new empirical maximum potential intensity (MPI) has been developed, which includes the combined negative effect of translational speed and vertical shear as the environmental dynamical control in addition to the positive contribution of SST as the thermodynamic control. The new empirical MPI can better explain the observed behavior of the TC lifetime peak intensity, providing an understanding of the thermodynamic and environmental dynamical controls of TC intensity. Implications of the new empirical MPI are discussed.