Observational Evidence of Tropical Cyclone Intensification Driven by Vortex Stretching

Spin-up associated with vortex stretching is fundamental to tropical cyclone (TC) intensification; however, observational quantification of this process is lacking. Simple scaling analysis relates the magnitude of vortex stretching to a measure of surplus precipitation (S), which is defined as the inner core precipitation rate of a TC of intensity V minus the mean precipitation rate for TCs of the same intensity at neutral intensification rate. Our composite analysis of 17 years' TC observations reveals that 24-hour TC intensity change has a remarkably linear relation with intensity-weighted surplus precipitation (VS), indicating that vortex stretching predominantly determines TC intensification rate. We show that VS is an excellent predictor of TC rapid intensification (RI) and combining VS with environmental wind shear would increase the predictive skill for RI forecast by more than 40% in terms of the Peirce Skill Score (PSS) than using the shear alone. This study suggests that satellite measurements indicative of inner-core convective updraft strength may significantly improve the skills of operational TC intensity forecast.