Department of Atmospheric Sciences and International Pacific Research Center
School of Ocean and Earth Sciences
University of Hawaii at Manoa
In this presentation, the relationship between the observed rapid eyewall contraction (in terms of the radius of maximum wind–RMW) and the rapid intensification of hurricanes over the North Atlantic and Eastern North Pacific is statistically analyzed based on the best-track data. Results show that the RMW (eyewall) contracts rapidly well preceding the rapid intensification in hurricanes. To further understand the involved dynamics, we performed ensemble axisymmetric numerical simulations, which reproduced the observed feature. Further analysis indicates that because the absolute angular momentum (AAM) is not conserved following the RMW, the phenomenon could not be understood based on the AAM-based dynamics. Instead, results from the budgets of tangential wind following the RMW and the rate of change in the RMW provide dynamical insights into the simulated relationship between the intensification rate and RMW contraction rate. During the rapid RMW contraction stage, due to the weak hurricane intensity and large RMW, the negative radial gradient of radial vorticity flux and small curvature of tangential wind distribution near the RMW favor rapid RMW contraction but weak diabatic heating far inside the RMW leads to weak low-level inflow and small radial absolute vorticity flux near the RMW and thus relatively small intensification rate. As the RMW contracts rapidly together with the increase in hurricane intensity, diabatic heating inside the RMW and radial inflow near the RMW increase, leading to substantial increase in radial absolute vorticity flux near the RMW and thus the rapid hurricane intensification. However, the RMW contraction rate decreases rapidly due to the rapid increase in the curvature of tangential wind distribution near the RMW as the hurricane intensifies rapidly together with the decrease in the RMW. These findings help better understand hurricane structure and intensity changes.