Abstract

The prediction of rapid intensification (RI) in tropical cyclones (TCs) is a challenging problem. In this study, the RI process and factors contributing to it are compared for two TCs: an axis-symmetric case (Hurricane Irma, 2017) and an asymmetric case (Hurricane Michael, 2018). Both Irma and Michael became major hurricanes that made significant impacts in the United States. The Hurricane Weather Research and Forecasting (HWRF) Model was used to examine the connection between RI with forcing from the large-scale environment and the subsequent evolution of TC structure and convection. The observed large-scale environment was reasonably reproduced by HWRF forecasts. Hurricane Irma rapidly intensified in an environment with weak-to-moderate vertical wind shear (VWS), typically favorable for RI, leading to the symmetric development of vortical convective clouds in the cyclonic, vorticity-rich environment. Conversely, Hurricane Michael rapidly intensified in an environment of strong VWS, typically unfavorable for RI, leading to major asymmetries in the development of vortical convective clouds. The tangential wind momentum budget was analyzed for these two hurricanes to identify similarities and differences in the pathways to RI. Results suggest that eddy transport terms associated with convective processes positively contributed to vortex spin up in the early stages of RI and inhibited spin up in the later stages of RI in both TCs. In the early stages of RI, the mean transport terms exhibited notable differences in these TCs; they dominated the spin-up process in Irma and were of secondary importance to the spin-up process in Michael. Favorable aspects of the environment surrounding Michael appeared to aid in the RI process despite hostile VWS.

Highlights

  • A tropical cyclone (TC) is one of the most dangerous forms of natural disaster to impact society and coastal communities annually

  • Simulated track and intensity produced by Hurricane Weather Research and Forecasting (HWRF) for both TCs are compared with the National Hurricane Center (NHC) best track [34]

  • Forecasts of Irma and Michael from the HWRF model were analyzed to study how differences in both inner core structure and the large-scale environment were linked to rapid intensification (RI)

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Summary

Introduction

A tropical cyclone (TC) is one of the most dangerous forms of natural disaster to impact society and coastal communities annually. A great number of aircraft observations in the inner core were used to verify the forecast of Earl, an asymmetric TC that rapidly intensified in a hostile environment They concluded that the triggering mechanism for RI was the development of an upper-level warm core over the TC surface center. Despite hostile VWS, the key factor behind Gabrielle’s RI was an intense convective cell This intense convective cell developed in the downshear left quadrant of the TC and moved cyclonically inward to a radius of 17 km within the radius of maximum winds. In the case of Earl, despite persistent environmental shear and a lack of symmetric convection, a positive eddy vorticity flux in the middle to upper troposphere created by mesoscale convective complexes had a profound influence in accelerating the TC spin-up process.

Case Descriptions
Tangential Wind Momentum Budget Terms
Model Verification
Tangential Wind Momentum Budgets
Discussions and Conclusions
Full Text
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