Possible Environmental Influence on Eyewall Expansion During the Rapid Intensification of Hurricane Helene (2006)

Nannan Qin,Liguang Wu

doi:10.3389/feart.2021.715012

Abstract

Tropical cyclone (TC) rapid intensification (RI) is usually accompanied by a rapid eyewall contraction, followed by a slow contraction, and then a nearly steady eyewall. However, this study shows that Hurricane Helene (2006) exhibited an eyewall expansion during its 30-h rapid intensification period. The possible environmental influence on the eyewall expansion during the RI of Helene is examined. It is found that the synoptic-scale circulations led to additional low-level inflows and upper-level outflows that may play an important role in the eyewall expansion during the RI of Helene. Examination of the divergence of the absolute angular momentum flux (AAMF) associated with the environmental circulation suggests that the synoptic-scale atmospheric circulation played an important role in the eyewall expansion during the RI of Helene. In the lower and middle troposphere, the synoptic-scale cross-equatorial flow, which was enhanced by the Helene-induced wave train, led to the horizontal convergence of absolute angular momentum flux, while the TC-trough interaction and the related outflow in the upper troposphere resulted in the divergence of AAMF. The environment-induced low-level convergence and upper-level divergence of AAMF were superimposed on the secondary circulation of Helene and may be important to the eyewall expansion during the RI by accelerating the tangential wind outside of the eyewall. This study suggests that RI can occur with an eyewall expansion.

Highlights

The improvement of tropical cyclone (TC) intensity forecast is lagging far behind the improvement of the TC track forecast (Kaplan and DeMaria 2003; Rogers et al, 2006; Rogers et al, 2013; Huang et al, 2021), especially when TCs undergo rapid intensification (RI), which is defined by Kaplan and DeMaria (2003) as an intensification rate of not less than 15.4 m s−1 (24 h) −1 in the maximum surface wind (VMAX)
Based on the method proposed by Kurihara et al (1993, 1995), the TC vortex is removed from the Climate Forecast System Reanalysis (CFSR) data to exclude the influence of TC circulation since we focus on the environmental factors and the TC circulation is inaccurately represented in the coarse data
This study reveals that the synoptic-scale atmospheric circulation was likely responsible for the eyewall expansion during the RI of Helene through the environmental influences on the angular momentum (AAM) transport

Summary

INTRODUCTION

The improvement of tropical cyclone (TC) intensity forecast is lagging far behind the improvement of the TC track forecast (Kaplan and DeMaria 2003; Rogers et al, 2006; Rogers et al, 2013; Huang et al, 2021), especially when TCs undergo rapid intensification (RI), which is defined by Kaplan and DeMaria (2003) as an intensification rate of not less than 15.4 m s−1 (24 h) −1 in the maximum surface wind (VMAX). The storm movement is removed from the wind field to avoid the FIGURE 6 | Time-radius cross-section of the azimuthal-mean integrated divergence of the AAM flux (AAMFD, shaded, 105 m3 s−1 h−1) over low levels (averaged from 1,000 hPa to 900 hPa) during the RI period of Helene (A) total AAMFD (B) AAMFD associated with the low-frequency circulation, and (C) AAMFD associated with the synoptic-scale circulation. The synoptic-scale anticyclonic circulation is important to the middle-level convergence of the AAMF, which determines the intensification of the middle-level wind of Helene, especially outside the eyewall since the AAM is materially conserved (Shapiro and Willoughby 1982; Montgomery and Smith 2011). The upper-level outflow was enhanced with the maximum outflow core located around the radius of 500 km, which favors TC intensification especially outside of the eyewall

SUMMARY

Findings

DATA AVAILABILITY STATEMENT