A dynamical mechanism for secondary eyewall formation in tropical cyclones

Research output: Contribution to journalArticle

Abstract

This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

Original languageEnglish
Pages (from-to)3965-3986
Number of pages22
JournalJournal of the Atmospheric Sciences
Volume75
Issue number11
DOIs
Publication statusPublished - 2018 Nov 1

Fingerprint

tropical cyclone
vorticity
Ekman pumping
Rossby number
atmosphere
eigenvalue
shallow water
physics
wind velocity

Keywords

  • Ekman pumping
  • Ekman pumping/transport
  • Hurricanes
  • Hurricanes/typhoons
  • Hurricanes/typhoons
  • Tropical cyclones

ASJC Scopus subject areas

  • Atmospheric Science

Cite this

A dynamical mechanism for secondary eyewall formation in tropical cyclones. / Miyamoto, Yoshiaki; Nolan, David S.; Sugimoto, Norihiko.

In: Journal of the Atmospheric Sciences, Vol. 75, No. 11, 01.11.2018, p. 3965-3986.

Research output: Contribution to journalArticle

@article{2b1acad4daa342c691624cc1c60cdc98,
title = "A dynamical mechanism for secondary eyewall formation in tropical cyclones",
abstract = "This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.",
keywords = "Ekman pumping, Ekman pumping/transport, Hurricanes, Hurricanes/typhoons, Hurricanes/typhoons, Tropical cyclones",
author = "Yoshiaki Miyamoto and Nolan, {David S.} and Norihiko Sugimoto",
year = "2018",
month = "11",
day = "1",
doi = "10.1175/JAS-D-18-0042.1",
language = "English",
volume = "75",
pages = "3965--3986",
journal = "Journals of the Atmospheric Sciences",
issn = "0022-4928",
publisher = "American Meteorological Society",
number = "11",

}

TY - JOUR

T1 - A dynamical mechanism for secondary eyewall formation in tropical cyclones

AU - Miyamoto, Yoshiaki

AU - Nolan, David S.

AU - Sugimoto, Norihiko

PY - 2018/11/1

Y1 - 2018/11/1

N2 - This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

AB - This study proposes that secondary eyewall formation (SEF) of tropical cyclones (TCs) can be attributed to an instability of flow in the free atmosphere coupled with Ekman pumping. Unstable solutions of a 1.5-layer shallow-water system are obtained under fast-wind speed conditions in the free atmosphere. The instability condition derived in the linear model indicates the importance of the ratio of angular velocity to vorticity, and the condition is more likely to be satisfied when the ratio is large and its radial gradient is positive. Thus, fast angular velocity, low absolute vertical vorticity, small negative radial gradient of angular velocity, and large negative gradient of vertical vorticity are favorable. Eigenvalue analyses are performed over a wide range of parameters using a vorticity profile with an infinitesimal secondary maximum. The growth rate increases with vorticity outside the radius of maximum wind (RMW), the radius of the secondary vorticity maximum, its magnitude, and the Rossby number defined by maximum tangential velocity, the RMW, and the Coriolis parameter. Furthermore, the growth rate is positive only between 2 and 7 times the RMW, and it is negative close to or far outside the RMW. These features are consistent with previous studies on SEF. A dimensionless quantity γ obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models;γ increases several hours before a secondary peak of tangential velocity forms, suggesting that the initial process of SEF can be attributed to the proposed mechanism.

KW - Ekman pumping

KW - Ekman pumping/transport

KW - Hurricanes

KW - Hurricanes/typhoons

KW - Hurricanes/typhoons

KW - Tropical cyclones

UR - http://www.scopus.com/inward/record.url?scp=85056085586&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85056085586&partnerID=8YFLogxK

U2 - 10.1175/JAS-D-18-0042.1

DO - 10.1175/JAS-D-18-0042.1

M3 - Article

AN - SCOPUS:85056085586

VL - 75

SP - 3965

EP - 3986

JO - Journals of the Atmospheric Sciences

JF - Journals of the Atmospheric Sciences

SN - 0022-4928

IS - 11

ER -