The Stationary Banding Complex and Secondary Eyewall Formation in Tropical Cyclones

Abstract This study uses idealized numerical simulations to show that the interaction between tropical cyclones and a midlatitude jet can result in secondary eyewall formation. It is argued that the eddy activity by the outflow–jet interaction can enhance the upper-level outflow, thereby creating an asymmetric stratiform region outside of the primary eyewall. Numerous long-lasting deep convective cells are able to form in the stratiform cloud, creating forcing necessary for the secondary eyewall. The low-level inflow and the TC’s primary circulation advect the deep convective cells inward and cyclonically. The secondary eyewall forms after the deep convection has surrounded the TC. In contrast, numerical simulations without the jet do not show secondary eyewall formation. For moderately strong jets of wind speed 15–30 m s−1, there is little sensitivity to the jet strength. There is sensitivity to the distance between the jet and the TC, with secondary eyewall formation evident when their separation is 15° latitude but not when the separation exceeds 20°.

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A Dynamical Mechanism for Secondary Eyewall Formation in Tropical Cyclones

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0042.1 ◽

2018 ◽

Vol 75 (11) ◽

pp. 3965-3986

Author(s):

Yoshiaki Miyamoto ◽

David S. Nolan ◽

Norihiko Sugimoto

Keyword(s):

Growth Rate ◽

Angular Velocity ◽

Tropical Cyclones ◽

Tangential Velocity ◽

Free Atmosphere ◽

Vertical Vorticity ◽

Radial Gradient ◽

Secondary Eyewall Formation ◽

Wide Range ◽

Fast Wind

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 obtained from the unstable condition in the linear theory is applied to SEF events simulated by two different full-physics numerical models; 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.

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Roles of the Inner Eyewall Structure in the Secondary Eyewall Formation of Simulated Tropical Cyclones

10.5194/acp-2021-147 ◽

2021 ◽

Author(s):

Nannan Qin ◽

Liguang Wu ◽

Qingyuan Liu

Keyword(s):

Tropical Cyclones ◽

Numerical Experiments ◽

Large Scale ◽

Forecasting Model ◽

Diagnostic Analysis ◽

Grid Spacing ◽

Secondary Eyewall Formation ◽

Spiral Rainband ◽

Upper Level

Abstract. It has been suggested that the inner eyewall structure may play an important role in the secondary eyewall formation (SEF) of tropical cyclones (TCs). This study is to further examine the role of the inner eyewall structure by comparing two numerical experiments, which were conducted with the same large-scale environment and initial and boundary conditions but different grid sizes. The SEF was simulated in the experiment with the finer grid spacing, but not in the other.Comparing the eyewall structure in the simulated TCs with and without the SEF indicates that the eyewall structure can play an important role in the SEF. For the simulated TC with the SEF, the eyewall is more upright with stronger updrafts, accompanied by a wide eyewall anvil at a higher altitude. Compared to the simulated TC without the SEF, diagnostic analysis reveals that the cooling outside the inner eyewall is induced by the sublimation, melting and evaporation of hydrometeors falling from the eyewall anvil. The cooling also induces upper-level dry, cool inflow below the anvil, prompting the subsidence and moat formation between the inner eyewall and the spiral rainband. In the simulated TC without the SEF, the cooling induced by the falling hydrometeors is significantly reduced and offset by the diabatic warming. There is no upper-level dry inflow below the anvil and no moat formation between the inner eyewall and the spiral rainband. This study suggests that a realistic simulation of the intense eyewall convection is important to the prediction of the SEF in the numerical forecasting model.

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Dynamics and predictability of secondary eyewall formation in sheared tropical cyclones

Journal of Advances in Modeling Earth Systems ◽

10.1002/2016ms000729 ◽

2017 ◽

Vol 9 (1) ◽

pp. 89-112 ◽

Cited By ~ 20

Author(s):

Fuqing Zhang ◽

Dandan Tao ◽

Y. Qiang Sun ◽

Jeffrey D. Kepert

Keyword(s):

Tropical Cyclones ◽

Secondary Eyewall Formation

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Outer Rainbands–Driven Secondary Eyewall Formation of Tropical Cyclones

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0304.1 ◽

2020 ◽

Vol 77 (6) ◽

pp. 2217-2236

Author(s):

Yi-Fan Wang ◽

Zhe-Min Tan

Keyword(s):

Boundary Layer ◽

Tropical Cyclones ◽

Positive Feedback ◽

Outer Core ◽

Relative Vorticity ◽

Wind Maximum ◽

Formation Pathway ◽

Physical Conditions ◽

Secondary Eyewall Formation ◽

Tangential Wind

Abstract Secondary eyewall formation (SEF) could be considered as the aggregation of a convective-ring coupling with a tangential wind maximum outside the primary eyewall of a tropical cyclone (TC). The dynamics of SEF are investigated using idealized simulations based on a set of triplet experiments, whose differences are only in the initial outer-core wind speed. The triplet experiments indicate that the unbalanced boundary layer (BL) process driven by outer rainbands (ORBs) is essential for the canonical SEF. The developments of a secondary tangential wind maximum and a secondary convective ring are governed by two different pathways, which are well coupled in the canonical SEF. Compared with inner/suppressed rainbands, the downwind stratiform sectors of ORBs drive significant stronger BL convergence at its radially inward side, which fastens up the SEF region and links the two pathways. In the wind-maximum formation pathway, the positive feedback among the BL convergence, supergradient force, and relative vorticity within the BL dominates the spinup of a secondary tangential wind maximum. In the convective-ring formation pathway, the BL convergence contributes to the ascending motion through the frictional-forced updraft and accelerated outflow associated with the supergradient force above the BL. Driven only by inner rainbands, the simulated vortex develops a fake SEF with only the secondary convective ring since the rainband-driven BL convergence is less enhanced and thus fails to maintain the BL positive feedback in the wind-maximum pathway. Therefore, only ORBs can promote the canonical SEF. It also infers that any environmental/physical conditions favorable for the development of ORBs will ultimately contribute to SEF.

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Analyzing Tropical Cyclone Structures during Secondary Eyewall Formation Using Aircraft In Situ Observations

Monthly Weather Review ◽

10.1175/mwr-d-18-0197.1 ◽

2018 ◽

Vol 146 (12) ◽

pp. 3977-3993 ◽

Cited By ~ 1

Author(s):

Katharine E. D. Wunsch ◽

Anthony C. Didlake

Keyword(s):

Angular Momentum ◽

Tropical Cyclones ◽

Composite Analysis ◽

Convective Structures ◽

Asymmetric Structures ◽

Secondary Eyewall Formation ◽

Tangential Wind ◽

Flight Level ◽

In Situ Observations

Abstract The dynamical mechanisms for secondary eyewall formation (SEF) in tropical cyclones (TCs) are not yet fully understood. Most hypotheses for SEF rely on the early presence of persistent and widespread rainband convection outside of the primary eyewall. This convection eventually coalesces into a secondary eyewall through both axisymmetric and asymmetric processes, but the extent and importance of these dynamical processes and their associated convective structures remain unclear. This study examines the evolution of axisymmetric and asymmetric structures in a composite analysis of Atlantic TCs from 1999 to 2015 using aircraft reconnaissance observations from the Extended Flight-Level Dataset for Tropical Cyclones (FLIGHT+). Compared to intensifying TCs that did not experience SEF, TCs undergoing SEF showed axisymmetric broadening of the outer wind field in the tangential wind and angular momentum profiles before SEF. Thermodynamic observations indicated features consistent with strengthening eyewall convection. We also analyzed TCs in shear-relative quadrants to examine the evolution of asymmetric kinematic and thermodynamic structures during SEF. Utilizing a new normalization technique based on the radii of both eyewalls, we isolated the structures surrounding the secondary eyewall before and during SEF. Using this technique, we found that kinematic structures of the developing secondary eyewall were most prominent in the left-of-shear half, and the thermodynamic structures of the secondary eyewall became more axisymmetric during SEF. Asymmetries developed in the primary eyewall thermodynamics as it decayed. Understanding the evolution of these observed structures characteristic to SEF will improve our ability to predict SEF and the resulting changes in TC intensity and structure.

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Effect of Beta Shear on Simulated Tropical Cyclones

Monthly Weather Review ◽

10.1175/mwr-d-10-05021.1 ◽

2012 ◽

Vol 140 (10) ◽

pp. 3327-3346 ◽

Cited By ~ 43

Author(s):

Juan Fang ◽

Fuqing Zhang

Keyword(s):

Tropical Cyclones ◽

Potential Temperature ◽

Coriolis Parameter ◽

Second Stage ◽

Secondary Eyewall Formation ◽

Stratiform Region ◽

Equivalent Potential ◽

Third Stage ◽

Stratiform Precipitation ◽

Active Convection

Abstract Through cloud-resolving simulations, this study examines the effect of β on the evolution of tropical cyclones (TCs). It is found that the TC simulated on a β plane with variable Coriolis parameter f is weaker in intensity but larger in size and strength than the TC simulated on an f plane with constant f. Such differences result mainly from the effect of the β shear rather than from the variation of f due to the latitudinal change of the TC position, as illustrated in a three-stage conceptual model developed herein. The first stage begins with the establishment of the β shear and the emergence of asymmetries as the TC intensifies. The β shear peaks in value during the second stage that subsequently leads to the formation of an extensive stratiform region outside of the primary eyewall. The evaporative cooling associated with the stratiform precipitation acts to sharpen the low-level equivalent potential temperature gradient into a frontlike zone outside of the eyewall region, which leads to the burst of convection outside of the primary eyewall. The third stage is characterized by a weakening β shear and the corresponding TC vortex axisymmetrization and expansion. The convection on the inner edge of the stratiform region becomes more organized in the azimuthal direction and eventually causes the TC structure to evolve in a manner similar to the secondary eyewall formation and eyewall replacement usually observed in TCs. It is the active convection outside of the primary eyewall that contributes to a relatively weaker but larger TC on the β plane than that on the f plane.

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Secondary eyewall formation in tropical cyclones

Dynamics and Predictability of Large-Scale, High-Impact Weather and Climate Events ◽

10.1017/cbo9781107775541.014 ◽

2016 ◽

pp. 168-175 ◽

Cited By ~ 5

Author(s):

Chun-Chieh Wu ◽

Yi-Hsuan Huang ◽

Zhemin Tan

Keyword(s):

Tropical Cyclones ◽

Secondary Eyewall Formation

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Evolution of the Moat Associated with the Secondary Eyewall Formation in a Simulated Tropical Cyclone

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0375.1 ◽

2021 ◽

Author(s):

Nannan Qin ◽

Liguang Wu ◽

Qingyuan Liu

Keyword(s):

Tropical Cyclones ◽

Vertical Wind Shear ◽

Outer Edge ◽

Vertical Shear ◽

Subsequent Evolution ◽

Secondary Eyewall Formation ◽

Spiral Rainband ◽

Spiral Rainbands ◽

Upper Level ◽

Negative Buoyancy

AbstractPrevious studies have focused on the formation and maintenance of spiral rainbands in the secondary eyewall formation (SEF) of tropical cyclones (TCs). However, the evolution of the moat, a region with weak precipitation separating spiral rainbands from the inner eyewall, is also essential for the SEF. In this study, a semi-idealized numerical experiment is conducted to understand the SEF by focusing on the evolution of the moat. In the simulated TC, a secondary eyewall forms around 32 h, and then intensifies and replaces the inner eyewall at 46 h.It is found that the occurrence and subsequent evolution of the moat in the simulated TC are closely associated with the inner-eyewall structure. As the eyewall updraft becomes strong and the eyewall anvil is well developed, the upper-level inflow develops below the eyewall anvil in response to the diabatic warming in the eyewall anvil. The warming-induced inflow causes a drying effect and promotes the sublimation cooling below the anvil, inducing strong subsidence between the inner eyewall and the spiral rainband through the resulting negative buoyancy. Moreover, the resulting subsidence is enhanced by the compensated downward motion in the outer edge of the inner eyewall. Further analysis indicates that the rapidly decreasing vertical shear of environmental wind and the rapid filamentation zone outside the inner eyewall also play important role in the axisymmetrization of the rainband and the moat subsidence. Our results demonstrate that an intense inner eyewall with a wide upper-level anvil is favorable for the SEF in an environment with decreasing vertical wind shear.

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