Essential Dynamics of Secondary Eyewall Formation

Abstract The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field. The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary layer model is forced with the aforementioned mesoscale model's radial profile of pressure at the top of the boundary layer, it generates a secondary tangential wind maximum consistent with that from the full-physics, mesoscale simulation. These findings demonstrate that the boundary layer dynamics alone are capable of developing secondary wind maxima without prescribed secondary heat sources and/or invocation of special inertial stability properties of the swirling flow either within or above the boundary layer. Finally, the time-dependent slab model reveals that the simulated secondary wind maximum contracts inward, as secondary eyewalls do in mesoscale models and in nature, pointing to a hitherto unrecognized role of unbalanced dynamics in the eyewall replacement cycle.

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Departures from Axisymmetric Balance Dynamics during Secondary Eyewall Formation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0018.1 ◽

2014 ◽

Vol 71 (10) ◽

pp. 3723-3738 ◽

Cited By ~ 29

Author(s):

Sergio F. Abarca ◽

Michael T. Montgomery

Keyword(s):

Boundary Layer ◽

Tropical Cyclone ◽

Three Dimensional ◽

Balance Model ◽

Wind Maximum ◽

Secondary Eyewall Formation ◽

Tangential Wind ◽

Eyewall Replacement Cycle ◽

Tangential Momentum ◽

Balance Dynamics

Abstract Departures from axisymmetric balance dynamics are quantified during a case of secondary eyewall formation. The case occurred in a three-dimensional mesoscale convection-permitting numerical simulation of a tropical cyclone, integrated from an initial weak mesoscale vortex in an idealized quiescent environment. The simulation exhibits a canonical eyewall replacement cycle. Departures from balance dynamics are quantified by comparing the azimuthally averaged secondary circulation and corresponding tangential wind tendencies of the mesoscale integration with those diagnosed as the axisymmetric balanced response of a vortex subject to diabatic and tangential momentum forcing. Balance dynamics is defined here, following the tropical cyclone literature, as those processes that maintain a vortex in axisymmetric thermal wind balance. The dynamical and thermodynamical fields needed to characterize the background vortex for the Sawyer–Eliassen inversion are obtained by azimuthally averaging the relevant quantities in the mesoscale integration and by computing their corresponding balanced fields. Substantial differences between azimuthal averages and their homologous balance-derived fields are found in the boundary layer. These differences illustrate the inappropriateness of the balance assumption in this region of the vortex (where the secondary eyewall tangential wind maximum emerges). Although the balance model does broadly capture the sense of the forced transverse (overturning) circulation, the balance model is shown to significantly underestimate the inflow in the boundary layer. This difference translates to unexpected qualitative differences in the tangential wind tendency. The main finding is that balance dynamics does not capture the tangential wind spinup during the simulated secondary eyewall formation event.

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The Unconventional Eyewall Replacement Cycle of Hurricane Ophelia (2005)

Monthly Weather Review ◽

10.1175/mwr-d-20-0181.1 ◽

2021 ◽

Author(s):

Muhammad Naufal Razin ◽

Michael M. Bell

Keyword(s):

Intensity Change ◽

Wind Maximum ◽

Low Level ◽

Tangential Wind ◽

Eyewall Replacement Cycle ◽

Flight Level ◽

Intensive Observation ◽

Observation Period ◽

Replacement Cycle

AbstractHurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a Category 1 storm located over cold sea surface temperatures near 23°C. The ERC was analyzed using airborne radar, flight-level, and dropsonde data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) intensive observation period on 11 September 2005. Results showed that the spin-up of the secondary tangential wind maximum during the ERC can be attributed to the efficient convergence of absolute angular momentum by the mid-level inflow of Ophelia’s dominantly stratiform rainbands. This secondary tangential wind maximum strongly contributed to the azimuthal mean tangential wind field, which is conducive for increased low-level supergradient winds and corresponding outflow. The low-level supergradient forcing enhanced convergence to form a secondary eyewall. Ophelia provides a unique example of an ERC occurring in a weaker storm with predominantly stratiform rainbands, suggesting an important role of stratiform precipitation processes in the development of secondary eyewalls.

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Are Eyewall Replacement Cycles Governed Largely by Axisymmetric Balance Dynamics?

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0151.1 ◽

2015 ◽

Vol 72 (1) ◽

pp. 82-87 ◽

Cited By ~ 15

Author(s):

Sergio F. Abarca ◽

Michael T. Montgomery

Keyword(s):

Boundary Layer ◽

Recent Work ◽

Balance Model ◽

Tangential Wind ◽

Eyewall Replacement Cycle ◽

Physics Simulation ◽

Physics Model ◽

Balance Dynamics ◽

Eyewall Replacement Cycles ◽

Replacement Cycle

Abstract The authors question the widely held view that radial contraction of a secondary eyewall during an eyewall replacement cycle is well understood and governed largely by the classical theory of axisymmetric balance dynamics. The investigation is based on a comparison of the secondary circulation and derived tangential wind tendency between a full-physics simulation and the Sawyer–Eliassen balance model. The comparison is made at a time when the full-physics model exhibits radial contraction of the secondary eyewall during a canonical eyewall replacement cycle. It is shown that the Sawyer–Eliassen model is unable to capture the phenomenology of secondary eyewall radial contraction because it predicts a net spindown of the boundary layer tangential winds and does not represent the boundary layer spinup mechanism that has been articulated in recent work.

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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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Dynamics of the Transition from Spiral Rainbands to a Secondary Eyewall in Hurricane Earl (2010)

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-17-0348.1 ◽

2018 ◽

Vol 75 (9) ◽

pp. 2909-2929 ◽

Cited By ~ 15

Author(s):

Anthony C. Didlake ◽

Paul D. Reasor ◽

Robert F. Rogers ◽

Wen-Chau Lee

Keyword(s):

Boundary Layer ◽

Inner Core ◽

Doppler Radar ◽

Buoyant Jet ◽

Wind Maximum ◽

Low Level ◽

Secondary Eyewall Formation ◽

Tangential Wind ◽

Negatively Buoyant ◽

Left Quadrant

Abstract Airborne Doppler radar captured the inner core of Hurricane Earl during the early stages of secondary eyewall formation (SEF), providing needed insight into the SEF dynamics. An organized rainband complex outside of the primary eyewall transitioned into an axisymmetric secondary eyewall containing a low-level tangential wind maximum. During this transition, the downshear-left quadrant of the storm exhibited several notable features. A mesoscale descending inflow (MDI) jet persistently occurred across broad stretches of stratiform precipitation in a pattern similar to previous studies. This negatively buoyant jet traveled radially inward and descended into the boundary layer. Farther inward, enhanced low-level inflow and intense updrafts appeared. The updraft adjacent to the MDI was likely triggered by a region of convergence and upward acceleration (induced by the negatively buoyant MDI) entering the high-θe boundary layer. This updraft and the MDI in the downshear-left quadrant accelerated the tangential winds in a radial range where the axisymmetric wind maximum of the secondary eyewall soon developed. This same quadrant eventually exhibited the strongest overturning circulation and wind maximum of the forming secondary eyewall. Given these features occurring in succession in the downshear-left quadrant, we hypothesize that the MDI plays a significant dynamical role in SEF. The MDI within a mature rainband complex persistently perturbs the boundary layer, which locally forces enhanced convection and tangential winds. These perturbations provide steady low-level forcing that projects strongly onto the axisymmetric field, and forges the way for secondary eyewall development via one of several SEF theories that invoke axisymmetric dynamical processes.

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