Mechanism of Concentric Eyewall Replacement Cycles and Associated Intensity Change*

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 SawyerEliassen 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.

Section: A Secondary Eyewall Formation In the Rams Integrationsupporting

confidence: 78%

Departures from Axisymmetric Balance Dynamics during Secondary Eyewall Formation

Abarca

Montgomery

2014

“…It is difficult to distinguish eyewall formation, maturation, and decay with the temporal resolution of the RAINEX observations. However, this analysis suggests that the main reason for the decay of the inner eyewall is a reduction in the inflow to the primary eyewall as opposed to the direct effects of forced subsidence (Rozoff et al 2008;Zhou and Wang 2011). Deep subsidence is observed in the moat where the downward branches of the independent secondary circulations coincide, but the radar analysis suggests that forced subsidence from the secondary eyewall is not strong enough to significantly suppress deep convection at the primary eyewall.…”

Section: Discussionmentioning

confidence: 85%

“…This contrasts with the life cycle of Hurricane Isabel (2003), which displayed an expanding eyewall that stayed at or near category 5 intensity for three consecutive days (Bell and Montgomery 2008) in a low vertical shear, low ocean heat content environment (Mainelli et al 2008). The reasons for these different evolutions are not currently well understood but may include differences in environmental influences (Nong and Emanuel 2003) or microphysical processes (Zhou and Wang 2011;Zhou et al 2011). …”

Section: Discussionmentioning

confidence: 99%

An Axisymmetric View of Concentric Eyewall Evolution in Hurricane Rita (2005)

Bell

Montgomery

Lee

2012

Multiplatform observations of Hurricane Rita (2005) were collected as part of the Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign during a concentric eyewall stage of the storm's life cycle that occurred during 21-22 September. Satellite, aircraft, dropwindsonde, and Doppler radar data are used here to examine the symmetric evolution of the hurricane as it underwent eyewall replacement.During the approximately 1-day observation period, developing convection associated with the secondary eyewall became more symmetric and contracted inward. Latent heating in the emergent secondary eyewall led to the development of a distinct toroidal (overturning) circulation with inertially constrained radial inflow above the boundary layer and compensating subsidence in the moat region, properties that are consistent broadly with the balanced vortex response to an imposed ring of diabatic heating outside the primary eyewall. The primary eyewall's convection became more asymmetric during the observation period, but the primary eyewall was still the dominant swirling wind and vorticity structure throughout the period.The observed structure and evolution of Rita's secondary eyewall suggest that spinup of the tangential winds occurred both within and above the boundary layer, and that both balanced and unbalanced dynamical processes played an important role. Although Rita's core intensity decreased during the observation period, the observations indicate a 125% increase in areal extent of hurricane-force winds and a 19% increase in integrated kinetic energy resulting from the eyewall replacement.

“…It turns out that the SEF is also found at a similar time and location in the sensitive experiment. Therefore, the ice process may not be critical in the case of Sinlaku (2008), though it is in the other case (Zhou and Wang 2011;Fang and Zhang 2012). …”

Section: ) Burst and Maintenance Of Deep Convection And Sefmentioning

confidence: 99%

The Governing Dynamics of the Secondary Eyewall Formation of Typhoon Sinlaku (2008)

Sun

Jiang

Tan

et al. 2013

Through successful convection-permitting simulations of Typhoon Sinlaku (2008) using a high-resolution nonhydrostatic model, this study examines the role of peripheral convection in the storm's secondary eyewall formation (SEF) and its eyewall replacement cycle (ERC). The study demonstrates that before SEF the simulated storm intensifies via an expansion of the tangential winds and an increase in the boundary layer inflow, which are accompanied by peripheral convective cells outside the primary eyewall. These convective cells, which initially formed in the outer rainbands under favorable environmental conditions and move in an inward spiral, play a crucial role in the formation of the secondary eyewall. It is hypothesized that SEF and ERC ultimately arise from the convective heating released from the inward-moving rainbands, the balanced response in the transverse circulation, and the unbalanced dynamics in the atmospheric boundary layer, along with the positive feedback between these processes.