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ABSTRACT
Multiple secondary eyewall formations (SEFs) and eyewall replacement cycles (ERCs) are simulated with the fifth-generation Pennsylvania State University (PSU)-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) at horizontal grid spacing of 0.67 km. The simulated hurricane is initialized from a weak, synthetic vortex in a quiescent environment on an f plane. After spinup and rapid intensification, the hurricane enters a mature phase during which the intensity change is relatively slow. Convective clouds then organize into a ring with a secondary tangential wind maximum at radii beyond the hurricane's primary eyewall. This secondary eyewall (SE) then contracts and strengthens. The primary eyewall weakens and is eventually replaced by the SE. The hurricane grows in size and the radius of maximum wind (RMW) increases as similar ERCs repeat 5 times during the simulation.
Two existing hypotheses on SEF are evaluated using the simulation output. Then, model diagnostics are used to reveal that crucial linked components of SEF are (i) a broadening of the swirling flow, (ii) the structure of the evolving secondary circulation, and (iii) the structure of the net radial force (NRF) in the boundary layer (with largest contributions from the agradient and frictional forces). During SEF, there exists strong positive NRF in the region of the primary eyewall, a secondary positive maximum over the SEF region, and a minimum between the two. As a response of the boundary layer depth-integrated radial flow to the NRF, a secondary maximum convergence zone (SMCZ) in the boundary layer develops at the SEF radii. Eventually moist convection in the SMCZ becomes active as the SEF develops.
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1. Introduction
Secondary eyewall formation (SEF) and eyewall replacement cycles (ERCs) are important phenomena in hurricane inner-core dynamics. SEF and ERCs cause rapid structure and intensity changes in mature hurricanes, which make the study of SEF and ERCs crucial to understanding and forecasting the intensity, structure, and impacts of hurricanes (Willoughby 2007; Sitkowski et al. 2011). The ERC phenomena were observed several decades ago (Fortner 1958; Hoose and Colón 1970). The detailed characteristics of SEF and ERCs are described in Willoughby et al. (1982, hereafter WCS82), Willoughby (1990), Black and Willoughby (1992), and Samsury and Zipser (1995). More recently, passive microwave imagery (Hawkins et al....