Secondary Eyewall Formation and Eyewall Replacement Cycles in a Simulated Hurricane: Effect of the Net Radial Force in the Hurricane Boundary Layer

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.

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

“…Moreover, other than Qiu et al (2010), it is found that there is no clear link between outer rainbands and the VRWs in our simulation, which means the environment, even far from the TC center, can be important for SEF. Our study also complements recent studies of Rozoff et al (2012), Huang et al (2012), Qiu and Tan (2013), and Wang et al (2013) in understanding the interplay and positive feedback between balanced and unbalanced dynamic processes in SEF. It is believed 1) that the development of the outer rainbands and the unbalanced boundary layer processes characterized by supergradient winds and convergence should be taken as a precursor of SEF and as a basis for predicting the SEF and 2) that the development of the outer rainbands and the coupled boundary layer processes should be taken into full consideration.…”

Passive Microwave Quantification of Tropical Cyclone Inner-Core Cloud Populations Relative to Subsequent Intensity Change

“…DeMaria et al (2014) note that for many time scales intensity change predictions robustly improved in recent years; however, gains were typically weaker and not robust at #48 h. Across time scales, events occurring within the tails of the TC intensity change distribution inherently have reduced predictability. While rapid weakening episodes not associated with landfall are becoming increasingly well understood because of processes associated with secondary eyewall development (e.g., Houze et al 2007;Rozoff et al 2008;Wang et al 2013) or environmental influences (e.g., Wood and Ritchie. 2015), rapid intensification (RI) episodes continue to lack causal consensus.…”

Departures from Axisymmetric Balance Dynamics during Secondary Eyewall Formation

“…Sun et al (2013) proposed that SEF arises from a feedback between inward-moving rainbands, the balanced response to their source of heat, and unbalanced dynamics. Wang et al (2013) proposed that a ''net'' radial force in the boundary layer is relevant in the process and Qiu and Tan (2013) emphasized the role of the asymmetric boundary layer. Kepert (2013) presented theoretical arguments, based mostly on linearized Ekman theory, to propose a feedback mechanism for secondary eyewall formation that involves a local enhancement of the radial vorticity gradient, frictional updraft, and convection.…”