Updraft maintenance and axisymmetrization during secondary eyewall formation in a model simulation of Hurricane Matthew (2016)

As a follow-on to a previous study on secondary eyewall formation (SEF) in a simulation of Hurricane Matthew (2016), this study investigates the emergence and maintenance of an asymmetric rainband updraft region that leads to the SEF event. Under moderate deep-layer environmental wind shear, the storm develops a quasi-stationary rainband complex with intense, persistent updrafts in its left-of-shear, downwind end. Using a budget of equivalent potential temperature \theta_E, it is demonstrated that the updraft maintenance is aided by a mesoscale cold pool induced by rainband stratiform cooling which intrudes into the storm’s inner core envelope of high-\theta_E air. An extended period of decreased static stability occurs through differential horizontal advection of \theta_E in the boundary layer, which continuously replenishes the moist instability that would otherwise be depleted by the updrafts. A potential vorticity (PV) budget analysis shows that these left-of-shear updrafts generate low-to-midlevel PV through diabatic processes, which is carried vertically and cyclonically downwind. The convergence of eddy PV fluxes along with PV generation in the rainband region substantially increases the azimuthal mean PV at the radius of the developing secondary eyewall; these processes are followed by the axisymmetrization of the secondary eyewall in the subsequent hour. The low-level updrafts driven by left-of-shear stratiform rainband processes are shown to be primarily responsible for the PV generation and axisymmetrization that forms the secondary eyewall.