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 HurricaneMatthew (2016), this study investigates the emergence and maintenance of an asymmetric rainband updraft region that leads to an 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 maintenance of the left-of-shear updraft is aided by a mesoscale cold pool induced by rainband stratiform cooling which interacts with the storm's moist envelope of high-theta(E) air. An extended period of destabilization 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. The initial lifting of the updraft is found to be the result of buoyancy advection resulting from the density contrast between the surface cold pool and the inner-core high-theta(E) air. A potential vorticity (PV) budget analysis shows that these left-of-shear updrafts generate low- to midlevel PV through diabatic heating and boundary layer processes, which shapes the local PV enhancement and propagates cyclonically downwind. Meanwhile, in the mid- to upper levels, eddy PV flux convergence and PV generation continue to occur in the stratiform precipitation extending downwind into the upshear quadrants, which substantially increases the azimuthal mean PV at the radius of the developing secondary eyewall and marks the occurrence of the axisymmetrization process.

Automated summary

This study investigates the emergence and maintenance of an asymmetric rainband updraft region that leads to secondary eyewall formation. The results show that the maintenance of the left-of-shear updraft is aided by a mesoscale cold pool induced by rainband stratiform cooling and an extended period of destabilization occurs through differential horizontal advection of equivalent potential temperature. Buoyancy advection resulting from the density contrast between the surface cold pool and the inner-core high-theta(E) air leads to the initial lifting of the updraft. Potential vorticity budget analysis reveals that these updrafts generate low- to midlevel PV through diabatic heating and boundary layer processes.