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Abstract 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 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 θ 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- θ E air. An extended period of destabilization occurs through differential horizontal advection of θ 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- θ 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. 

Abstract The dynamics of an asymmetric rainband complex leading into secondary eyewall formation (SEF) are examined in a simulation of Hurricane Matthew (2016), with particular focus on the tangential wind field evolution. Prior to SEF, the storm experiences an axisymmetric broadening of the tangential wind field as a stationary rainband complex in the downshear quadrants intensifies. The axisymmetric acceleration pattern that causes this broadening is an inward-descending structure of positive acceleration nearly 100 km wide in radial extent and maximizes in the low levels near 50 km radius. Vertical advection from convective updrafts in the downshear-right quadrant largely contributes to the low-level acceleration maximum, while the broader inward-descending pattern is due to horizontal advection within stratiform precipitation in the downshear-left quadrant. This broad slantwise pattern of positive acceleration is due to a mesoscale descending inflow (MDI) that is driven by midlevel cooling within the stratiform regions and draws absolute angular momentum inward. The MDI is further revealed by examining the irrotational component of the radial velocity, which shows the MDI extending downwind into the upshear-left quadrant. Here, the MDI connects with the boundary layer, where new convective updrafts are triggered along its inner edge; these new upshear-left updrafts are found to be important to the subsequent axisymmetrization of the low-level tangential wind maximum within the incipient secondary eyewall. 

Abstract Using idealized simulations, we examine the storm-scale wind field response of a dry, hurricane-like vortex to prescribed stratiform heating profiles that mimic tropical cyclone (TC) spiral rainbands. These profiles were stationary with respect to the storm center to represent the diabatic forcing imposed by a quasi-stationary rainband complex. The first profile was typical of stratiform precipitation with heating above and cooling below the melting level. The vortex response included a mesoscale descending inflow and a midlevel tangential jet, consistent with previous studies. An additional response was an inward-spiraling low-level updraft radially inside the rainband heating. The second profile was a modified stratiform heating structure derived from observations and consisted of a diagonal dipole of heating and cooling. The same features were found with stronger magnitudes and larger vertical extents. The dynamics and implications of the forced low-level updraft were examined. This updraft was driven by buoyancy advection because of the stratiform-induced low-level cold pool. The stationary nature of the rainband diabatic forcing played an important role in modulating the required temperature and pressure anomalies to sustain this updraft. Simulations with moisture and full microphysics confirmed that this low-level updraft response was robust and capable of triggering sustained deep convection that could further impact the storm evolution, including having a potential role in secondary eyewall formation.