Examining the Role of Spiral Rainbands in Secondary Eyewall Formation

Several theories for secondary eyewall formation (SEF) rely on preconditioning of the axisymmetric vortex field by rainband processes that project strongly onto the azimuthal mean. But the roles and variation of these spiral rainbands that lead to SEF remain a source of uncertainty in our understanding. In this study, we explore the dynamics, structure, and impacts of TC rainbands before and during SEF using a combination of airborne radar observations and convection-permitting modeling simulations with data assimilation. In airborne radar observations of Hurricane Earl (2010), we find that the rainband complex in the downshear-left quadrant, which is predominantly stratiform precipitation, exhibits a mesoscale descending inflow that perturbs the boundary layer and is associated with an intense long-lasting updraft and low-level tangential wind maximum. This quadrant experiences the most rapid broadening of the tangential wind field, and it is also in this quadrant that the developing secondary eyewall later experiences its strongest updraft, inflow, and tangential winds. Using the Penn State WRF-EnKF modeling system, a numerical simulation of Hurricane Matthew (2016) was run and results similar to the observations are found. A mesoscale descending inflow, tangential wind field broadening, and intense updrafts also occur in the downshear-left quadrant prior to SEF. A momentum budget analysis shows that radial flux of absolute vorticity is particularly effective at building the low-level tangential winds here throughout the lifetime of the spiral rainband complex and into the stages of clear secondary eyewall development. Model sensitivity analyses are performed on this deterministic run of Matthew and the ubiquity of these results is examined.