Advisor: Anthony Didlake

Abstract: 

Eyewall replacement cycles (ERCs) are a frequently occurring phenomenon in tropical cyclones (TCs) that can significantly affect both the structure and intensity of TCs. The physical processes that govern the evolution of different stages of an ERC are not yet fully understood. In particular, asymmetric structures within the TC inner core are complex features with an uncertain role ERC dynamics. To help fill these gaps in understanding, this study analyzes the kinematic and precipitation asymmetric structures during successive ERCs in Hurricane Ivan (2004) using airborne Doppler radar observations. The azimuthal location of these asymmetries are analyzed relative to the deep-layer (850-200 hPa) environmental wind shear vector. During the first ERC stage (secondary eyewall onset), the developing secondary eyewall region exhibited a defined convective rainband in the right-of-shear half where the strongest updrafts and reflectivities occurred. The left-of-shear half exhibited more stratiform precipitation, consistent with properties of a shear-aligned rainband complex that often precedes SEF events. During the second ERC stage (concentric eyewall period), the outer eyewall updrafts were strongest in the left-of-shear half. These updrafts were collocated with mesoscale descending inflow (MDI) and was downwind of an inward spiraling low-level convergence zone in the downshear quadrants. These observations suggest that the updrafts (and resulting vertical velocity asymmetry) were likely forced by a stratiform-induced cold pool in a similar manner to MDI impacts seen in past studies. During the final ERC stage (post-replacement adjustment), the outer eyewall (now the singular primary eyewall) experienced an upwind shift in the precipitation and vertical velocity asymmetries. The updraft maximum shifted from the downshear-left quadrant to the downshear-right quadrant, and the precipitation maximum (downwind of the updraft maximum) shifted from left-of-shear to the downshear direction. This shift corroborates previous studies, which suggest that at the end of an ERC, the asymmetric forcing mechanism that drives the eyewall updraft maximum transitions from MDI/cold pool processes to direct interaction with the environmental wind shear. Results of this study will further understanding of the changing asymmetries and underlying dynamics during ERCs.