Secondary eyewall formation (SEF) is a major mode of inner-core reorganization in tropical cyclones (TCs) under vertical wind shear (VWS), but the environmental controls on its pathways remain incompletely understood. Using a suite of idealized numerical simulations with identical VWS magnitude but varying directional wind profiles, this study examines how environmental helicity and midlevel flow orientation regulate SEF. Consistent with previous studies, positive environmental helicity suppresses SEF, whereas negative helicity favors SEF. More importantly, negative helicity does not correspond to a single SEF mechanism. Instead, SEF follows distinct pathways under different midlevel flow orientations. In experiments with a northeasterly midlevel flow, enhanced low-level winds increase surface enthalpy fluxes and favor widespread stratiform cloud development in the upshear-left quadrant. The resulting diabatic cooling induces persistent subsidence, gradually reduces reflectivity in the inner-rainband region, and produces a moat before SEF occurs. In contrast, under a northwesterly midlevel flow, stronger shear-induced radial inflow and associated dry-air intrusion produce more pronounced subsidence in the upshear-left quadrant, while supergradient forcing, low-level convergence, and ascent promote convective organization and azimuthal homogenization at larger radii. In this pathway, the moat and secondary eyewall develop nearly simultaneously through convective axisymmetrization. These results indicate that the sign of environmental helicity alone is insufficient to determine the SEF pathway. Rather, subtle differences in midlevel flow orientation govern the relative roles of stratiform-cooling-induced moat formation and dynamically forced convective organization. These findings provide a process-based framework for understanding the diversity of SEF in vertically sheared TCs.
 

tropical cyclone,vertical wind shear,secondary eyewall