Secondary eyewall formation in upper- and lower-layer vertical wind shear simulated in idealized tropical cyclones

This study investigates the characteristics of secondary eyewall formation (SEF) in idealized tropical cyclones embedded in vertical wind shear (VWS) at different heights. The results show that upper-layer VWS at a relatively low shear height is more favorable for SEF than upper-layer VWS at a relatively high shear height and lower-layer VWS. In the experiments with upper-layer VWS at a relatively low shear height, better-organized stratiform clouds are located in the downwind sector of outer rainbands. The low-level descending inflow associated with the stratiform sector is stronger in these experiments than in the experiments with upper-layer VWS at a relatively high shear height and lower-layer VWS. The enhanced descending inflow can trigger supergradient winds and convergence near the top of the boundary layer, close to three times the radius of the maximum wind, where convection is locally forced. The subsequent convection axisymmetrization leads to SEF.