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. 不同高度垂直风切变下热带气旋外眼墙形成特征存在明显差异.研究发现较低的高层切变比较高的高层切变和低层切变更有利于外眼墙形成.结果表明, 较低的高层切变环境下外雨带下游的层云更具组织化, 与层云相关的下沉入流更强, 有利于在边界层上方三倍最大风半径附近产生更强的超梯度风和辐合, 有利于强迫出对流以及之后的轴对称化过程最终形成外眼墙.