This study examines axisymmetric and asymmetric aspects of secondary eyewall formation (SEF) in tropical cyclones (TCs) by applying a nonlinear boundary layer model to tangential wind composites of observed TCs with and without SEF. SEF storms were further analyzed at times prior to and after SEF, as defined by the emergence of a secondary maximum in axisymmetric tangential wind. The model is used to investigate the steady-state boundary layer response to the free-tropospheric pressure forcing derived from observed tangential wind fields. The axisymmetric response to the Post-SEF wind field displayed a secondary updraft maximum associated with a mature secondary eyewall; the model correctly produced no secondary updraft for non-SEF storms. The Pre-SEF response also exhibited a secondary updraft associated with an incipient secondary eyewall largely due to the broadened outer tangential wind field that commonly precedes SEF events. The asymmetric wind fields and model response were analyzed relative to the 850–200 hPa environmental wind shear vector. In Pre-SEF storms, the tangential wind field displayed a broadened tangential wind structure in the downshear quadrants. The boundary layer response shows a downwind shift toward the left-of-shear quadrants, exhibiting the clearest secondary maxima in updrafts, tangential wind, and radial inflow. This left-of-shear response was the leading contributor to the secondary eyewall signals in the Pre-SEF axisymmetric response. Sensitivity analyses confirmed the robustness of these asymmetric signals. These findings suggest that enhanced tangential wind and boundary layer updrafts in the left-of-shear sectors may be early indicators and critical features of SEF in sheared TCs.

中文翻译：

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使用基于观测的热带气旋边界层建模研究二次眼墙地层的轴对称和非对称信号

本研究通过将非线性边界层模型应用于观测到的有和没有 SEF 的 TC 的切向风复合材料，研究热带气旋 (TC) 中次级眼墙形成 (SEF) 的轴对称和不对称方面。SEF 风暴在 SEF 之前和之后进行了进一步分析，定义为轴对称切向风中出现次要最大值。该模型用于研究稳态边界层对来自观察到的切向风场的自由对流层压力强迫的响应。对后 SEF 风场的轴对称响应显示出与成熟的次级眼墙相关的次级上升气流最大值；该模型正确地为非 SEF 风暴没有产生二次上升气流。Pre-SEF 响应还表现出与初期次生眼壁相关的二次上升气流，这主要是由于通常在 SEF 事件之前扩大的外切向风场。相对于 850-200 hPa 环境风切变矢量分析了不对称风场和模型响应。在 Pre-SEF 风暴中，切向风场在下切象限中显示出加宽的切向风结构。边界层响应显示顺风向剪切象限左侧移动，在上升气流、切向风和径向流入中表现出最清晰的次生最大值。这种左切变响应是 Pre-SEF 轴对称响应中次生眼壁信号的主要贡献者。灵敏度分析证实了这些不对称信号的稳健性。