Investigating Axisymmetric and Asymmetric Signals of Secondary Eyewall Formation Using Observations‐Based Modeling of the Tropical Cyclone Boundary Layer
Investigating Axisymmetric and Asymmetric Signals of Secondary Eyewall Formation Using Observations‐Based Modeling of the Tropical Cyclone Boundary Layer
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DOI：
10.1029/2020jd034027
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发表时间：
2021-07
期刊：
影响因子：
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通讯作者：
Chau-Lam Yu;A. Didlake;J. Kepert;Fuqing Zhang
中科院分区：
文献类型：
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作者：
Chau-Lam Yu;A. Didlake;J. Kepert;Fuqing Zhang
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，研究了热带气旋 (TC) 中次级眼墙形成 (SEF) 的轴对称和不对称方面。在 SEF 之前和之后，有时对 SEF 风暴进行了进一步分析，定义为轴对称切向风中次要极大值的出现。该模型用于研究稳态边界层对来自观测到的切向风场的自由对流层压力强迫的响应。对 SEF 后风场的轴对称响应显示出与成熟的次生眼墙相关的次生上升气流最大值；该模型正确地没有产生非 SEF 风暴的二次上升气流。海啸前响应还表现出与初期次级眼壁相关的次级上升气流，这主要是由于通常在海啸事件发生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。在海风暴前的风暴中，切向风场在下切变象限中显示出扩大的切向风结构。边界层响应显示顺风向剪切力左侧象限移动，在上升气流、切向风和径向流入中表现出最清晰的次要极大值。这种左剪切响应是 Pre-SEF 轴对称响应中次级眼壁信号的主要贡献者。敏感性分析证实了这些不对称信号的稳健性。这些发现表明，左切变扇区增强的切向风和边界层上升气流可能是切变热带气旋中SEF的早期指标和关键特征。
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.