成熟发展的热带气旋通常会形成双眼墙结构。本文选取了2019年发生于西北太平洋的超强台风“利奇马”事件作为研究个例，通过WRF数值模式模拟，对“利奇马”双眼墙形成过程进行了分析，研究探讨了外眼墙形成和台风强度之间的联系。再分析数据分析表明合适的海洋热力分布条件、垂直风切变、大尺度背景环流和水汽分布是造成台风“利奇马”生成和迅速发展的必要条件，并为“利奇马”快速增强期之后的双眼墙事件提供了有利条件。随后设计了WRF数值模式试验，成功复现了“利奇马”生成期和发展期的背景环流场及其强度和路径变化，模拟了“利奇马”在登陆前的双眼墙形成过程。然后就“利奇马”双眼墙事件过程进行了重点分析。降水场、垂直速度、非绝热加热和降水粒子等物理要素场的演化过程表明，在双眼墙形成的关键时期，台风“利奇马”内眼墙外侧的不连续降水区逐渐与上游的强降水带连接，形成较为完整的次外层环状降水密集带。在非绝热加热的强迫下，台风“利奇马”的径向环流形势发生显著调整，在外眼墙处形成一支独立于内眼墙的完整的边界层入流，入流携带的水汽在外眼墙对流上升区被抬升凝结导致潜热释放，进一步加快径向环流的调整过程，同时切向风风速也在这一阶段得到迅速加强。边界层中切向风的加强使得非梯度力分布发生了显著调整，在边界层入流的共同作用下，水平辐合和抬升运动在外眼墙区域不断增强，为该区域的对流活动和外眼墙的稳定维持提供了有利的动力条件。敏感性试验表明，云微物理参数化方案对外眼墙能否建立和建立后的强度变化有着至关重要的作用。其中显式考虑降水粒子和降冰粒子分布的参数化方案均能有效地模拟出双眼墙事件以及随后的眼墙替换过程。通过影响降水和降冰粒子及其非绝热加热的空间分布，不同参数化方案直接对双眼墙事件的强度变化产生影响，表明外眼墙的形成不仅是台风动力调整的结果，同时也与云微物理过程带来的热力条件分布的调整有着显著联系。
Mature and fully developed tropical cyclones typically form a double eyewall structure. This study selects the super typhoon "Lekima" in the northwestern Pacific in 2019 as a research case to analyze its double eyewall formation process using WRF simulations. The relationship between the formation of outer eyewall and the evolution of intensity was also investigated.Based on the analysis of reanalysis data, it is found that the favorable marine thermal distribution, vertical wind shear, large-scale background circulation and water vapor distribution are the necessary conditions for the formation and rapid development of typhoon "Lekima"，and thereafter provide favorable conditions for the double eyewall formation after the rapid intensification period. A WRF numerical model simulation was designed, and the results showed that the simulation not only successfully reproduced the background flow field，intensity and path changes during the genesis and development stages of "Lekima", but also captured the formation process of "Lekima"‘s double eyewall before its landfall. Important physical factors, such as precipitation , vertical velocity, diabatic heating and precipitation particle distribution during "Lekima" double eyewall period show that the discontinuous precipitation area outside the inner eyewall of "Lekima" gradually connects with the upstream heavy precipitation zone, forming a relatively complete secondary outer ring of heavy precipitation. Under the thermal forcing of diabatic heating, the secondary circulation of "Lekima" was also significantly adjusted accordingly. A boundary layer inflow independent of the inner eyewall is formed gradually at the outer eyewall. Large amounts of moist air inflow in the boundary layer is lifted and condensed in the outer eyewall, resulting in latent heat release, further accelerating the secondary circulation. At the same time, the tangential wind speed is also rapidly strengthened at this stage, which lead to significantly adjusted agradient force. Under the combined effect of boundary layer inflow, the horizontal convergence and upward motion in the outer eyewall area are continuously strengthened, providing favorable conditions for the convection and maintenance of the outer eyewall.A series of sensitivity experiments using different microphysics schemes are conducted. It is found that cloud microphysics plays a crucial role in the establishment and strength of outer eyewall via varying distribution of cloud and precipitation particles, thereby affecting the distribution of diabatic heating. The parameterization scheme that explicitly prognoses precipitation particles and ice particles can effectively capture the double eyewall formation and the subsequent eyewall replacement. It is shown that the double eyewall process is strongly regulated by the diabatic heating distribution, which is closely related to the distribution of cloud and precipitation particles. The formation of outer eyewall is not only influenced by the dynamic adjustment, but also significantly related to the diabatic heating associated with cloud and precipitation microphysical processes.
