Roles of Upper-Level Descending Inflow in Moat Development in Simulated Tropical Cyclones with Secondary Eyewall Formation

Abstract

This study investigated the effects of upper-level descending inflow (ULDI) associated with inner-eyewall convection on the formation of the moat in tropical cyclones (TCs) with secondary eyewall formation (SEF). In our numerical experiments, a clear moat with SEF occurred in TCs with a significant ULDI, while no SEF occurred in TCs without a significant ULDI. The eyewall convection developed more vigorously in the control run. A ULDI occurred outside the inner-eyewall convection, where it was symmetrically unstable. The ULDI was initially triggered by the diabatic warming released by the inner eyewall and later enhanced by the cooling below the anvil cloud. The ULDI penetrated the outer edge of the inner eyewall with relatively dry air and prevented excessive solid-phase hydrometeors from being advected further outward. It produced extensive sublimation cooling of falling hydrometeors between the eyewall and the outer convection. The sublimation cooling resulted in negative buoyancy and further induced strong subsidence between the eyewall and the outer convection. As a result, a clear moat was generated. Development of the moat in the ongoing SEF prevented the outer rainband from moving farther inward, helping the outer rainband to symmetrize into an outer eyewall. In the sensitivity experiment, no significant ULDI formed since the eyewall convection was weaker, and the eyewall anvil developed relatively lower, meaning the formation of a moat and thus an outer eyewall was less likely. This study suggests that a better-represented simulation of inner-eyewall convective structures and distribution of the solid-phase hydrometeors is important to the prediction of SEF.

摘要

本文研究了与内眼墙对流相联系的高层下沉入流(ULDI)对热带气旋(TC)外眼墙生成过程中弱回波区(Moat)形成的影响机理. 数值试验结果指出, 双眼墙和Moat通常出现在有显著ULDI的TC中, 而在没有显著ULDI的TC中没有出现双眼墙结构. 在控制试验中, 内眼墙对流发展更加旺盛, 内眼墙非绝热加热会在内眼墙高层云毡下方的对称不稳定区域激发出一支ULDI, ULDI生成后由于受到云毡下方水凝物相变过程中冷却作用的影响而增强. ULDI对Moat区形成的重要作用具体表现在: ULDI一方面使干空气入侵到内外眼墙之间的区域; 另一方面在一定程度上阻止了高层出流对水凝物的径向向外平流. 这使得更多的水凝物下落到内外眼墙之间的区域, 由于受到水凝物升华、 融化和蒸发冷却造成的负浮力的影响, 该区域的空气粒子下沉运动增强, 导致Moat区形成. Moat区的发展通过阻止外围雨带向内眼墙的收缩和合并过程, 有助于外围雨带在Moat区外侧发展成为对称的外眼墙. 在敏感性试验中, 由于TC眼墙对流较弱, 云毡发展相对较低, 没有出现明显的ULDI, 因此不太可能形成Moat区和外眼墙结构. 研究表明, 更好地再现眼墙对流结构和水凝物分布对双眼墙预报具有重要意义.