Intensification Rates of Tropical Cyclone–Like Vortices in a Model with Downtilt Diabatic Forcing and Oceanic Surface Drag
Intensification Rates of Tropical Cyclone–Like Vortices in a Model with Downtilt Diabatic Forcing and Oceanic Surface Drag
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DOI：
10.1175/jas-d-22-0188.1
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发表时间：
2023-07
影响因子：
3.1
通讯作者：
D. Schecter
中科院分区：
文献类型：
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作者：
D. Schecter
通常观察到热带气旋在成为全强度飓风之前有明显的垂直偏差。热带气旋的垂直错位（倾斜）通常与内核对流的明显不对称性有关，最强的对流往往集中在表面涡旋中心的下倾处。倾斜热带气旋增强的机制以及这种机制运作的时间尺度都尚未被完全了解。本研究通过在 3D 非静水力数值模型中检查倾斜热带气旋类涡旋对下倾非绝热强迫（加热）的响应，提供了对不对称强化过程的一些见解。调节加热的幅度以改变其产生的下倾对流的强度。在不同的模拟组中发现了相当一致的强化图，这些模拟组的初始涡流配置、环境剪切流和下倾加热的具体位置不同。强化机制通常取决于在下倾热源附近产生的低水平收敛σb是否超过临界值，该临界值取决于随热源漂移的参考系中低水平非发散背景流的局部速度。超临界 σb 导致由下倾核心更换引发的快速旋转。亚临界 σb 导致强化过程变慢。如本文所测量的，超临界强化率大约与σb成比例。亚临界强化率具有更微妙的缩放比例，并且当 σb 降至摩擦旋转下降占主导地位的阈值以下时，预计会变为负值。讨论了上述结果与现实世界热带气旋的相关性。
Tropical cyclones are commonly observed to have appreciable vertical misalignments prior to becoming full-strength hurricanes. The vertical misalignment (tilt) of a tropical cyclone is generally coupled to a pronounced asymmetry of inner-core convection, with the strongest convection tending to concentrate downtilt of the surface vortex center. Neither the mechanisms by which tilted tropical cyclones intensify nor the time scales over which such mechanisms operate are fully understood. The present study offers some insight into the asymmetric intensification process by examining the responses of tilted tropical cyclone–like vortices to downtilt diabatic forcing (heating) in a 3D nonhydrostatic numerical model. The magnitude of the heating is adjusted so as to vary the strength of the downtilt convection that it generates. A fairly consistent picture of intensification is found in various simulation groups that differ in their initial vortex configurations, environmental shear flows, and specific positionings of downtilt heating. The intensification mechanism generally depends on whether the low-level convergence σb produced in the vicinity of the downtilt heat source exceeds a critical value dependent on the local velocity of the low-level nondivergent background flow in a reference frame that drifts with the heat source. Supercritical σb causes fast spinup initiated by downtilt core replacement. Subcritical σb causes a slower intensification process. As measured herein, the supercritical intensification rate is approximately proportional to σb. The subcritical intensification rate has a more subtle scaling, and expectedly becomes negative when σb drops below a threshold for frictional spindown to dominate. The relevance of the foregoing results to real-world tropical cyclones is discussed.