Influence of Low-Level Shear Orientation and Magnitude on the Evolution and Rotation of Idealized Squall Lines. Part I: Storm Morphology and Bulk Updraft/Mesovortex Attributes Monthly Weather Review

Observational and modeling efforts have explored the formation and maintenance of mesovortices, which contribute to severe hazards in quasi-linear convective systems (QLCSs). There exists an important interplay between environmental shear and cold-pool-induced circulations which, when balanced, allow for upright QLCS updrafts with maximized lift along storm outflow boundaries. Numerical simulations have primarily tested the sensitivity of squall lines to zonally varying low-level (LL) shear profiles (i.e., purely line-normal, assuming a north–south-oriented system), but observed near-storm environments of mesovortex-producing QLCSs exhibit substantial LL hodograph curvature (i.e., line-parallel shear). Therefore, previous QLCS simulations may fail to capture the full impacts of LL shear variability on mesovortex characteristics. To this end, this study employs an ensemble of idealized QLCS simulations with systematic variations in the orientation and magnitude of the ambient LL shear vector, all while holding 0–3-km line-normal shear constant. This allows for a nuanced examination of how line-parallel shear modulates system structure, as well as mesovortex strength, size, and longevity. Results indicate that hodographs with LL curvature support squall lines with prominent bowing segments and wider, more intense rotating updrafts. Shear orientation also impacts mesovortex characteristics, with curved hodographs favoring cyclonic vortices that are stronger, wider, deeper, and longer-lived than those produced with straight-line wind profiles. These results provide a more complete physical understanding of how LL shear variability influences the generation of rotation in squall lines. Significance Statement Research related to linear storms has largely focused on vertical changes in winds (i.e., shear) oriented perpendicular to squall lines given its ability to balance storm cold pools and keep updrafts upright, thus promoting long-lived storms that presumably can go on to produce rotation. However, squall lines that produce a great deal of rotation often have a component of low-level shear oriented parallel to storms. This study gauges the sensitivity of simulated squall lines to changes in the direction and strength of shear close to the surface. We find that shear oriented parallel to linear storms creates stronger and larger updrafts that in turn support the development of intense and persistent rotation with characteristics supportive of tornadoes. These insights have impacts on both our physical understanding and prediction of the rotation and associated hazards of linear storms.

观测与模拟研究已针对中涡旋（mesovortices）的形成与维持机制开展探索，这类涡旋会在准线性对流系统（quasi-linear convective systems，QLCSs）中引发严重灾害。环境风切变与冷池诱导环流之间存在重要的相互作用，当二者达到平衡时，可形成垂直向上的准线性对流系统上升气流，并在风暴外流边界处实现最大抬升。以往数值模拟主要针对纬向变化的低层（low-level，LL）风切变廓线（即纯垂直于飑线的切变，假设系统呈南北走向）检验飑线的敏感性，但观测显示，产生中涡旋的准线性对流系统的近风暴环境存在显著的低层风矢端图（hodograph）曲率（即平行于飑线的风切变）。因此，以往的准线性对流系统模拟可能无法完整刻画低层风切变变率对中涡旋特征的影响。为此，本研究开展理想化准线性对流系统集合模拟，系统改变环境低层风切变矢量的方向与强度，同时保持0~3 km垂直于飑线的风切变恒定。这使得我们能够细致分析平行于飑线的风切变如何调控系统结构，以及中涡旋的强度、尺度与持续时间。研究结果表明，带有低层风矢端图曲率的风廓线可形成具有显著弓形段的飑线，以及更宽广、更强的旋转上升气流。风切变方向同样会影响中涡旋特征，相较于直线型风廓线产生的涡旋，带有风矢端图曲率的切变更易生成强度更强、尺度更大、深度更深且持续时间更久的气旋性涡旋（cyclonic vortices）。上述结果让我们对低层风切变变率如何影响飑线内旋转的生成机制有了更全面的物理认知。
研究意义：此前针对线性风暴的研究大多聚焦于垂直于飑线的风的垂直变化（即风切变），因为这类切变能够平衡风暴冷池，维持上升气流垂直向上，从而延长风暴生命周期，进而可能产生旋转现象。但产生强烈旋转的飑线往往存在平行于风暴的低层风切变分量。本研究针对近地面风切变的方向与强度变化，检验模拟飑线的响应敏感性。研究发现，平行于线性风暴的风切变可形成更强、更宽广的上升气流，进而助力形成具有龙卷特征的强烈且持久的旋转现象。这些研究结果既深化了我们对线性风暴旋转及相关灾害的物理认知，也对其预报工作具有参考价值。