Disentangling the Influences of Storm-Relative Flow and Horizontal Streamwise Vorticity on Low-Level Mesocyclones in Supercells
Disentangling the Influences of Storm-Relative Flow and Horizontal Streamwise Vorticity on Low-Level Mesocyclones in Supercells
复制标题
解开风暴相对流和水平流向涡度对超级单体中低层中气旋的影响
DOI：
10.1175/jas-d-22-0114.1
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发表时间：
2023
影响因子：
3.1
通讯作者：
Allen, John T.
中科院分区：
文献类型：
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作者：
Peters, John M.;Coffer, Brice E.;Parker, Matthew D.;Nowotarski, Christopher J.;Mulholland, Jake P.;Nixon, Cameron J.;Allen, John T.
足够的低层风暴相对流是维持超级单体雷暴的必要因素，并与超级单体上升气流宽度有关。假设超级单体存在，低层风暴相对气流在调节超级单体低层中气旋强度方面的作用还不太清楚。本文考虑的一种可能性是，风暴相对流通过对总体上升气流范围的调节来控制中气旋和龙卷风的宽度。这一假设依赖于先前假定的上升气流宽度、中气旋宽度和龙卷风宽度之间的正对应关系。另一种假设是，中气旋的特征主要是由水平流向涡度调节风暴相对流量无关。超级单体模拟矩阵进行了分析，以解决上述假设，其中水平流向涡量和风暴相对流量独立变化。在这些模拟中，中气旋的宽度和强度与水平流向涡度有很强的相关性，与风暴相对流的相关性相对较弱，支持第二个假设。伴随的理论和轨迹分析提供了物理解释，当风暴相对流量大，上升气流宽，垂直倾斜流向涡预计在一个更广泛的区域，但具有较小的平均幅度时，这些参数都很小。这些因素部分地相互抵消，降低了风暴相对流与上升气流环流和旋转速度的对应关系，而上升气流环流和旋转速度是与龙卷风关系最密切的中气旋属性。这些结果反驳了先前声称的上升气流宽度，中气旋宽度和龙卷风宽度之间的联系，并强调水平流向涡度作为持续supercells.Significance StatementThe强度的低层中气旋超级单体雷暴的低层旋转，被称为“中气旋”，被认为是影响龙卷风的可能性。中气旋强度取决于许多环境属性，这些属性往往相互关联，难以区分。这项研究使用了大量的数值模拟来研究低空空气进入超级单体的速度（风暴相对流），进入雷暴的环境空气的水平旋转（流向涡度）和风暴上升气流的宽度的影响。我们的研究结果表明，在超级单体中的中气旋的旋转主要是由流向涡度的影响，与风暴相对流和上升气流宽度的连接相对较弱。这些发现为我们科学地了解风暴环境如何影响其中气旋的旋转速度以及相关的龙卷风威胁提供了重要的澄清。
Sufficient low-level storm-relative flow is a necessary ingredient for sustained supercell thunderstorms and is connected to supercell updraft width. Assuming a supercell exists, the role of low-level storm-relative flow in regulating supercells’ low-level mesocyclone intensity is less clear. One possibility considered in this article is that storm-relative flow controls mesocyclone and tornado width via its modulation of overall updraft extent. This hypothesis relies on a previously postulated positive correspondence between updraft width, mesocyclone width, and tornado width. An alternative hypothesis is that mesocyclone characteristics are primarily regulated by horizontal streamwise vorticity irrespective of storm-relative flow. A matrix of supercell simulations was analyzed to address the aforementioned hypotheses, wherein horizontal streamwise vorticity and storm-relative flow were independently varied. Among these simulations, mesocyclone width and intensity were strongly correlated with horizontal streamwise vorticity, and comparatively weakly correlated with storm-relative flow, supporting the second hypothesis. Accompanying theory and trajectory analysis offers the physical explanation that, when storm-relative flow is large and updrafts are wide, vertically tilted streamwise vorticity is projected over a wider area but with a lesser average magnitude than when these parameters are small. These factors partially offset one another, degrading the correspondence of storm-relative flow with updraft circulation and rotational velocity, which are the mesocyclone attributes most closely tied to tornadoes. These results refute the previously purported connections between updraft width, mesocyclone width, and tornado width, and emphasize horizontal streamwise vorticity as the primary control on low-level mesocyclones in sustained supercells.Significance StatementThe intensity of a supercell thunderstorm’s low-level rotation, known as the “mesocyclone,” is thought to influence tornado likelihood. Mesocyclone intensity depends on many environmental attributes that are often correlated with one another and difficult to disentangle. This study used a large body of numerical simulations to investigate the influence of the speed of low-level air entering a supercell (storm-relative flow), the horizontal spin of the ambient air entering the thunderstorm (streamwise vorticity), and the width of the storm’s updraft. Our results suggest that the rotation of the mesocyclone in supercells is primarily influenced by streamwise vorticity, with comparatively weaker connections to storm-relative flow and updraft width. These findings provide important clarification in our scientific understanding of how a storm’s environment influences the rate of rotation of its mesocyclone, and the associated tornado threat.