Synoptic-scale and mesoscale controls for tornadogenesis on cold fronts: Shear-zone vortex-genesis in a developing frontal wave
Synoptic-scale and mesoscale controls for tornadogenesis on cold fronts: Shear-zone vortex-genesis in a developing frontal wave
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冷锋龙卷风发生的天气尺度和中尺度控制：锋面波发展中的剪切带涡旋发生
DOI：
10.1002/qj.4164
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发表时间：
2021
影响因子：
8.9
通讯作者：
Clark M
中科院分区：
文献类型：
--
作者：
Clark M
高分辨率模型模拟和雷达观测用于研究龙卷风窄冷锋雨带（NCFR）中涡旋的发生。涡旋产生的时间和位置受到正在发展的锋面波的强烈限制，锋面波于 2011 年 10 月 17 日向东北方向穿过英国和爱尔兰。在模拟中，涡旋优先发生在波发展的早期阶段和波中心的正下方，其中垂直涡度的大幅增加与跨锋面汇合的减少同时发生。由于额叶断裂的发生，随着额波的成熟，涡旋的产生停止。记录了两种不同尺度的涡旋发生：中观-γ尺度上的初级涡旋发生和味噌尺度上的次生涡旋发生。我们表明，水平剪切不稳定性是最有可能的涡旋生成机制，这与之前关于存在水平拉伸变形的垂直涡带稳定性的理论工作一致。次级涡流沿着主涡流之间的辫状区域发生，其中剪切带变得特别狭窄和强烈。在模型中，这些涡旋发展得极其迅速（在 5-15 分钟内从小的扰动到最大垂直涡度），最强的涡旋表现出接近 10−1s−1 的近地表垂直涡度最大值。两种尺度的涡旋都与 NCFR 中的特征性局部扰动相关，通过与雷达反射率数据的比较，我们表明，主涡旋和次涡旋可能存在于真实的 NCFR 中。龙卷风报告与小的 NCFR 扰动相关，例如与模型模拟中的次级涡流相关的扰动。对单个模拟涡旋子结构的分析表明，龙卷风最有可能发生在次级涡旋北侧或西北侧延伸的强烈近地表垂直涡旋区域内。
High‐resolution model simulations and radar observations are used to investigate the onset of vortex‐genesis in a tornadic narrow cold‐frontal rain band (NCFR). The timing and location of vortex‐genesis was strongly constrained by a developing frontal wave, which tracked northeast across the United Kingdom and Ireland on 17 October 2011. In the simulations, vortices occurred preferentially during the early stages of wave development and just down‐front of the wave centre, where large increases in vertical vorticity occurred in concert with decreases in the cross‐frontal confluence. Vortex‐genesis ceased as the frontal wave matured, due to the onset of frontal fracture. Two distinct scales of vortex‐genesis are documented: primary vortex‐genesis on the meso‐γ‐scale, and secondary vortex‐genesis on the miso‐scale. We show that horizontal shearing instability is the most likely vortex‐genesis mechanism, consistent with previous theoretical work on the stability of vertical vortex strips in the presence of horizontal stretching deformation. Secondary vortices occurred along the braid regions between primary vortices where the shear zone became particularly narrow and intense. In the model, these vortices developed extremely rapidly (from small perturbations to maximum vertical vorticity in 5–15 min) and the strongest exhibited near‐surface vertical vorticity maxima approaching 10−1s−1. Vortices of both scales were associated with characteristic local perturbations in the NCFR and we show, by comparison with radar reflectivity data, that primary and secondary vortices were likely present in the real NCFR. Tornado reports were associated with small NCFR perturbations like those associated with the secondary vortices in the model simulations. Analysis of the sub‐structure of individual simulated vortices suggests that tornado‐genesis is most likely within a region of intense near‐surface vertical vorticity stretching at the north or northwest flank of the secondary vortices.