Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: A generalised measure of tornado risk and identification of synoptic types,Quarterly Journal of the Royal Meteorological Society

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Synoptic‐scale and mesoscale controls for tornadogenesis on cold fronts: A generalised measure of tornado risk and identification of synoptic types
Quarterly Journal of the Royal Meteorological Society ( IF 3.0 ) Pub Date : 2020-09-03 , DOI: 10.1002/qj.3898
Matthew R. Clark _{1,

2,

3} , Douglas J. Parker ₂

Affiliation

Environments of tornadic and non‐tornadic narrow cold‐frontal rain bands (NCFRs) are investigated using ERA‐Interim reanalyses for a sample of 114 events over the United Kingdom and Ireland (44 tornadic). The results offer a practical tool for prediction of the likelihood of tornadoes in these potentially high‐impact events. Of 22 analysed parameters, a bulk measure of shear vorticity, and the front‐normal wind component on the cold side of the front, yield the best discrimination between event classes, showing significantly larger values in tornadic events. A generalised measure of tornado probability, p[TN], is obtained using the distribution of points within the two‐dimensional parameter space defined by these parameters. Synoptic situations commonly associated with tornadic NCFRs are identified and conceptual models describing the large‐scale evolution are developed. Most events are associated with developing secondary cyclones (i.e., frontal waves) along trailing cold fronts (≥54.5%), generally within west to southwesterly large‐scale flow. Another significant class of event corresponds to situations where a strong mid‐ to upper‐level jet streak cuts across the front within an amplifying large‐scale flow pattern (upstream ridge building and downstream trough extension), generally within northwesterly flow (27.3%). In frontal waves, tornadoes occurred relatively early in the wave's development and just down‐front of the wave centre, where rapid increases in p[TN] occurred as the wave amplified. In northwesterly flow cases, tornadoes occurred along a well‐defined NCFR bulge close to where the mid‐ to upper‐level jet streak and an associated positive potential‐vorticity anomaly intersected the front. Analysis of a high‐tornadic subset of tornadic events (NCFRs producing ≥7 tornadoes) revealed an even stronger association with frontal waves (72.2% of cases), suggesting that the highest‐impact events are usually associated with secondary cyclogenesis. The possible relevance of identified environmental parameters to candidate vortex‐genesis and tornadogenesis mechanisms within NCFRs and quasi‐linear convective systems is discussed.

中文翻译：

冷锋龙卷风形成的天气尺度和中尺度控制：龙卷风风险的通用度量和天气类型的识别

利用ERA-Interim再分析法对英国和爱尔兰（44个龙卷风）的114个事件进行了采样，研究了龙卷风和非龙卷风的窄冷锋雨带（NCFR）的环境。结果为预测这些潜在高影响事件中龙卷风的可能性提供了一种实用工具。在22个分析参数中，大量测量剪切涡度以及在前部冷侧的前法向风分量在事件类别之间产生最佳区分，显示出较大的龙卷风事件值。龙卷风概率的广义度量p[TN]是使用由这些参数定义的二维参数空间内的点分布来获得的。确定了与气旋NCFR通常相关的天气情况，并开发了描述大规模演变的概念模型。大多数事件与沿后冷锋（≥54.5％）的次级旋风（即前波）的发展有关，通常发生在西向西南的大尺度流动内。另一类重要的事件对应于这样的情况，即在放大的大流量模式（上游山脊形成和下游波谷延伸）内，通常在西北向气流（27.3％）内，强烈的中高层喷气条纹横穿了前部。在正面波浪中，龙卷风发生在波浪发展的相对较早的时候，并且正好位于波浪中心的下方，p [TN]随着波的放大而发生。在西北向流动的情况下，龙卷风沿着明确界定的NCFR隆起发生，该隆起靠近中高层射流条纹和相关的正势涡度异常与前缘相交的地方。对一次高旋风事件（NCFR产生≥7个龙卷风）的分析表明，其与额波的关联更强（72.2％的病例），这表明影响最大的事件通常与继发性回旋作用有关。讨论了已确定的环境参数与NCFR和准线性对流系统内候选涡旋生成和旋流成因机制的可能相关性。

更新日期：2020-09-03

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