ANALYSIS OF THE FORMATION OF MINI-SUPERCELLS AND ASSOCIATED TORNADOGENESIS WITHIN A MESOSCALE VORTEX EMBEDDED IN A MEIYU FRONT
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摘要: 对一次梅雨锋中尺度涡旋上发展的系列龙卷过程的形势背景、近风暴环境、龙卷演变过程和形成机理进行了研究。在高空强辐散区、中低层西南风急流的强风速辐合背景下，北侧干空气卷入后静止锋上形成了具有弓状结构的“逗点”状中尺度对流系统(MCS)，MCS北部γ中尺度涡旋明显发展，产生多个龙卷。相较于S波段多普勒天气雷达，高分辨率(30 m距离库、30秒体扫描)相控阵雷达探测到上海崇明陈家镇龙卷及其母体“微型”超级单体的精细结构和形成演变过程，及涡旋速度对、反射率空洞、高谱宽等典型标记；正、负径向速度对的结构和演变揭示涡旋源于近地面，发展后其高度最高达到2.0 km；龙卷接地前2~3分钟，收缩拉伸的龙卷涡旋又呈现出从上向下接地的过程。分析表明：极低的抬升凝结高度和极强的低层垂直风切变为龙卷形成提供了环境条件，龙卷发生前低层垂直风切变显著加强；在中尺度涡旋东部降水回波后缘，西南干急流与东南湿急流汇集激发形成“微型”超级单体，其指状回波快速向单体主体靠近、收缩逐渐演变成钩状回波，形成具有空洞特征的龙卷涡旋区。该龙卷形成的可能机制是：指状回波包围区内上升运动与外围下沉运动分别作用于由低层垂直风切变形成的水平涡管上，使水平涡管倾斜向上逐渐转为垂直涡管，在上升运动的作用下不断拉伸加强，涡度迅速加大形成龙卷。针对该次过程，建立了中尺度涡旋上“大涡套小涡”的系列龙卷分布概念模型和“微型”超级单体上龙卷涡旋形成的概念模型。Abstract: The synoptic situation, near storm environment, and tornado evolution process and formation mechanism of a series of tornadoes within a mesoscale vortex embedded in a Meiyu front are studied. In the aloft strong divergence zone and the strong wind speed convergence of low-level southwesterly jets, a mesoscale vortex on the stationary front developed after the dry air on the north side was involved, forming a comma-like mesoscale convective system (MCS) with a bow echo. A meso-γ-scale vortex in the MCS's northern part developed, producing several tornadoes. Compared with the S-band Doppler weather radar, the high-resolution (30m range bin, 30-second volume scan) phased array radar at Chenjiazhen, Chongming District, Shanghai, detected the fine structure and evolution of the Chenjiazhen tornado and its parent mini-supercell which was much lower and smaller than classic supercells. Typical characteristics can be found, such as radial velocity pairs, reflectivity holes, high spectral width, etc. The positive and negative radial velocity pair structure and evolution process revealed that the vortex originated near the ground, and then its height reached a height of up to 2.0 km. Only 2~3 minutes before the tornado touched down, the contracted and stretched tornado vortex dropped rapidly from as high as 1.5~2.0 km to near surface. The analysis shows that the extremely low lifted condensation level (LCL) and the extremely strong low-level wind shear provided suitable conditions for the formation of tornadoes. Before the tornado formed, the low-level wind vertical shear was significantly strengthened at the rear edge of the precipitation echo in the eastern part of the meso - γ - scale vortex. The southwestern dry flows and the southeast wet flows converged to form a mini-supercell. The finger-shaped echo quickly approached the parent storm, shrank, and gradually evolved into a hook-shaped echo, forming a tornado vortex area with a hole echo area. It is hypothesized that the ascending motion and the peripheral sinking motion in the encircled area by the hook echo acted on the horizontal vortex formed by the low-altitude vertical wind shear, respectively. As a result, the horizontal vortex tube tilted upward and gradually turned into a vertical vortex, which was continuously pulled under the action of the ascending motion. Finally, the vorticity rapidly increased to form a tornado near Chenjiazhen. A conceptual model of the distribution of tornadoes in the mesoscale vortex and a conceptual model of tornado vortex formation in mini-supercells are established.
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Key words:
- tornado /
- mesoscale vortex /
- mini-supercell /
- dual-polarization phased-array weather radar /
- Meiyu front
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图 1 2020年7月6日上海崇明港沿镇和陈家镇龙卷路径、龙卷目击视频(崇明港沿镇石材店庄志强拍摄)和灾情等
a. 港沿镇龙卷路径；b. 漏斗云视频抓屏(来自网络，已在现场与发布者确认)；c~d. 港沿镇龙卷灾情；e~f. 陈家镇龙卷路径和灾情。
图 3 2020年7月6日08:00天气形势综合分析图(a，包括500 hPa等高线和槽线，850 hPa填图，200 hPa、500 hPa、700 hPa和850 hPa的急流(分别为紫色、蓝色、褐色和红色线)，700 hPa干线(黄色加短标曲线)、地面锋面等)和09:36华东雷达拼图(b)
图 4 2020年7月6日08:00宝山探空分析图(a)和采用陈家镇相控阵子阵雷达垂直风反演产品(VWP)修订后的宝山探空分析图(b)；陈家镇子阵雷达获取的10:33—10:53的VWP水平风垂直分布时间序列图(c)
图 5 青浦雷达2020年7月6日08:41(a, i)、09:15(b, j)、09:44(c, k)、10:01(d, l)、10:12(e, m)、10:23(f, n)、10:40(g，o)及10:51 (h, p)的0.5 °仰角的反射率因子(a~h)和径向速度(i~p)
其中09:44、10:23和10:40反射率因子图上“T1”、“T2”、“T3”和“T4”标记分别对应江苏太仓浏河中燕村及上海崇明港沿镇、陈家镇和向化镇龙卷目击报告位置。
图 6 2020年7月6日产生系列龙卷的“逗点状”对流系统所对应的南汇双偏振雷达0.5 °仰角产品
a. 10:27反射率因子(港沿镇龙卷发生前后)；b~g. 10:39(陈家镇龙卷发生前后)的反射率因子、径向速度、谱宽、Zdr(差分反射率)、φdp(差与相移率)和CC(相关系数)。其中，白色“T1”、“T2”、“T3”和“T4”分别对应江苏太仓浏河和上海崇明港沿镇、向化镇和陈家镇龙卷目击报告位置，黑色虚线+箭头为中尺度涡旋中心的移动轨迹示意。
图 7 2020年7月6日10:41上海南汇WSR-88D多普勒天气雷达0.5 °仰角的反射率因子(a)和径向速度(c)；上海陈家镇ETWS-X01相控阵雷达子阵探测到对应时刻的5.6 °仰角的反射率因子(b)和径向速度(d)
a、c图里褐色方框包含的区域与b、d图包含的区域对应，黑色和红色点划圆圈分别标记了微型超级单体钩状回波及对应的空洞回波(b)和小尺度涡旋速度对(d)的区域。
图 8 上海相控阵雷达网陈家镇子阵雷达探测到的陈家镇龙卷1分钟间隔的演变图(10:36—10:45)：1.4 °仰角的反射率因子(a~b，e~f，i~j，m~n和q~r)和径向速度产品图(c~d，g~h，k~l，o~p和s~t)
图中白(蓝)色箭头所指为指状(钩状)回波的位置，径向速度图上的红色点划圆圈中心为小尺度涡旋的速度对。图 8r中给出了5 km的距离标尺。
图 9 陈家镇龙卷发生中前期(10:34—10:53)不同仰角(0.0 °~23.91 °)上的旋转速度(a，单位m/s)、涡旋直径(b，单位m)、水平风切变值(c，单位: 10-3 s-1)10:34开始速度对距离雷达站约8.5 km，之后逐渐靠近，10:40—10:53期间距离雷达站4~6 km。
图 11 陈家镇龙卷形成的概念模型图
a. 微型超级单体发展加强过程中，后侧入流干急流(蓝色点流线)和东南暖湿急流(褐色点流线)汇合加强并形成气旋性弯曲；b. 指状回波“收缩”演变成钩状回波、龙卷涡旋形成的过程，类似“吹吹卷”效应的示意图。
表 1 上海相控阵阵列天气雷达子阵的相关技术指标
型号 ETWS-X01 ETWS-X02 子阵站点 崇明陈家镇、宝山南王、浦东七灶 松江东勤、崇明长征 技术体制 固态有源相控阵、数字多波束 双偏振固态有源相控阵、数字多波束 工作频率 9.3~9.5 GHz 9.3~9.5 GHz 峰值发射功率 ≥320 W ≥320 W 天线阵元数量 64 128(64 H、64 V) 探测距离 ≥60 km ≥60 km 距离分辨率 ≤30 m ≤30 m 波束 ≥16 ≥16 波束宽度 水平1.6 °，垂直1.6 ° 水平1.6 °，垂直1.6 ° 体扫范围 水平：0 °~360 °，垂直: 0 °~90 ° 水平：0 °~360 °，垂直: 0 °~72 ° 体扫时间 24 s/30 s/48 s/60 s(可设置) 24 s/30 s/48 s/60 s(可设置) 整机功耗 ≤3 kW(220 V/50HZ) ≤3 kW(220 V/50 HZ) 天线尺寸 长≤1.2 m、宽≤1.4 m 长≤1.2 m、宽≤1.4 m 尺寸 1 550 mm×1 390 mm×2 000 mm 1 550 mm×1 390 mm×2 000 mm 重量 ＜850 kg ＜930 kg 
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