POLARIMETRIC SIGNATURES IN TORNADIC SUPERCELL THUNDERSTORMS ON JULY 22, 2020 IN ANHUI PROVINCE
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摘要: 通过对2020年7月22日安徽省宿州市龙卷的现场灾情调查，并利用地面区域自动气象站、探空资料、欧洲中心再分析资料、蚌埠和阜阳站S波段双偏振雷达资料，分析此次超级单体龙卷风暴的天气背景条件，并探讨龙卷发生时双偏振雷达观测特征。(1)此次龙卷自西向东移动，路径长约60 km，持续时间约1小时，强度为EF2级。(2)龙卷发生于梅雨期，在地面锋面和高空急流围成的区域内，位于超低空急流和高空急流交界的西北象限。探空显示，除了具有2 243 J/kg的对流有效位能(CAPE)，底层丰富水汽之外，环境风速强并且垂直风切变异常偏大是此次过程大气环流背景场中最明显的特征。(3)雷达探测到超级单体钩状回波，中心反射率因子超过55 dBZ，但强反射率因子伸展高度偏低，45 dBZ伸展高度约7 km；35 m/s的TVS位于中气旋内部西侧。当龙卷发生于钩状回波的头部时，中气旋和TVS突然增强，直径缩小，底高降低；随后TVS向中气旋中心靠近，即钩状回波头部反射率因子梯度大值区。(4)龙卷发生时双偏振雷达观测到TVS附近ZDR偏低，维持于0 dB附近；CC异常偏低，仅有0.7，TDS特征明显，其伸展高度超过2 km。钩状回波前侧有超过2 dB的ZDR高值弧，其强度和范围逐渐缩小。因此，综合使用双偏振雷达的反射率因子、径向速度、ZDR和CC产品是对龙卷进行观测识别的有效途径之一。此次过程中，ZDR和KDP产品的高值区分别位于钩状回波前部和后部，表明钩状回波内部水凝物粒子大小、形状、数量分布较为复杂，其中KDP对于强降水有较好的指示作用，对龙卷的指示意义较弱。
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关键词:
- 梅雨期龙卷 /
- 垂直风切变 /
- 双偏振雷达 /
- 中气旋 /
- 龙卷涡旋特征(TVS) /
- 龙卷碎片特征(TDS)
Abstract: Based on the on-site disaster survey data, data from ground automatic stations and sounding stations, reanalysis data from the ECMWF, data from the S-band dual-polarization radars in Bengbu and Fuyang, the present study analyzed the atmospheric circulation and polarimetric signatures in tornadicsupercell thunderstorms on July 22, 2020 in Suzhou, Anhui Province. The results showed that: (1) The tornado moved from west to east, tracking about 60 kilometers long and lasted for about 1 hour, and its intensity was EF2. (2) The tornado occurred in Meiyu period and was in the area surrounded by the ground front and the upper-level jet; it was also in the northwest quadrant of the junction of the ultra-low jet and the upper-level jet. Sounding data showed that the area had convective effective potential energy (CAPE) of about 2 243 J/kg and rich water vapor. In particular, the ambient wind speed was strong, and the vertical wind shear was abnormally large. (3) Both radars detected the hook echo of the supercell, which had a central reflectivity of over 55 dBZ. However, the extension height of 45 dBZ reflectivity was low, which was nearly 7 km; the TVS of 35 m/s was in the west of the meso-cyclone. When the tornado occurred in the hook echo, the meso-cyclone and TVS suddenly enhanced, and the diameter and bottom height of the meso-cyclone both reduced. Then the TVS approached the center of the meso-cyclone, which had a large reflectivity gradient of the hook echo. (4) During the tornado, the dual polarization radar detected the low ZDR to be around 0 dB, and abnormally low CC, which was just 0.7, near the TVS. The extension height of TDS exceeded 2 km. There was an ZDR arc of more than 2 dB in front of the hook echo, and its strength and scope gradually reduced. Therefore, it was effective to observe and identify tornadoes by using reflectivity, velocity, ZDR and CC of the dual polarization radar. In this process, the high value areas of ZDR and KDP were in the front and rear of hook echo respectively, indicating that the size, shape, and quantity distribution of condensate particles in the hook echo were complicated. In addition, KDP was a good indicator of heavy precipitation. However, itappeared to be not very useful in indicating tornadoes.-
Key words:
- tornadoes in Meiyu period /
- vertical wind shear /
- dual-polarization radar /
- meso-cyclone /
- TVS /
- TDS
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图 4 2020年7月22日17:58蚌埠雷达基本反射率因子(a.0.5 °仰角，实线为0.5 °仰角反射率因子45 dBZ和50 dBZ等值线，三角形标识为0.5 °仰角上TVS位置，黑色圆圈为中气旋位置，下同；b.1.5 °仰角)和径向速度图(c.0.5 °仰角; d.1.5 °仰角)
图 5 2020年7月22日17:58蚌埠雷达ZDR产品(a.0.5 °仰角；b.1.5 °仰角)、CC产品(c.0.5 °仰角；d.1.5 °仰角)和KDP产品(e.0.5 °仰角；f.1.5 °仰角)
表 1 蚌埠双偏振雷达龙卷特征量
时间 17:35 17:40 17:46 17:52 17:58 18:03 18:09 18:15 18:20 18:26 中气旋旋转速度/(m/s) 17.5 17 16.5 17 21 24.5 24.5 21.5 18 18 中气旋直径/km 4.2 5.8 5.6 5.8 6 4.2 4 4.5 4.6 5.6 中气旋底高/km 1.1 1 1 1 0.9 0.9 0.9 0.9 0.9 0.9 TVS切变速度/(m/s) \ \ 23 26 35 31 37 29 21 22.5 ZDR/dB 3.6 2.7 2.5 2.5 -0.21 -0.01 -0.77 -0.66 0.21 1.59 CC 0.99 0.98 0.99 0.99 0.7 0.83 0.85 0.82 0.94 0.98 
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