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Observation of A Tornado Event in Outside-region of Typhoon Mangkhut by X-band Polarimetric Phased Array Radar in 2018
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摘要:
2018年9月17日台风山竹（1822）外围螺旋雨带中发生EF2级龙卷。在高湿度、高不稳定、强垂直风切变的环流背景下，龙卷在台风外围雨带上微型超级单体右后侧钩状回波顶端的弱回波区中发展起来。具有高时空分辨率的广州X波段双极化相控阵雷达，不仅观测到超级单体的发展过程，还呈现出龙卷涡旋演变特征：单体风暴尾端在右后方入流加强作用下，逐渐形成钩状回波形态，此时对流层中低层2~3 km高度附近的中气旋强度率先达到最大，随着旋转强度进一步加强和旋转中心高度逐步下降，低层强旋转特征越来越明显，当低层旋转速度达到峰值（超过21 m·s-1），旋转直径收缩到1 km范围，地面出现EF2级以上龙卷，旋转速度对区域出现清晰的弱回波龙卷眼区特征。X波段双极化相控阵雷达在龙卷观测中优势显著，弥补了多普勒天气雷达观测的不足。
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关键词:
- X波段双极化相控阵雷达;
- 台风山竹龙卷;
- 龙卷涡旋特征;
- 旋转直径;
- 龙卷弱回波眼区
Abstract:It is well realized that the phased array radar provides fine information for meso-scale weather system, e.g., tornados. The detecting capability of Guangzhou X-band polarimetric phased array radar for severe storms is investigated, focusing on a violent tornado induced by a miniature supercell in the outer rain band of typhoon Mangkhut near Foshan on 17 September 2018 after Mangkhut's landfall. The rare complete typical tornado is captured, which is of category 2 on the enhanced Fujita scale (EF2), and it lasts for 23 minutes and causes great national economy loss.The structure, evolution and environmental conditions of the tornadic miniature supercell are discussed based on coastal Doppler S-band radar measurements. Environment conditions in the outer rain band are consistent with those of typhoon tornadoes in previous studies, with moderate convection effective potential energy and large shear below 3 km. S-band radar analysis indicate that this tornadic, miniature supercell exhibits characteristics similar to those found in landfalling hurricanes, including a hook echo, a small and shallow mesocyclone, and a relative long lifespan (~3 h).However, limited by beam blockage and resolution, further tornadic features are only observed by Guangzhou X-band polarimetric phased array radar. With the strengthening of inflow from right rear of the miniature supercell, hook echo is formed when the tornado occurs in the shallow and strong mesocyclone with the depth below the height of 2-3 km. It touches down when its parent circulation reaches its peak intensity of about 21 m·s-1. Along with intensifying of strength and contraction of couplet diameter, the height of the rotation declines below 1 km and characteristics of tornado vortex signature (TVS) are detected. The echo eye of weak echo region indicating the tornado eye is first observed. The X-band phased array radar shows great advantage in tornado observation, capturing some key characteristics of tornado evolution: Continually declining strong meso-cyclone and the appearance of TVS. The strengthening and deepening of TVS and the appearance of weak echo eye is highly likely to indicate the increase of tornado intensity. Data observed in the experiment and the preliminary results will be used in studies of tornado mechanism.
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图 1 2018年9月17日09:30台风山竹(1822)路径及广东雷达网组合反射率因子
黑色矩形框为中气旋位置，台风图标上方标记时间及台风中心最低气压，其中红色台风图标为09：00位置，与09:42中气旋位置直线距离为447 km
Fig. 1 Composite reflectivity and track of typhoon Mangkhut at 0930 BT 17 Sep 2018
black rectangle indicates the region where tornado occurred, tropical cyclone symbols are marked with time and minimum pressure, the red tropical cyclone symbol is at 0900 BT, with the straight distance of 447 km to mesocyclone at 0942 BT
图 2 2018年9月17日高度场(等值线，单位：gpm)和风场 (a)02:00 500 hPa，(b)08:00 500 hPa, (c)02:00 850 hPa，(d)08:00 850 hPa
图 2a和图 2b中填色代表整层大气可降水量，图 2c和图 2d中填色代表风速
Fig. 2 Geopotential height(the contour, unit:gpm) and wind fields on 17 Sep 2018 (a)500 hPa, 0200 BT, (b)500 hPa, 0800 BT, (c)850 hPa, 0200 BT, (d)850 hPa, 0800 BT
the shaded represents precipitable water in Fig. 2a and Fig. 2b and wind speed in Fig. 2c and Fig. 2d
图 3 2018年9月17日09:12—09:42广州多普勒天气雷达1.5°仰角反射率因子演变
龙卷(EF2等级)发生位置用黑色×形表示
Fig. 3 Radar reflectivity at 1.5° elevation angle from Guangzhou Doppler radar during 0912-0942 BT on 17 Sep 2018
the position of EF2 tornado is indicated by black ×
图 4 2018年9月17日09:12—09:42广州多普勒天气雷达1.5°仰角径向速度演变
龙卷(EF2等级)发生位置用黑色×形表示
Fig. 4 Ground-relative radial velocities at 1.5° elevation angle from Guangzhou Doppler radar during 0912-0942 BT on 17 Sep 2018
the position of EF2 tornado is indicated by black ×
图 5 2018年9月17日09:31—09:48广州相控阵雷达0.9°仰角反射率因子演变
龙卷(EF2等级)发生位置用黑色×形表示
Fig. 5 Radar reflectivity at 0.9° elevation angle from Guangzhou phased array radar during 0931-0948 BT on 17 Sep 2018
the position of EF2 tornado is indicated by black ×
图 6 2018年9月17日09:40—09:46广州相控阵雷达0.9°仰角反射率因子
龙卷(EF2等级)发生位置用黑色×形表示
Fig. 6 Radar reflectivity at 0.9° elevation angle from Guangzhou phased array radar during 0940-0946 BT on 17 Sep 2018
the position of EF2 tornado is indicated by black ×
图 7 2018年9月17日09:40—09:46广州相控阵雷达0.9°，2.7°与4.5°仰角径向速度
龙卷(EF2等级)发生位置用黑色×形表示
Fig. 7 Ground-relative radial velocities at 0.9°, 2.7° and 4.5° elevation angles from Guangzhou phased array radar during 0940-0946 BT on 17 Sep 2018
the position of EF2 tornado is indicated by black ×
图 8 2018年9月17日09:00—10：00龙卷涡旋转动速度(单位：m·s-1)、旋转直径(单位：km)和转动速度与旋转直径之比(单位：m·s-1·km-1)的垂直廓线随时间演变
灾情调查龙卷结果用黑色粗线显示，EF2等级用实心黑点标示
Fig. 8 Time-height profiles of rotational velocity(unit:m·s-1), couplet diameter(unit:km) and rotational velocity versus couplet diameter(unit:m·s-1·km-1) for the tornado-producing mesocyclone(0900-1000 BT on 17 Sep 2018)
tornado presence through visual and damage track records is indicated by the black line, EF2 tornado is indicated by black dot
图 9 2018年9月17日09:44:23广州相控阵雷达0.9°仰角观测
Fig. 9 Measurements of Guangzhou phased array radar at 0.9° elevation angle at 094423 BT 17 Sep 2018
表 1 广州相控阵雷达主要性能指标
Table 1 Parameters of Guangzhou phased array radar
性能指标 参数 工作体制 一维电子相控扫描 最大探测距离 42 km 测速范围(单PRF) ±26 m·s-1 体扫模式 水平：0~360°，
垂直: 0~30° (17层等间隔)极化方式 水平、垂直双极化 波束宽度H/V 3.6°/1.8° 发射机峰值功率 256 W 脉冲宽度 20 μs 脉冲重复频率 400~4000 Hz 灵敏度 110 dBm(@1 MHz) 噪声系数 ≤3.3 dB 动态范围 ≥85 dB 径向最大分辨率 30 m 
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